1 00:00:00,866 --> 00:00:02,866 ♪ ♪ 2 00:00:06,366 --> 00:00:10,633 NARRATOR: They're the most mysterious particles ever discovered, 3 00:00:10,666 --> 00:00:14,833 {\an1}tiny ghosts hidden in our world. 4 00:00:14,866 --> 00:00:20,000 {\an1}Now scientists are on a mission to unlock their secrets. 5 00:00:20,033 --> 00:00:22,966 {\an1}They're called neutrinos. 6 00:00:24,666 --> 00:00:29,066 {\an1}The story of their discovery is almost impossible to believe. 7 00:00:29,100 --> 00:00:31,200 DAVID KAISER: If they had bolted the detector 8 00:00:31,233 --> 00:00:32,509 {\an1}in place, the nuclear bomb would've 9 00:00:32,533 --> 00:00:34,400 {\an1}just smashed it to smithereens. 10 00:00:34,433 --> 00:00:38,600 NARRATOR: With links to a dramatic Cold War defection. 11 00:00:38,633 --> 00:00:40,476 FRANK CLOSE: He disappeared through the Iron Curtain, 12 00:00:40,500 --> 00:00:41,733 {\an1}and for five years, 13 00:00:41,766 --> 00:00:43,433 {\an1}disappeared off the face of the planet. 14 00:00:43,466 --> 00:00:45,500 NARRATOR: And astonishing experiments 15 00:00:45,533 --> 00:00:47,900 {\an1}that keep defying the laws of physics. 16 00:00:47,933 --> 00:00:51,466 KERSTIN PEREZ: Even as someone who builds these experiments for a living, 17 00:00:51,500 --> 00:00:54,000 {\an1}it just seems mind-blowing that they ever work. 18 00:00:54,033 --> 00:00:56,933 NARRATOR: Today, scientists are using neutrinos 19 00:00:56,966 --> 00:01:00,633 {\an1}to probe the edges of our detectable universe. 20 00:01:00,666 --> 00:01:04,166 {\an1}They're on a mission to reveal a hidden world 21 00:01:04,200 --> 00:01:06,066 {\an1}of "Particles Unknown." 22 00:01:08,333 --> 00:01:10,766 {\an1}Right now, on "NOVA." 23 00:01:10,800 --> 00:01:11,800 ♪ ♪ 24 00:01:33,000 --> 00:01:36,200 NARRATOR: We live in a world of matter... 25 00:01:36,233 --> 00:01:41,966 {\an1}a realm of tiny particles far smaller than atoms 26 00:01:42,000 --> 00:01:43,900 {\an1}that build the universe that we know. 27 00:01:43,933 --> 00:01:47,633 {\an1}But there is a mystery. 28 00:01:47,666 --> 00:01:52,633 {\an1}Scientists theorize there exists a hidden, parallel world 29 00:01:52,666 --> 00:01:57,600 of particles... So-called dark matter. 30 00:01:57,633 --> 00:02:03,600 {\an1}So far, no one has managed to detect a single one. 31 00:02:03,633 --> 00:02:08,966 {\an1}But now there might be a way. 32 00:02:09,000 --> 00:02:12,033 {\an1}Of all the particles scientists have discovered, 33 00:02:12,066 --> 00:02:16,700 {\an1}the most elusive, on the very edge of detectability, 34 00:02:16,733 --> 00:02:19,700 are neutrinos. 35 00:02:19,733 --> 00:02:22,500 ♪ ♪ 36 00:02:22,533 --> 00:02:25,766 KAISER: Neutrinos are really remarkable particles. 37 00:02:25,800 --> 00:02:28,266 {\an1}There are trillions and trillions of them 38 00:02:28,300 --> 00:02:29,800 {\an1}streaming through our bodies, 39 00:02:29,833 --> 00:02:31,300 {\an1}and we don't even notice. 40 00:02:31,333 --> 00:02:34,000 {\an1}They are kind of ghost-like, and yet they're everywhere. 41 00:02:34,033 --> 00:02:36,600 NARRATOR: Everywhere and nowhere. 42 00:02:36,633 --> 00:02:40,366 {\an1}Neutrinos are so ghostly, they can pass 43 00:02:40,400 --> 00:02:44,766 {\an1}through solid matter as if it didn't exist. 44 00:02:44,800 --> 00:02:48,500 {\an1}And yet they hold the secrets to why the stars shine 45 00:02:48,533 --> 00:02:50,733 {\an1}and what our universe is made of. 46 00:02:50,766 --> 00:02:53,566 {\an1}RAY JAYAWARDHANA: The reason we care about these elusive particles 47 00:02:53,600 --> 00:02:56,966 {\an1}is because they do play a fundamentally important role 48 00:02:57,000 --> 00:03:01,333 in the universe, in the nature of matter... 49 00:03:01,366 --> 00:03:05,400 {\an1}in some of the most violent cosmic phenomena. 50 00:03:06,433 --> 00:03:09,300 NARRATOR: First theorized in the 1930s, 51 00:03:09,333 --> 00:03:12,700 {\an1}they would soon become linked to nuclear secrets 52 00:03:12,733 --> 00:03:15,266 {\an1}and a dramatic Cold War defection 53 00:03:15,300 --> 00:03:18,266 {\an1}behind the Iron Curtain. 54 00:03:18,300 --> 00:03:20,000 {\an1}He goes off to Europe 55 00:03:20,033 --> 00:03:21,800 {\an1}and never returns. 56 00:03:21,833 --> 00:03:24,533 NARRATOR: Now the quest to detect neutrinos 57 00:03:24,566 --> 00:03:29,333 {\an1}has triggered vast experiments all over the globe. 58 00:03:29,366 --> 00:03:31,642 {\an1}Even as someone who builds these experiments for a living, 59 00:03:31,666 --> 00:03:34,233 {\an1}it just seems mind-blowing that they ever work. 60 00:03:34,266 --> 00:03:36,900 NARRATOR: Today, scientists are on the cusp 61 00:03:36,933 --> 00:03:38,933 {\an1}of an astonishing discovery. 62 00:03:38,966 --> 00:03:42,066 {\an1}Tantalizing evidence suggests neutrinos 63 00:03:42,100 --> 00:03:45,766 {\an1}could be a doorway between our world of matter 64 00:03:45,800 --> 00:03:48,600 {\an1}and the hidden world of dark matter, 65 00:03:48,633 --> 00:03:50,933 {\an1}waiting to be discovered. 66 00:03:50,966 --> 00:03:53,233 {\an1}GEORGIA KARAGIORGI: It would be a game-changer. 67 00:03:53,266 --> 00:03:55,333 {\an1}What exactly are these particles? 68 00:03:55,366 --> 00:03:59,966 {\an1}What is its role in the evolution of our universe? 69 00:04:00,000 --> 00:04:01,833 NARRATOR: The quest for answers 70 00:04:01,866 --> 00:04:04,466 {\an1}has driven scientists to the edge 71 00:04:04,500 --> 00:04:06,733 {\an1}of what is experimentally possible 72 00:04:06,766 --> 00:04:12,600 {\an1}to reveal a universe we've never seen before. 73 00:04:12,633 --> 00:04:17,366 ♪ ♪ 74 00:04:24,000 --> 00:04:27,733 NARRATOR: Fermilab, in Batavia, Illinois. 75 00:04:27,766 --> 00:04:31,666 {\an1}World-renowned physics laboratory. 76 00:04:31,700 --> 00:04:35,466 {\an1}Thousands of scientists build enormous experiments 77 00:04:35,500 --> 00:04:37,833 {\an1}to probe the very smallest particles 78 00:04:37,866 --> 00:04:41,100 {\an1}that make up our universe. 79 00:04:41,133 --> 00:04:42,666 {\an1}(indistinct chatter) 80 00:04:42,700 --> 00:04:45,366 {\an1}Leading one of the teams is Sam Zeller. 81 00:04:45,400 --> 00:04:47,533 Hi, team. 82 00:04:47,566 --> 00:04:49,109 {\an7}My interest in physics started when I signed up 83 00:04:49,133 --> 00:04:51,833 {\an7}for a field trip to come to Fermilab in high school. 84 00:04:51,866 --> 00:04:53,233 {\an1}It just blew my mind. 85 00:04:53,266 --> 00:04:58,300 {\an1}From that point on, I was a particle physicist. 86 00:04:58,333 --> 00:05:00,666 ♪ ♪ 87 00:05:00,700 --> 00:05:03,700 {\an1}It turns out that the universe can be described 88 00:05:03,733 --> 00:05:06,100 {\an1}by a small number of subatomic particles. 89 00:05:06,133 --> 00:05:08,866 ♪ ♪ 90 00:05:08,900 --> 00:05:10,700 NARRATOR: Today, scientists have discovered 91 00:05:10,733 --> 00:05:14,633 {\an1}17 basic particles that make up our universe. 92 00:05:14,666 --> 00:05:16,400 ♪ ♪ 93 00:05:16,433 --> 00:05:20,666 {\an1}Some are the building blocks of atoms. 94 00:05:20,700 --> 00:05:25,700 {\an1}Others are the things that hold matter together. 95 00:05:25,733 --> 00:05:29,000 {\an7}It's an understanding of our world that physicists call 96 00:05:29,033 --> 00:05:31,400 {\an7}the Standard Model. 97 00:05:31,433 --> 00:05:33,566 {\an8}PEREZ: The Standard Model of particle physics 98 00:05:33,600 --> 00:05:36,733 {\an7}describes the most fundamental constituents of matter 99 00:05:36,766 --> 00:05:39,800 {\an7}and how they interact with each other. 100 00:05:39,833 --> 00:05:43,066 {\an7}It is in fact the most mathematically well-defined 101 00:05:43,100 --> 00:05:46,500 {\an7}physical theory we as humans have ever written down. 102 00:05:46,533 --> 00:05:48,000 ♪ ♪ 103 00:05:48,033 --> 00:05:49,266 NARRATOR: For 50 years, 104 00:05:49,300 --> 00:05:53,400 {\an1}the Standard Model has withstood test after test, 105 00:05:53,433 --> 00:05:56,666 {\an1}confirming the hierarchy of all the fundamental particles. 106 00:05:56,700 --> 00:05:58,433 (device beeping) 107 00:05:58,466 --> 00:06:04,066 {\an1}But one type remains far more mysterious than others. 108 00:06:05,166 --> 00:06:08,833 {\an1}They're called neutrinos. 109 00:06:08,866 --> 00:06:09,833 JAYAWARDHANA: A neutrino is a 110 00:06:09,866 --> 00:06:12,200 {\an7}type of elementary particle, 111 00:06:12,233 --> 00:06:16,300 {\an7}a basic fundamental building block of the universe, 112 00:06:16,333 --> 00:06:20,400 and they come in three different flavors. 113 00:06:20,433 --> 00:06:22,033 KARAGIORGI: Neutrinos are everywhere. 114 00:06:22,066 --> 00:06:24,866 {\an1}They are produced in the sun. 115 00:06:24,900 --> 00:06:29,033 {\an7}There are neutrinos that were left over after the Big Bang. 116 00:06:29,066 --> 00:06:33,033 {\an1}Humans emit neutrinos. 117 00:06:33,066 --> 00:06:35,666 CLOSE: Neutrinos have got no electric charge. 118 00:06:35,700 --> 00:06:38,533 {\an7}They've almost got no mass at all. 119 00:06:38,566 --> 00:06:40,500 {\an7}They're as near to nothing as you can imagine. 120 00:06:40,533 --> 00:06:44,066 {\an1}They're so reluctant to interact with stuff, 121 00:06:44,100 --> 00:06:47,666 {\an1}they pass through the Earth as if it wasn't there. 122 00:06:47,700 --> 00:06:51,500 NARRATOR: And yet, at Fermilab, scientists are constructing 123 00:06:51,533 --> 00:06:53,900 {\an1}a complex two-stage experiment 124 00:06:53,933 --> 00:06:58,100 {\an1}with the means to create them and study them. 125 00:06:58,133 --> 00:07:00,033 {\an8}♪ ♪ 126 00:07:00,066 --> 00:07:03,700 {\an7}In its first stage, a powerful ring of magnets 127 00:07:03,733 --> 00:07:07,400 {\an7}accelerates positively charged particles called protons 128 00:07:07,433 --> 00:07:13,466 {\an7}to colossal speeds, sending them smashing into a target. 129 00:07:13,500 --> 00:07:16,500 {\an7}The collision creates a shower of new particles, 130 00:07:16,533 --> 00:07:21,466 {\an7}including a powerful beam of neutrinos. 131 00:07:21,500 --> 00:07:25,066 {\an7}150 trillion per second pass through the Earth 132 00:07:25,100 --> 00:07:26,933 {\an7}at nearly the speed of light, 133 00:07:26,966 --> 00:07:29,266 {\an7}racing towards the second stage... 134 00:07:29,300 --> 00:07:32,133 {\an7}three giant neutrino detectors. 135 00:07:33,900 --> 00:07:38,266 {\an1}The largest is called ICARUS. 136 00:07:38,300 --> 00:07:39,566 Once complete, 137 00:07:39,600 --> 00:07:42,400 {\an1}this immense tank filled with a web of electronics 138 00:07:42,433 --> 00:07:44,266 {\an1}and cryogenic liquid 139 00:07:44,300 --> 00:07:48,766 {\an1}will be bombarded by hundreds of trillions of neutrinos, 140 00:07:48,800 --> 00:07:53,400 {\an1}all in the hope of catching just one each minute. 141 00:07:53,433 --> 00:07:55,100 ♪ ♪ 142 00:07:55,133 --> 00:07:58,100 {\an1}That alone will be a remarkable achievement. 143 00:07:58,133 --> 00:07:59,700 (device beeping) 144 00:07:59,733 --> 00:08:03,500 {\an1}But the scientists have even bigger ambitions. 145 00:08:03,533 --> 00:08:07,466 ZELLER: One of the big goals here at Fermilab is to try to search 146 00:08:07,500 --> 00:08:09,500 {\an1}for possibly a new type of neutrino 147 00:08:09,533 --> 00:08:11,100 {\an1}that no one has yet observed. 148 00:08:13,366 --> 00:08:14,742 NARRATOR: Experiments have hinted there could be 149 00:08:14,766 --> 00:08:17,033 {\an1}an even more elusive neutrino 150 00:08:17,066 --> 00:08:20,533 {\an1}beyond the three types already known to exist. 151 00:08:20,566 --> 00:08:23,733 {\an1}Some have suggested that it could be a link 152 00:08:23,766 --> 00:08:25,966 {\an1}to a hidden realm of particles 153 00:08:26,000 --> 00:08:27,533 {\an1}that could finally lead 154 00:08:27,566 --> 00:08:31,600 {\an1}to new discoveries beyond the Standard Model. 155 00:08:31,633 --> 00:08:33,833 ZELLER: If we found evidence for a new type of neutrino, 156 00:08:33,866 --> 00:08:35,900 {\an1}that would be really astounding. 157 00:08:35,933 --> 00:08:37,376 {\an1}That's what gets me excited in the morning. 158 00:08:37,400 --> 00:08:38,776 {\an1}That's what gets me coming in to work. 159 00:08:38,800 --> 00:08:41,566 {\an1}It would be a major and massive discovery. 160 00:08:43,366 --> 00:08:47,033 NARRATOR: Making that discovery would be groundbreaking. 161 00:08:47,066 --> 00:08:52,566 {\an1}Because while ordinary neutrinos are extremely hard to detect, 162 00:08:52,600 --> 00:08:57,466 {\an1}this fourth type of neutrino could break the Standard Model. 163 00:08:59,900 --> 00:09:01,176 {\an1}What brought them to this moment... 164 00:09:01,200 --> 00:09:04,100 {\an1}and possibly to the brink of upending 165 00:09:04,133 --> 00:09:06,800 {\an1}one of the bedrocks of modern physics? 166 00:09:06,833 --> 00:09:09,233 ♪ ♪ 167 00:09:09,266 --> 00:09:13,300 {\an1}That story begins almost 100 years ago 168 00:09:13,333 --> 00:09:15,166 {\an1}half a world away. 169 00:09:16,633 --> 00:09:19,333 In Rome. 170 00:09:21,766 --> 00:09:25,233 {\an1}Physicist and historian Professor David Kaiser 171 00:09:25,266 --> 00:09:27,666 {\an1}has traveled here, to the place where, 172 00:09:27,700 --> 00:09:30,633 in the 1930s, scientists were investigating 173 00:09:30,666 --> 00:09:33,800 {\an1}the inner workings of the atom. 174 00:09:33,833 --> 00:09:37,966 KAISER: For millennia, for thousands of years, 175 00:09:38,000 --> 00:09:41,233 {\an1}people had come to believe that the world was made of atoms, 176 00:09:41,266 --> 00:09:43,900 {\an7}and those atoms were the smallest thing there was. 177 00:09:43,933 --> 00:09:45,533 {\an7}In fact, the word atom even means 178 00:09:45,566 --> 00:09:48,033 {\an7}"unbreakable" or "indivisible"... The smallest piece. 179 00:09:48,066 --> 00:09:50,200 ♪ ♪ 180 00:09:50,233 --> 00:09:52,366 NARRATOR: But by the early 1900s, 181 00:09:52,400 --> 00:09:55,800 {\an1}scientists had revealed a deeper hidden structure. 182 00:09:57,333 --> 00:10:00,800 KAISER: If you think about an atom, it's about a nanometer, 183 00:10:00,833 --> 00:10:04,300 {\an1}about a billion times smaller than a meter, roughly. 184 00:10:04,333 --> 00:10:07,366 {\an1}The inside, the deep core of an atom, the nucleus, 185 00:10:07,400 --> 00:10:11,500 {\an1}is about 100,000 times smaller than that. 186 00:10:11,533 --> 00:10:15,233 {\an1}So we're really zooming in powers of ten, powers of ten, 187 00:10:15,266 --> 00:10:17,300 {\an1}getting to unimaginably tiny scales. 188 00:10:17,333 --> 00:10:20,933 NARRATOR: During the early 20th century, 189 00:10:20,966 --> 00:10:26,066 {\an1}scientists discovered the atom's tiny nucleus contained protons, 190 00:10:26,100 --> 00:10:29,333 particles with a positive electric charge. 191 00:10:29,366 --> 00:10:31,833 {\an1}These protons held in place 192 00:10:31,866 --> 00:10:35,200 {\an1}a cloud of negatively charged electrons 193 00:10:35,233 --> 00:10:37,766 {\an1}that formed the atom's outer limit. 194 00:10:40,966 --> 00:10:43,466 {\an1}It seemed that protons and electrons 195 00:10:43,500 --> 00:10:46,500 {\an1}were the only two components of all atoms... 196 00:10:46,533 --> 00:10:50,633 {\an1}permanent and fixed. 197 00:10:50,666 --> 00:10:54,500 {\an1}But scientists had also found something shocking: 198 00:10:54,533 --> 00:10:59,300 {\an1}some types of atoms seemed to break apart. 199 00:10:59,333 --> 00:11:01,100 KAISER: That was just jaw-dropping. 200 00:11:01,133 --> 00:11:03,966 {\an1}Literally, it contradicts the name of the thing itself. 201 00:11:04,000 --> 00:11:05,600 {\an1}Atoms are supposed to not break down. 202 00:11:05,633 --> 00:11:07,700 ♪ ♪ 203 00:11:07,733 --> 00:11:11,900 NARRATOR: It was as though certain atoms had too much energy. 204 00:11:11,933 --> 00:11:16,000 {\an1}The nucleus would spontaneously transform 205 00:11:16,033 --> 00:11:19,533 {\an1}and spit out an electron. 206 00:11:19,566 --> 00:11:22,866 {\an1}This phenomenon was a type of radioactivity 207 00:11:22,900 --> 00:11:26,333 {\an1}known as beta decay. 208 00:11:26,366 --> 00:11:27,800 JAYAWARDHANA: It appeared to be 209 00:11:27,833 --> 00:11:32,033 {\an1}this sort of mysterious energy leaking from or emanating from 210 00:11:32,066 --> 00:11:34,466 certain atoms. 211 00:11:34,500 --> 00:11:38,100 {\an8}NARRATOR: This process was remarkable in itself, 212 00:11:38,133 --> 00:11:40,166 {\an7}but when scientists measured the energy 213 00:11:40,200 --> 00:11:44,500 {\an7}of the electrons from beta decay, something was wrong. 214 00:11:44,533 --> 00:11:48,733 KARAGIORGI: One of the basic principles in all sciences 215 00:11:48,766 --> 00:11:53,000 {\an1}is that energy can change from one form to the other, 216 00:11:53,033 --> 00:11:56,066 {\an1}but the total sum must be conserved. 217 00:11:56,100 --> 00:11:58,333 ♪ ♪ 218 00:11:58,366 --> 00:12:03,366 NARRATOR: This is the principle of conservation of energy. 219 00:12:03,400 --> 00:12:05,800 {\an1}From collisions in the macro world 220 00:12:05,833 --> 00:12:07,900 to the behavior of tiny particles, 221 00:12:07,933 --> 00:12:12,633 {\an1}the principle states that energy should never disappear. 222 00:12:12,666 --> 00:12:16,000 {\an1}But when scientists measured the energy of the electrons 223 00:12:16,033 --> 00:12:20,633 {\an1}from beta decay, that's exactly what seemed to happen. 224 00:12:20,666 --> 00:12:24,933 KARAGIORGI: So every time, rather than having energy conserved, 225 00:12:24,966 --> 00:12:26,233 {\an1}what they were seeing is that 226 00:12:26,266 --> 00:12:29,033 {\an1}some amount of energy would be missing. 227 00:12:29,066 --> 00:12:33,200 NARRATOR: Where was the energy going? 228 00:12:33,233 --> 00:12:35,933 {\an1}It seemed that the particles themselves were breaking 229 00:12:35,966 --> 00:12:40,000 {\an1}the fundamental rules of physics. 230 00:12:42,800 --> 00:12:44,700 ♪ ♪ 231 00:12:44,733 --> 00:12:49,933 {\an1}In 1926, a young Italian physicist called Enrico Fermi 232 00:12:49,966 --> 00:12:54,100 {\an1}was working at the University of Rome's Physics Institute. 233 00:12:55,700 --> 00:12:57,533 {\an1}It was here that Fermi probed 234 00:12:57,566 --> 00:13:01,566 {\an1}into the developing field of nuclear physics. 235 00:13:04,000 --> 00:13:05,476 KAISER: Enrico Fermi was really a towering figure 236 00:13:05,500 --> 00:13:06,666 {\an1}of 20th-century physics... 237 00:13:06,700 --> 00:13:08,409 {\an1}by any measure, one of the greatest physicists 238 00:13:08,433 --> 00:13:09,866 {\an1}of the 20th century. 239 00:13:09,900 --> 00:13:12,600 This is the site where Fermi built what became 240 00:13:12,633 --> 00:13:16,233 {\an1}an absolutely world-class group of researchers. 241 00:13:16,266 --> 00:13:18,000 NARRATOR: They were known 242 00:13:18,033 --> 00:13:20,566 {\an1}as the Via Panisperna Boys. 243 00:13:20,600 --> 00:13:22,366 KAISER: This is really an iconic photograph. 244 00:13:22,400 --> 00:13:24,176 {\an1}It captures them in the middle of what would become 245 00:13:24,200 --> 00:13:26,233 {\an1}world-changing research. 246 00:13:26,266 --> 00:13:28,033 {\an1}Fermi himself was remarkably young... 247 00:13:28,066 --> 00:13:30,233 {\an1}he was just 26 years old, 248 00:13:30,266 --> 00:13:32,566 {\an1}and already he'd been made the big senior professor 249 00:13:32,600 --> 00:13:35,633 {\an1}around which this young group would come together. 250 00:13:35,666 --> 00:13:38,700 {\an1}They referred to Fermi as the Pope, he was the great leader. 251 00:13:38,733 --> 00:13:43,000 {\an1}Rasetti was next in line, he was a cardinal. 252 00:13:43,033 --> 00:13:44,076 {\an1}The person taking the photograph, 253 00:13:44,100 --> 00:13:45,500 {\an1}the very young Bruno Pontecorvo, 254 00:13:45,533 --> 00:13:46,933 {\an1}the youngest member of the group, 255 00:13:46,966 --> 00:13:51,100 {\an1}they called him the Puppy. 256 00:13:51,133 --> 00:13:56,200 {\an8}NARRATOR: The group's ideas would have a profound impact on the world. 257 00:13:57,333 --> 00:13:58,833 {\an8}♪ ♪ 258 00:13:58,866 --> 00:14:01,866 {\an8}In October 1931, 259 00:14:01,900 --> 00:14:04,500 {\an7}they invited a group of the world's leading physicists 260 00:14:04,533 --> 00:14:09,033 {\an7}to a conference held at the Physics Institute. 261 00:14:09,066 --> 00:14:11,633 {\an7}High on the agenda was the problem 262 00:14:11,666 --> 00:14:14,733 {\an8}of the missing radioactive energy. 263 00:14:16,500 --> 00:14:18,666 {\an7}One scientist at the conference, 264 00:14:18,700 --> 00:14:23,966 {\an7}the famous Wolfgang Pauli, proposed a radical idea. 265 00:14:24,000 --> 00:14:26,900 KAISER: Wolfgang Pauli had written a letter to colleagues. 266 00:14:26,933 --> 00:14:29,466 {\an1}And he put forward what he called a desperate remedy, 267 00:14:29,500 --> 00:14:33,333 {\an1}a "versweifelten Ausweg"... It was just ridiculous. 268 00:14:33,366 --> 00:14:35,266 {\an7}And he says so in his letter. 269 00:14:35,300 --> 00:14:38,566 {\an7}It's a really quite strange-sounding idea. 270 00:14:38,600 --> 00:14:41,100 {\an1}What if there was a new type of particle in the world 271 00:14:41,133 --> 00:14:44,800 {\an1}that no one had ever seen or detected before? 272 00:14:44,833 --> 00:14:46,766 {\an8}♪ ♪ 273 00:14:46,800 --> 00:14:51,333 {\an8}NARRATOR: Pauli suggested that instead of just an electron, 274 00:14:51,366 --> 00:14:54,066 {\an7}perhaps there was an unknown particle 275 00:14:54,100 --> 00:14:58,666 {\an7}that was carrying away the missing energy. 276 00:14:58,700 --> 00:15:00,766 KAISER: Very few people seem to have been convinced 277 00:15:00,800 --> 00:15:02,633 {\an1}that this was the right way to go. 278 00:15:02,666 --> 00:15:05,400 At that time, physicists were quite confident 279 00:15:05,433 --> 00:15:07,566 {\an1}there existed two basic kinds of particles, 280 00:15:07,600 --> 00:15:09,333 {\an1}electrons and protons. 281 00:15:09,366 --> 00:15:14,066 {\an1}But Pauli was suggesting, "Let's make this enormous leap." 282 00:15:14,100 --> 00:15:18,066 NARRATOR: A new particle of matter seemed a step too far. 283 00:15:18,100 --> 00:15:19,633 ♪ ♪ 284 00:15:19,666 --> 00:15:23,566 {\an1}But for Enrico Fermi, the Pope of Via Panisperna, 285 00:15:23,600 --> 00:15:29,100 {\an1}he took the wacky idea and ran with it. 286 00:15:29,133 --> 00:15:32,000 {\an1}Fermi dedicated the next two years of his life 287 00:15:32,033 --> 00:15:34,966 {\an1}to describe the obscure ghost particle. 288 00:15:35,000 --> 00:15:38,266 {\an1}It would be neutral, and carry no electric charge. 289 00:15:38,300 --> 00:15:42,433 {\an1}It would be tiny, far smaller than an electron. 290 00:15:42,466 --> 00:15:47,166 {\an1}And it would pass through atoms as if they weren't there at all. 291 00:15:47,200 --> 00:15:50,800 {\an1}He named the particle the neutrino, 292 00:15:50,833 --> 00:15:54,100 Italian for "little neutral one." 293 00:15:56,500 --> 00:15:59,800 KAISER: This was a really quite remarkable step. 294 00:15:59,833 --> 00:16:02,233 {\an1}But many physicists, Fermi included, thought 295 00:16:02,266 --> 00:16:03,542 {\an1}that it should be nearly impossible... 296 00:16:03,566 --> 00:16:05,500 {\an1}perhaps impossible forever... 297 00:16:05,533 --> 00:16:10,233 {\an1}to detect such a particle even if it really exists. 298 00:16:10,266 --> 00:16:13,066 ♪ ♪ 299 00:16:13,100 --> 00:16:15,600 NARRATOR: Outside the intellectual fervor of the lab, 300 00:16:15,633 --> 00:16:17,766 {\an1}fascism was about to cast a shadow 301 00:16:17,800 --> 00:16:20,533 {\an1}over the neutrino mystery. 302 00:16:20,566 --> 00:16:25,033 {\an1}In 1939, Fermi immigrated to the U.S.A. 303 00:16:25,066 --> 00:16:27,166 {\an1}and was quickly put to work. 304 00:16:27,200 --> 00:16:29,166 {\an1}He helped to develop 305 00:16:29,200 --> 00:16:31,733 {\an1}the first operational nuclear reactor 306 00:16:31,766 --> 00:16:36,700 {\an1}that led eventually to the atomic bomb. 307 00:16:38,433 --> 00:16:43,733 {\an1}But not everybody had forgotten about the elusive neutrino. 308 00:16:43,766 --> 00:16:46,400 ♪ ♪ 309 00:16:46,433 --> 00:16:51,866 {\an1}Bruno Pontecorvo, the Puppy of the Via Panisperna Boys. 310 00:16:51,900 --> 00:16:55,533 {\an1}Upon moving to England after the Second World War, 311 00:16:55,566 --> 00:16:58,200 {\an1}he continued to think about neutrinos 312 00:16:58,233 --> 00:17:02,233 {\an1}until his life took a shocking turn. 313 00:17:02,266 --> 00:17:06,766 CLOSE: Pontecorvo was a man who created big ideas. 314 00:17:06,800 --> 00:17:10,600 {\an7}The work that he did on neutrinos alone 315 00:17:10,633 --> 00:17:12,600 {\an7}could have won him 316 00:17:12,633 --> 00:17:14,900 {\an1}certainly one Nobel Prize, 317 00:17:14,933 --> 00:17:16,800 {\an1}and been a candidate maybe for two. 318 00:17:16,833 --> 00:17:20,533 NARRATOR: But it wasn't to be. 319 00:17:20,566 --> 00:17:24,033 {\an1}In 1950, in the midst of the Cold War, 320 00:17:24,066 --> 00:17:28,733 {\an1}Pontecorvo and his family mysteriously went missing. 321 00:17:28,766 --> 00:17:30,833 Bruno Pontecorvo 322 00:17:30,866 --> 00:17:33,700 {\an1}disappeared through the Iron Curtain in 1950, 323 00:17:33,733 --> 00:17:36,400 {\an1}and for five years, 324 00:17:36,433 --> 00:17:38,533 {\an1}disappeared off the face of the planet. 325 00:17:40,100 --> 00:17:42,600 NARRATOR: Only after five years of silence 326 00:17:42,633 --> 00:17:46,100 did he reappear in the Soviet Union. 327 00:17:46,133 --> 00:17:49,800 ♪ ♪ 328 00:17:49,833 --> 00:17:52,466 {\an1}So, what happened? 329 00:17:52,500 --> 00:17:55,066 {\an1}Was he kidnapped? 330 00:17:55,100 --> 00:17:58,333 Was he a spy? 331 00:17:58,366 --> 00:18:00,533 {\an1}Professor Frank Close has spent years 332 00:18:00,566 --> 00:18:05,733 {\an1}researching Pontecorvo and his mysterious disappearance. 333 00:18:05,766 --> 00:18:10,100 {\an1}He has come to the British National Archives in London. 334 00:18:11,300 --> 00:18:13,200 {\an1}Earlier in his life, 335 00:18:13,233 --> 00:18:16,433 {\an1}Pontecorvo had been a member of a communist party. 336 00:18:16,466 --> 00:18:19,166 {\an1}And there are now British intelligence files 337 00:18:19,200 --> 00:18:21,766 under his name. 338 00:18:21,800 --> 00:18:23,000 CLOSE: Looking at these 339 00:18:23,033 --> 00:18:25,733 old folders, they're worn down the sides. 340 00:18:25,766 --> 00:18:27,700 {\an1}They have red stamps, "top secret." 341 00:18:27,733 --> 00:18:31,366 {\an1}The case of Pontecorvo. 342 00:18:31,400 --> 00:18:34,066 {\an1}It is dripping with intrigue. (chuckles) 343 00:18:34,100 --> 00:18:35,500 ♪ ♪ 344 00:18:35,533 --> 00:18:36,900 NARRATOR: After the war, 345 00:18:36,933 --> 00:18:40,200 {\an1}while working for the U.K.'s atomic energy program, 346 00:18:40,233 --> 00:18:45,133 {\an1}Pontecorvo devised a method to try and detect neutrinos. 347 00:18:45,166 --> 00:18:48,166 He reasoned that nuclear reactors... 348 00:18:48,200 --> 00:18:51,066 {\an1}which derive energy from splitting atoms... 349 00:18:51,100 --> 00:18:54,766 {\an1}should produce neutrinos in vast quantities. 350 00:18:54,800 --> 00:18:59,700 {\an1}But the government classified his paper. 351 00:18:59,733 --> 00:19:04,366 {\an1}Now, I conjecture that this paper was classified secret 352 00:19:04,400 --> 00:19:09,100 {\an1}because, if you could indeed detect neutrinos 353 00:19:09,133 --> 00:19:11,333 {\an1}coming from a nuclear reactor, 354 00:19:11,366 --> 00:19:12,966 {\an1}you would be able to work out 355 00:19:13,000 --> 00:19:14,733 how powerful the nuclear reactor was. 356 00:19:14,766 --> 00:19:16,866 {\an1}So they classified it. 357 00:19:16,900 --> 00:19:19,433 ♪ ♪ 358 00:19:19,466 --> 00:19:21,466 NARRATOR: As the Cold War escalated, 359 00:19:21,500 --> 00:19:27,066 {\an1}the U.S.A. became paranoid of atomic espionage. 360 00:19:27,100 --> 00:19:31,533 {\an1}In 1950, the Rosenberg spy ring was uncovered. 361 00:19:31,566 --> 00:19:34,666 And it triggered a communist witch hunt. 362 00:19:37,033 --> 00:19:39,133 {\an1}A secret letter reveals the FBI 363 00:19:39,166 --> 00:19:41,500 {\an1}wrote to a British intelligence service 364 00:19:41,533 --> 00:19:43,900 {\an1}about Pontecorvo. 365 00:19:43,933 --> 00:19:47,666 CLOSE: "The FBI now ask if we can send them any information 366 00:19:47,700 --> 00:19:49,500 {\an1}"which would indicate that Pontecorvo 367 00:19:49,533 --> 00:19:53,166 may be engaged in communist activities." 368 00:19:53,200 --> 00:19:57,233 {\an1}The letter was received in London on the 19th of July. 369 00:19:57,266 --> 00:19:58,666 Five days later, 370 00:19:58,700 --> 00:20:02,333 {\an1}Pontecorvo goes off to Europe and never returns. 371 00:20:02,366 --> 00:20:03,866 ♪ ♪ 372 00:20:03,900 --> 00:20:06,900 NARRATOR: Flight manifests reveal Pontecorvo and his family 373 00:20:06,933 --> 00:20:10,866 {\an1}flew from Rome, across Europe, to Helsinki, 374 00:20:10,900 --> 00:20:14,900 {\an1}alongside two suspected KGB agents. 375 00:20:14,933 --> 00:20:18,733 {\an1}Pontecorvo's son, just 12 years old at the time, 376 00:20:18,766 --> 00:20:22,566 {\an1}revealed they were then driven across the border to Moscow... 377 00:20:22,600 --> 00:20:25,433 {\an1}with Bruno in the trunk. 378 00:20:25,466 --> 00:20:26,600 CLOSE: He said to me, 379 00:20:26,633 --> 00:20:29,400 {\an1}"I knew something was up." (chuckles) 380 00:20:29,433 --> 00:20:34,933 NARRATOR: Frank believes a Soviet mole passed the FBI letter to Moscow, 381 00:20:34,966 --> 00:20:40,266 {\an1}who then pressured Pontecorvo to defect. 382 00:20:40,300 --> 00:20:43,700 {\an1}There's no clear evidence that he had been a spy, 383 00:20:43,733 --> 00:20:46,366 {\an1}but whatever his reason for leaving, 384 00:20:46,400 --> 00:20:50,233 {\an1}Bruno's time in the West was over. 385 00:20:50,266 --> 00:20:51,933 CLOSE: Was he a spy or not? 386 00:20:51,966 --> 00:20:53,066 {\an1}We don't yet know. 387 00:20:53,100 --> 00:20:54,533 {\an1}In any event, it was clear 388 00:20:54,566 --> 00:20:57,333 that Pontecorvo was a top-quality scientist 389 00:20:57,366 --> 00:21:00,100 {\an1}who had taken his brain to the Soviet Union. 390 00:21:06,300 --> 00:21:09,933 NARRATOR: By 1950, the U.S.A. and the Soviet Union 391 00:21:09,966 --> 00:21:13,900 were engaged in a nuclear arms race. 392 00:21:13,933 --> 00:21:18,366 {\an1}With it came a new opportunity to hunt for neutrinos. 393 00:21:21,166 --> 00:21:25,200 KARAGIORGI: When a nuclear bomb goes off, 394 00:21:25,233 --> 00:21:29,933 {\an1}there is this huge cascade of particles 395 00:21:29,966 --> 00:21:34,166 that spews out: protons, electrons, 396 00:21:34,200 --> 00:21:37,566 {\an1}a lot of light particles carrying off energy. 397 00:21:37,600 --> 00:21:40,833 {\an7}And along with these particles spewing out, 398 00:21:40,866 --> 00:21:44,233 {\an7}lots and lots of neutrinos come out for free. 399 00:21:45,966 --> 00:21:50,266 NARRATOR: If neutrinos were real, could a nuclear weapon finally be 400 00:21:50,300 --> 00:21:52,533 {\an1}the key to detect them? 401 00:21:52,566 --> 00:21:57,633 {\an1}In 1951, a young American called Fred Reines 402 00:21:57,666 --> 00:21:59,866 {\an1}was working on the U.S. nuclear program 403 00:21:59,900 --> 00:22:03,433 {\an1}at Los Alamos National Laboratory. 404 00:22:03,466 --> 00:22:07,700 {\an1}It was here that Reines, along with his colleague Clyde Cowan, 405 00:22:07,733 --> 00:22:11,333 {\an1}decided to take advantage of destructive bomb tests 406 00:22:11,366 --> 00:22:16,066 {\an1}to investigate the mystery of the missing neutrino. 407 00:22:16,100 --> 00:22:17,509 KAISER: Reines went back to a question 408 00:22:17,533 --> 00:22:19,766 {\an1}that had been kind of abandoned in the decades 409 00:22:19,800 --> 00:22:21,433 {\an1}before the Second World War, 410 00:22:21,466 --> 00:22:24,066 {\an7}the question of, could physicists ever actually detect 411 00:22:24,100 --> 00:22:28,566 {\an7}these very strange, elusive, ghost-like particles? 412 00:22:28,600 --> 00:22:34,000 NARRATOR: They called their mission Project Poltergeist. 413 00:22:34,033 --> 00:22:36,266 {\an1}For detecting the neutrino, the good news was, 414 00:22:36,300 --> 00:22:38,800 {\an1}you could calculate the chance of doing it. 415 00:22:38,833 --> 00:22:41,600 {\an1}And the bad news was, it was almost zero. 416 00:22:41,633 --> 00:22:47,033 NARRATOR: Reines and Cowan needed to tip the odds in their favor, 417 00:22:47,066 --> 00:22:50,866 {\an1}and knew a nuclear bomb test could be the key. 418 00:22:50,900 --> 00:22:54,200 {\an1}An atom bomb should produce thousands of times 419 00:22:54,233 --> 00:22:57,933 {\an1}more neutrinos than even the biggest nuclear reactor. 420 00:22:59,000 --> 00:23:02,466 {\an1}But it also created a problem. 421 00:23:02,500 --> 00:23:04,433 {\an1}If they had bolted the detector in place, 422 00:23:04,466 --> 00:23:05,509 the nuclear bomb would've just 423 00:23:05,533 --> 00:23:07,233 {\an1}smashed it to smithereens. 424 00:23:07,266 --> 00:23:08,766 {\an1}So instead, the proposal 425 00:23:08,800 --> 00:23:11,700 {\an1}was to dig a shaft about 150 feet deep 426 00:23:11,733 --> 00:23:13,700 {\an7}right near where the bomb would eventually 427 00:23:13,733 --> 00:23:16,766 {\an7}be detonated above ground. 428 00:23:16,800 --> 00:23:18,700 {\an8}NARRATOR: The team planned to drop 429 00:23:18,733 --> 00:23:24,000 {\an7}a detector down the shaft to avoid the shockwave of the bomb. 430 00:23:24,033 --> 00:23:27,000 {\an8}KAISER: Inside that shaft, they would pad the bottom with foam 431 00:23:27,033 --> 00:23:30,466 {\an7}and feathers and kind of, like, mattress cushions. 432 00:23:32,800 --> 00:23:34,866 {\an1}It was, I mean... (chuckles) 433 00:23:34,900 --> 00:23:36,900 {\an1}...a creative, ambitious, 434 00:23:36,933 --> 00:23:39,300 {\an1}and maybe slightly crazy kind of idea 435 00:23:39,333 --> 00:23:41,133 {\an1}to try to catch these neutrinos in the midst 436 00:23:41,166 --> 00:23:43,866 {\an1}of this very dramatic, very worldly set of events 437 00:23:43,900 --> 00:23:46,000 {\an1}in the early years of the Cold War. 438 00:23:46,033 --> 00:23:47,733 ♪ ♪ 439 00:23:47,766 --> 00:23:50,166 NARRATOR: Work digging the shaft had begun, 440 00:23:50,200 --> 00:23:53,533 {\an1}but the head of physics at Los Alamos was concerned 441 00:23:53,566 --> 00:23:56,866 {\an1}that the experiment couldn't be repeated. 442 00:23:56,900 --> 00:24:01,000 {\an1}He urged the team to find another way. 443 00:24:01,033 --> 00:24:04,866 {\an1}Couldn't they use a nuclear reactor instead? 444 00:24:04,900 --> 00:24:09,333 {\an1}Late one evening, Reines and Cowan had a realization. 445 00:24:12,100 --> 00:24:15,566 {\an1}In the same way that the nucleus of an atom could decay 446 00:24:15,600 --> 00:24:19,533 {\an1}and release a neutrino, they knew in theory 447 00:24:19,566 --> 00:24:23,233 {\an1}the process should be reversible. 448 00:24:23,266 --> 00:24:27,633 {\an1}On the rare occasion a neutrino could interact with a nucleus, 449 00:24:27,666 --> 00:24:30,866 {\an1}it should produce two new particles, 450 00:24:30,900 --> 00:24:34,100 called a neutron and a positron. 451 00:24:34,133 --> 00:24:37,233 {\an1}And if they traveled through the right medium, 452 00:24:37,266 --> 00:24:40,633 {\an1}those two telltale particles should produce 453 00:24:40,666 --> 00:24:44,166 {\an1}two distinctive flashes of light. 454 00:24:44,200 --> 00:24:48,700 KARAGIORGI: So Reines and Cowan built a detector, 455 00:24:48,733 --> 00:24:53,566 {\an1}essentially a big tank filled with a solvent 456 00:24:53,600 --> 00:24:55,566 {\an1}that could pick up 457 00:24:55,600 --> 00:25:00,200 {\an1}this two coincident signal blip 458 00:25:00,233 --> 00:25:03,300 {\an1}deep under a nuclear reactor. 459 00:25:03,333 --> 00:25:08,666 ♪ ♪ 460 00:25:08,700 --> 00:25:11,500 NARRATOR: After five years of experiments, 461 00:25:11,533 --> 00:25:16,566 in 1956, finally, they got their answer. 462 00:25:18,933 --> 00:25:21,433 ♪ ♪ 463 00:25:21,466 --> 00:25:24,966 {\an1}They recorded the two telltale flashes of light. 464 00:25:25,000 --> 00:25:26,766 ♪ ♪ 465 00:25:26,800 --> 00:25:29,500 {\an1}For the first time, they saw evidence 466 00:25:29,533 --> 00:25:32,533 {\an1}of the elusive neutrino. 467 00:25:32,566 --> 00:25:35,033 {\an7}What they had done was a remarkable achievement, 468 00:25:35,066 --> 00:25:38,366 {\an7}one that seemed impossible. 469 00:25:38,400 --> 00:25:39,566 ♪ ♪ 470 00:25:39,600 --> 00:25:41,200 KAISER: Neutrinos exist. 471 00:25:41,233 --> 00:25:44,133 {\an1}They're real and they're part of the world. 472 00:25:44,166 --> 00:25:46,166 {\an1}They're not only a clever idea. 473 00:25:51,033 --> 00:25:52,833 {\an1}Knowing neutrinos exist 474 00:25:52,866 --> 00:25:55,200 {\an1}put a whole extra set of investigations 475 00:25:55,233 --> 00:25:57,333 {\an1}on a kind of firmer path. 476 00:25:57,366 --> 00:25:59,133 ♪ ♪ 477 00:25:59,166 --> 00:26:04,833 NARRATOR: If neutrinos were pouring from nuclear reactors on Earth, 478 00:26:04,866 --> 00:26:06,400 {\an1}then surely they would be generated 479 00:26:06,433 --> 00:26:10,900 {\an1}in abundance in the largest nuclear furnaces of all. 480 00:26:12,466 --> 00:26:15,100 Stars. 481 00:26:15,133 --> 00:26:16,766 KAISER: For a long, long time, 482 00:26:16,800 --> 00:26:19,600 {\an1}scientists have been wondering, what makes the stars shine? 483 00:26:19,633 --> 00:26:22,966 {\an1}What drives that enormous output of energy? 484 00:26:23,000 --> 00:26:24,733 ♪ ♪ 485 00:26:24,766 --> 00:26:30,400 KARAGIORGI: People theorized that our sun is like a giant nuclear reactor, 486 00:26:30,433 --> 00:26:34,700 {\an1}except, rather than heavier elements breaking down 487 00:26:34,733 --> 00:26:38,333 {\an1}into smaller ones and releasing energy, 488 00:26:38,366 --> 00:26:41,766 {\an1}you have lighter elements that fuse together 489 00:26:41,800 --> 00:26:43,166 {\an1}through nuclear fusion. 490 00:26:43,200 --> 00:26:45,833 ♪ ♪ 491 00:26:45,866 --> 00:26:47,466 NARRATOR: In the heart of the sun, 492 00:26:47,500 --> 00:26:50,933 {\an1}tremendous heat and pressure force hydrogen nuclei 493 00:26:50,966 --> 00:26:53,833 to fuse together to make helium. 494 00:26:55,466 --> 00:27:00,400 And, in theory, vast quantities of neutrinos 495 00:27:00,433 --> 00:27:05,800 {\an1}that pass freely through the sun and out into space. 496 00:27:09,066 --> 00:27:12,533 {\an1}So if we could detect neutrinos from the sun, 497 00:27:12,566 --> 00:27:16,700 {\an1}we could learn about the processes that fuel it. 498 00:27:16,733 --> 00:27:21,733 {\an1}We could peek inside the core of our sun. 499 00:27:24,900 --> 00:27:28,166 {\an8}NARRATOR: In the historic gold mining town of Lead, 500 00:27:28,200 --> 00:27:31,933 {\an1}people descend into the depths of the Earth. 501 00:27:31,966 --> 00:27:33,833 {\an1}(indistinct chatter) 502 00:27:33,866 --> 00:27:36,633 NARRATOR: But no longer to mine precious metal. 503 00:27:36,666 --> 00:27:41,333 They're hunting for neutrinos. 504 00:27:41,366 --> 00:27:44,366 {\an1}It was here in 1965 505 00:27:44,400 --> 00:27:47,500 {\an1}that an experimentalist called Ray Davis 506 00:27:47,533 --> 00:27:52,000 {\an1}came to try and prove what makes the sun shine. 507 00:27:52,033 --> 00:27:54,266 KAISER: Ray Davis got very excited 508 00:27:54,300 --> 00:27:57,400 {\an1}that there is this new thing in the world called a neutrino. 509 00:27:57,433 --> 00:28:00,333 {\an1}He began realizing that other kinds of nuclear reactors 510 00:28:00,366 --> 00:28:03,100 {\an1}that occur throughout the universe, like stars, 511 00:28:03,133 --> 00:28:06,100 {\an1}they should be spewing out these neutrinos all the time. 512 00:28:06,133 --> 00:28:10,900 NARRATOR: But catching them wouldn't be easy. 513 00:28:10,933 --> 00:28:14,400 {\an1}Calculations showed that neutrinos from the sun 514 00:28:14,433 --> 00:28:17,800 {\an1}would be so faint, a detector near the Earth's surface 515 00:28:17,833 --> 00:28:21,100 {\an1}would be overwhelmed by background radiation. 516 00:28:21,133 --> 00:28:26,600 {\an1}His only option was to go to the bottom of a mine. 517 00:28:26,633 --> 00:28:30,400 {\an1}Beneath almost a mile of solid rock, Davis's team built 518 00:28:30,433 --> 00:28:32,900 {\an1}a steel tank the size of a house 519 00:28:32,933 --> 00:28:35,666 {\an1}and filled it with 100,000 gallons 520 00:28:35,700 --> 00:28:38,300 {\an1}of dry-cleaning fluid. 521 00:28:40,366 --> 00:28:43,433 In theory, if a neutrino from the sun 522 00:28:43,466 --> 00:28:47,633 {\an1}collided with a chlorine atom inside the tank, 523 00:28:47,666 --> 00:28:52,266 {\an1}it would cause a reaction that Ray Davis could detect. 524 00:28:52,300 --> 00:28:55,900 CLOSE: Here was something that was completely fresh. 525 00:28:55,933 --> 00:28:58,066 {\an1}Nobody knew anything about it. 526 00:28:58,100 --> 00:29:02,633 {\an1}But the key thing was that if neutrinos hit chlorine, 527 00:29:02,666 --> 00:29:04,600 {\an7}which you could get in cleaning fluid, 528 00:29:04,633 --> 00:29:06,200 {\an7}it would turn the atoms of chlorine 529 00:29:06,233 --> 00:29:08,866 {\an7}into a radioactive form of argon. 530 00:29:08,900 --> 00:29:10,400 {\an7}And that's when Davis got excited, 531 00:29:10,433 --> 00:29:13,800 {\an1}because he was a radiochemist, and for him, 532 00:29:13,833 --> 00:29:19,766 {\an1}detecting radioactive forms of argon was easy street. 533 00:29:21,200 --> 00:29:23,000 NARRATOR: Scientists had calculated 534 00:29:23,033 --> 00:29:26,600 {\an1}that around a million trillion neutrinos from the sun 535 00:29:26,633 --> 00:29:30,866 {\an1}should pass through Davis's tank each minute. 536 00:29:30,900 --> 00:29:33,300 {\an1}But the probability of them hitting the fluid 537 00:29:33,333 --> 00:29:37,166 {\an1}and making an argon atom was so small, 538 00:29:37,200 --> 00:29:39,500 {\an1}Ray Davis could only expect to find 539 00:29:39,533 --> 00:29:42,200 {\an1}ten individual atoms of argon 540 00:29:42,233 --> 00:29:46,533 {\an1}from ten neutrino collisions per week. 541 00:29:46,566 --> 00:29:49,500 JAYAWARDHANA: His task was almost impossible. 542 00:29:49,533 --> 00:29:52,833 {\an1}Many of his own physicist colleagues doubted 543 00:29:52,866 --> 00:29:55,100 {\an1}this experiment would ever work. 544 00:29:55,133 --> 00:29:57,100 ♪ ♪ 545 00:29:57,133 --> 00:29:58,542 CLOSE: He was having to convince people 546 00:29:58,566 --> 00:29:59,809 {\an1}that out of these millions and millions and millions 547 00:29:59,833 --> 00:30:02,066 {\an1}and millions of atoms inside this tank, 548 00:30:02,100 --> 00:30:05,533 {\an1}he could identify the collisions of one or two 549 00:30:05,566 --> 00:30:08,933 {\an1}and convince you that these were neutrinos coming from the sun. 550 00:30:08,966 --> 00:30:13,433 NARRATOR: Around each month, Davis flushed out the giant tank 551 00:30:13,466 --> 00:30:16,133 {\an1}to extract the argon atoms. 552 00:30:17,466 --> 00:30:19,366 {\an1}To everybody's amazement, 553 00:30:19,400 --> 00:30:21,933 he found them. 554 00:30:21,966 --> 00:30:26,166 {\an1}(machine whirring) 555 00:30:27,200 --> 00:30:29,733 {\an1}But there was a problem. 556 00:30:29,766 --> 00:30:34,133 {\an1}Instead of detecting the number of atoms that theory predicted, 557 00:30:34,166 --> 00:30:37,366 {\an1}his measurements fell short. 558 00:30:37,400 --> 00:30:38,976 KAISER: They knew the target number based on 559 00:30:39,000 --> 00:30:41,666 {\an1}the nuclear physics theoretical explanation 560 00:30:41,700 --> 00:30:43,300 {\an1}of how stars shine, 561 00:30:43,333 --> 00:30:46,266 {\an1}and that led to a very particular target number. 562 00:30:46,300 --> 00:30:48,366 {\an1}And Davis's remarkable experiment 563 00:30:48,400 --> 00:30:51,100 {\an1}kept coming in not close to it, not 80 percent, 564 00:30:51,133 --> 00:30:54,000 {\an1}but only at one-third of that target number. 565 00:30:54,033 --> 00:30:57,333 NARRATOR: What happened? 566 00:30:57,366 --> 00:30:59,566 {\an1}Had the experiment gone wrong? 567 00:30:59,600 --> 00:31:03,066 {\an1}Another scientist carried out a blind trial 568 00:31:03,100 --> 00:31:05,966 {\an1}to test the accuracy of Ray's atom detection. 569 00:31:06,000 --> 00:31:10,300 KAISER: A colleague put in 500 kind of rogue atoms 570 00:31:10,333 --> 00:31:12,233 {\an1}without telling Davis the number. 571 00:31:12,266 --> 00:31:14,400 {\an1}And Davis was able to go through the whole process, 572 00:31:14,433 --> 00:31:15,933 sift it through, 573 00:31:15,966 --> 00:31:18,009 {\an1}and he counted exactly the number that had been put in. 574 00:31:18,033 --> 00:31:21,733 NARRATOR: If the experimental results were accurate, 575 00:31:21,766 --> 00:31:24,666 {\an1}then perhaps scientists had gotten their theory 576 00:31:24,700 --> 00:31:28,000 {\an1}about neutrinos from the sun wrong. 577 00:31:28,033 --> 00:31:29,609 CLOSE: Everybody was blaming everybody else. 578 00:31:29,633 --> 00:31:31,266 {\an1}There were even suggestions, 579 00:31:31,300 --> 00:31:34,566 {\an1}has the sun already burnt out in the core? 580 00:31:34,600 --> 00:31:36,133 {\an1}It was just an enormous puzzle. 581 00:31:36,166 --> 00:31:39,200 {\an1}All these advances in understanding how stars shine, 582 00:31:39,233 --> 00:31:41,433 {\an1}and then hitting this kind of brick wall 583 00:31:41,466 --> 00:31:44,933 {\an1}where theory and experiment just would not agree with each other. 584 00:31:47,333 --> 00:31:51,833 NARRATOR: The puzzle became known as the solar neutrino problem. 585 00:31:54,266 --> 00:31:56,400 ♪ ♪ 586 00:31:56,433 --> 00:31:58,033 1970, 587 00:31:58,066 --> 00:32:00,500 20 years since Bruno Pontecorvo 588 00:32:00,533 --> 00:32:03,500 {\an1}defected to the Soviet Union. 589 00:32:03,533 --> 00:32:05,000 ♪ ♪ 590 00:32:05,033 --> 00:32:07,033 {\an1}Even after all that time, 591 00:32:07,066 --> 00:32:10,933 {\an1}his life behind the Iron Curtain remained shrouded in secrecy. 592 00:32:12,733 --> 00:32:15,833 {\an1}But in a government lab outside Moscow, 593 00:32:15,866 --> 00:32:18,400 {\an1}Pontecorvo worked tirelessly to explain 594 00:32:18,433 --> 00:32:21,566 {\an1}the puzzling behavior of neutrinos. 595 00:32:24,100 --> 00:32:28,200 {\an1}He suggested that instead of just one, 596 00:32:28,233 --> 00:32:32,400 {\an1}there may be two or even three different kinds of neutrino... 597 00:32:32,433 --> 00:32:36,900 {\an1}known as different flavors. 598 00:32:36,933 --> 00:32:40,033 ♪ ♪ 599 00:32:40,066 --> 00:32:43,333 {\an1}If this wasn't strange enough, he calculated that something 600 00:32:43,366 --> 00:32:47,366 {\an1}peculiar might happen as they traveled through space. 601 00:32:50,400 --> 00:32:55,533 {\an1}A neutrino would always be born as one definite flavor, 602 00:32:55,566 --> 00:32:59,666 but over time, it would change its identity. 603 00:32:59,700 --> 00:33:03,900 {\an1}It would transform, mixing back and forth 604 00:33:03,933 --> 00:33:08,100 {\an1}between the three different types. 605 00:33:08,133 --> 00:33:12,233 This was called neutrino oscillation. 606 00:33:12,266 --> 00:33:16,833 ♪ ♪ 607 00:33:16,866 --> 00:33:20,366 {\an7}Pontecorvo's idea really is, it's, it's sort of delicious. 608 00:33:20,400 --> 00:33:25,000 {\an1}These neutrinos could be not taking one identity, 609 00:33:25,033 --> 00:33:28,533 {\an1}dropping that, adopting another one, dropping that, 610 00:33:28,566 --> 00:33:30,206 {\an1}but going into this even stranger mixture, 611 00:33:30,233 --> 00:33:33,100 {\an1}where they're in neither and both states at once. 612 00:33:33,133 --> 00:33:35,633 NARRATOR: It was a bold idea. 613 00:33:35,666 --> 00:33:37,800 {\an1}No other fundamental particle 614 00:33:37,833 --> 00:33:41,300 {\an1}seemed to spontaneously change its identity. 615 00:33:41,333 --> 00:33:43,766 {\an1}But if neutrinos were transforming into flavors 616 00:33:43,800 --> 00:33:46,966 {\an1}that Ray Davis's detector couldn't see, 617 00:33:47,000 --> 00:33:50,233 {\an1}it might explain why two-thirds of the neutrinos 618 00:33:50,266 --> 00:33:53,300 {\an1}from the sun appeared to be missing. 619 00:33:55,166 --> 00:33:56,933 {\an7}But there was a catch. 620 00:33:56,966 --> 00:33:58,900 {\an7}The Standard Model, 621 00:33:58,933 --> 00:34:02,300 {\an7}the most precise scientific theory in human history, 622 00:34:02,333 --> 00:34:05,933 {\an7}made one important prediction that stood in the way. 623 00:34:08,266 --> 00:34:09,766 {\an8}PEREZ: The Standard Model anticipated 624 00:34:09,800 --> 00:34:12,333 {\an7}neutrinos would be completely massless. 625 00:34:12,366 --> 00:34:15,766 {\an7}They would have no mass at all, much like the photon of light. 626 00:34:15,800 --> 00:34:18,400 {\an7}And if they had no mass, 627 00:34:18,433 --> 00:34:21,200 {\an7}that meant that they could not oscillate. 628 00:34:21,233 --> 00:34:24,433 NARRATOR: If neutrinos had no mass, 629 00:34:24,466 --> 00:34:27,366 {\an1}one of Albert Einstein's most important theories 630 00:34:27,400 --> 00:34:31,433 {\an1}predicted that neutrinos could not possibly oscillate. 631 00:34:34,166 --> 00:34:35,909 KAISER: There is this mind-boggling phenomenon 632 00:34:35,933 --> 00:34:37,966 {\an1}from Einstein's relativity 633 00:34:38,000 --> 00:34:40,433 {\an1}that says that a clock that is moving closer 634 00:34:40,466 --> 00:34:41,933 {\an1}and closer to the speed of light 635 00:34:41,966 --> 00:34:45,266 {\an1}will tick at a slower and slower rate. 636 00:34:45,300 --> 00:34:47,766 {\an1}If that clock were moving literally at the speed of light, 637 00:34:47,800 --> 00:34:49,333 {\an1}it would never tick at all. 638 00:34:49,366 --> 00:34:51,433 {\an1}No time would pass for that object 639 00:34:51,466 --> 00:34:53,900 {\an1}that moves at exactly the speed of light. 640 00:34:53,933 --> 00:34:56,833 NARRATOR: According to Einstein's theories, 641 00:34:56,866 --> 00:34:59,200 {\an1}the faster a particle travels, 642 00:34:59,233 --> 00:35:02,933 {\an1}the more its internal clock slows down. 643 00:35:02,966 --> 00:35:08,200 {\an1}A particle with no mass can only travel at the speed of light, 644 00:35:08,233 --> 00:35:10,366 {\an1}which is where time stops. 645 00:35:13,033 --> 00:35:15,666 {\an1}So if a neutrino had zero mass, 646 00:35:15,700 --> 00:35:18,500 {\an1}it would not experience the passage of time, 647 00:35:18,533 --> 00:35:24,300 {\an1}and would never be able to change. 648 00:35:24,333 --> 00:35:26,666 {\an1}If a particle has zero mass, 649 00:35:26,700 --> 00:35:30,400 {\an1}what that means is that its internal clock is not ticking. 650 00:35:30,433 --> 00:35:34,433 {\an1}There's no way for that particle to experience time. 651 00:35:34,466 --> 00:35:36,400 {\an1}If there's no passage of time, 652 00:35:36,433 --> 00:35:39,466 {\an1}then how could they change over time from one identity 653 00:35:39,500 --> 00:35:42,100 to another? 654 00:35:42,133 --> 00:35:45,633 NARRATOR: If neutrino oscillation was real, 655 00:35:45,666 --> 00:35:49,066 {\an1}neutrinos must have some mass. 656 00:35:49,100 --> 00:35:54,433 {\an1}But could the Standard Model really be wrong? 657 00:35:54,466 --> 00:35:57,600 ♪ ♪ 658 00:35:57,633 --> 00:36:01,400 {\an7}Throughout the 1950s and '60s, clues from experiments 659 00:36:01,433 --> 00:36:04,800 {\an1}performed at CERN, alongside Fermilab, 660 00:36:04,833 --> 00:36:08,900 {\an1}helped to lay the foundation of the Standard Model. 661 00:36:08,933 --> 00:36:12,666 What they found revolutionized our understanding 662 00:36:12,700 --> 00:36:15,433 {\an1}of the particles that make up our universe. 663 00:36:15,466 --> 00:36:19,400 FILM NARRATOR: By means of this machine, it is possible to see 664 00:36:19,433 --> 00:36:20,576 the tracks of sub-nuclear particles, 665 00:36:20,600 --> 00:36:23,966 {\an1}the smallest particles known to man: 666 00:36:24,000 --> 00:36:26,933 {\an1}the electron, the positron, 667 00:36:26,966 --> 00:36:29,666 {\an1}the photon, and the neutrino... 668 00:36:32,300 --> 00:36:34,300 NARRATOR: Over the years, work at CERN 669 00:36:34,333 --> 00:36:36,300 {\an1}led to groundbreaking new technologies: 670 00:36:36,333 --> 00:36:40,666 medical advances like PET scans; 671 00:36:40,700 --> 00:36:44,566 {\an1}even the birth of the World Wide Web. 672 00:36:46,866 --> 00:36:51,366 {\an1}Perhaps CERN's biggest success came in 2012. 673 00:36:51,400 --> 00:36:54,466 {\an1}Nearly 50 years after the Standard Model was proposed, 674 00:36:54,500 --> 00:36:57,566 {\an1}physicists detected the final particle 675 00:36:57,600 --> 00:37:01,933 {\an1}it predicted... the Higgs boson. 676 00:37:03,233 --> 00:37:05,366 {\an1}I think we have it. 677 00:37:05,400 --> 00:37:07,366 {\an1}(cheers and applause) 678 00:37:16,700 --> 00:37:19,366 NARRATOR: Finally, all the pieces needed 679 00:37:19,400 --> 00:37:22,166 {\an1}to describe the detectable physical universe 680 00:37:22,200 --> 00:37:25,733 {\an1}seemed to be in place. 681 00:37:25,766 --> 00:37:29,266 {\an1}Along with the Higgs boson, there are force carriers, 682 00:37:29,300 --> 00:37:32,133 {\an1}like the photon of light. 683 00:37:32,166 --> 00:37:36,366 {\an1}Quarks, which form the nuclei of atoms. 684 00:37:36,400 --> 00:37:41,933 {\an1}Leptons, including the electron, muon, and tau. 685 00:37:41,966 --> 00:37:46,400 {\an1}And three corresponding flavors of neutrinos. 686 00:37:46,433 --> 00:37:49,233 KAISER: It is a map of what's out there, 687 00:37:49,266 --> 00:37:52,400 {\an7}what we're made of, and how we fit... all of us. 688 00:37:52,433 --> 00:37:55,533 {\an7}We are made of these things. 689 00:37:55,566 --> 00:37:57,200 {\an1}And that is a kind of basic understanding 690 00:37:57,233 --> 00:37:59,133 {\an1}of nature, of our own world, 691 00:37:59,166 --> 00:38:01,700 {\an1}that I, I think is, is just a remarkable 692 00:38:01,733 --> 00:38:03,933 {\an1}human achievement. 693 00:38:05,700 --> 00:38:07,966 NARRATOR: And yet, for all its success, 694 00:38:08,000 --> 00:38:10,933 {\an1}the Standard Model had no equations to explain 695 00:38:10,966 --> 00:38:15,000 {\an1}how or why the neutrinos would have mass. 696 00:38:19,700 --> 00:38:23,033 For Ray Davis and his missing solar neutrinos, 697 00:38:23,066 --> 00:38:27,566 {\an1}it seemed an unsolvable paradox. 698 00:38:28,866 --> 00:38:32,233 {\an7}For decades, Davis persists, 699 00:38:32,266 --> 00:38:35,333 {\an7}but he still only finds one-third of the neutrinos 700 00:38:35,366 --> 00:38:38,266 {\an7}that were supposed to be coming from the sun. 701 00:38:39,733 --> 00:38:43,133 {\an7}Well, we've been carrying on this experiment 702 00:38:43,166 --> 00:38:45,966 {\an7}for about 20 years right here. 703 00:38:46,000 --> 00:38:51,300 {\an7}But we're still observing a low flux of neutrinos. 704 00:38:52,800 --> 00:38:56,566 {\an8}NARRATOR: Eventually, the problem is too big to ignore. 705 00:38:56,600 --> 00:39:00,800 {\an8}In the 1990s, scientists in Canada and Japan 706 00:39:00,833 --> 00:39:05,200 {\an7}construct a new generation of supersized neutrino detectors 707 00:39:05,233 --> 00:39:07,966 {\an7}to finally settle the mystery. 708 00:39:08,000 --> 00:39:10,866 {\an1}(explosion roars) 709 00:39:10,900 --> 00:39:15,500 {\an1}One of them lies deep beneath Japan's Ikeno Mountain. 710 00:39:15,533 --> 00:39:19,266 {\an1}Scientists fit 11,000 light detectors 711 00:39:19,300 --> 00:39:21,666 to the inside of a gigantic container 712 00:39:21,700 --> 00:39:28,200 {\an1}and fill it with 50,000 tons of ultra-pure water. 713 00:39:28,233 --> 00:39:34,633 {\an1}This $100 million detector is named Super-K. 714 00:39:34,666 --> 00:39:38,266 {\an1}The Super-K experiment ended up being a game-changer. 715 00:39:38,300 --> 00:39:41,933 NARRATOR: In the rare event that a neutrino collides 716 00:39:41,966 --> 00:39:44,566 {\an1}with the liquid in Super-K, 717 00:39:44,600 --> 00:39:46,400 {\an1}the reaction produces a trail of light 718 00:39:46,433 --> 00:39:49,400 {\an1}which the sensors can pick up. 719 00:39:49,433 --> 00:39:51,400 {\an1}Unlike Davis's detector, 720 00:39:51,433 --> 00:39:54,733 {\an1}this signal allows scientists to calculate 721 00:39:54,766 --> 00:39:57,200 {\an1}which type of neutrino has hit 722 00:39:57,233 --> 00:39:59,100 {\an1}and the direction it came from. 723 00:39:59,133 --> 00:40:02,400 {\an1}Super-K allows scientists 724 00:40:02,433 --> 00:40:05,833 {\an1}to test the theory of neutrino oscillation 725 00:40:05,866 --> 00:40:08,300 {\an1}by catching them from a new source: 726 00:40:08,333 --> 00:40:10,033 {\an1}the Earth's atmosphere. 727 00:40:10,066 --> 00:40:12,800 ♪ ♪ 728 00:40:12,833 --> 00:40:15,500 {\an1}Theory suggests that when radiation from space 729 00:40:15,533 --> 00:40:18,866 {\an1}hits the atmosphere, it creates neutrinos 730 00:40:18,900 --> 00:40:23,233 {\an1}that travel directly through the Earth. 731 00:40:23,266 --> 00:40:26,033 {\an1}Some travel a short distance, 732 00:40:26,066 --> 00:40:30,533 {\an1}but others will come from the other side of the planet 733 00:40:30,566 --> 00:40:33,666 {\an1}to reach the detector. 734 00:40:33,700 --> 00:40:36,233 {\an1}If the neutrinos are not changing, 735 00:40:36,266 --> 00:40:38,433 {\an1}the combination of flavors they record 736 00:40:38,466 --> 00:40:41,066 {\an1}coming from a short distance will be the same 737 00:40:41,100 --> 00:40:44,333 {\an1}as those coming from afar. 738 00:40:44,366 --> 00:40:47,800 {\an1}If they are changing over a long distance, 739 00:40:47,833 --> 00:40:53,033 {\an1}the combination of flavors will be different. 740 00:40:55,300 --> 00:40:58,133 After two years of recording data, 741 00:40:58,166 --> 00:41:00,833 {\an1}the team finally has an answer. 742 00:41:02,900 --> 00:41:04,800 KARAGIORGI: What they were seeing was that 743 00:41:04,833 --> 00:41:07,800 {\an1}one type of neutrinos was depleting 744 00:41:07,833 --> 00:41:11,700 {\an1}when traveling through the Earth. 745 00:41:11,733 --> 00:41:16,266 {\an1}The Super-K results combined with results 746 00:41:16,300 --> 00:41:17,433 {\an1}from another experiment 747 00:41:17,466 --> 00:41:20,966 {\an1}were able to definitively show 748 00:41:21,000 --> 00:41:25,900 {\an1}that neutrinos can change from one type to the other. 749 00:41:25,933 --> 00:41:28,433 {\an1}For that to happen, 750 00:41:28,466 --> 00:41:31,133 {\an1}you must have non-zero neutrino mass. 751 00:41:31,166 --> 00:41:35,166 NARRATOR: The results are groundbreaking. 752 00:41:35,200 --> 00:41:38,300 {\an1}Neutrinos change their identity. 753 00:41:38,333 --> 00:41:41,900 {\an1}Neutrinos have mass after all. 754 00:41:41,933 --> 00:41:43,800 {\an1}And the Standard Model's prediction 755 00:41:43,833 --> 00:41:48,200 {\an1}of the nature of neutrinos must be wrong. 756 00:41:48,233 --> 00:41:49,533 KAISER: With the new input, 757 00:41:49,566 --> 00:41:52,066 {\an1}the evidence that neutrinos really oscillate, 758 00:41:52,100 --> 00:41:53,566 {\an1}they really change their identities, 759 00:41:53,600 --> 00:41:56,000 {\an1}therefore they really, really have a mass, 760 00:41:56,033 --> 00:41:58,800 {\an1}this long-standing, decades-long challenge 761 00:41:58,833 --> 00:42:00,376 {\an1}to understand the solar neutrino problem 762 00:42:00,400 --> 00:42:04,200 {\an7}finally fell into place. 763 00:42:04,233 --> 00:42:06,700 {\an8}NARRATOR: Nuclear fusion in the sun 764 00:42:06,733 --> 00:42:09,933 {\an7}produces one type of neutrino. 765 00:42:09,966 --> 00:42:12,900 {\an7}But on the long journey through space, 766 00:42:12,933 --> 00:42:14,866 {\an7}the neutrinos oscillate, 767 00:42:14,900 --> 00:42:19,133 {\an7}and turn into a mixture of all three. 768 00:42:19,166 --> 00:42:21,800 {\an8}On Earth, 769 00:42:21,833 --> 00:42:26,766 {\an7}Ray Davis's detector only picked out one flavor. 770 00:42:26,800 --> 00:42:31,300 {\an7}His results had been accurate all along. 771 00:42:34,600 --> 00:42:38,100 37 years after the experiment began, 772 00:42:38,133 --> 00:42:40,466 {\an1}Ray Davis was awarded the Nobel Prize. 773 00:42:40,500 --> 00:42:44,600 {\an1}(cheers and applause) 774 00:42:46,500 --> 00:42:50,233 {\an1}For Bruno Pontecorvo and his theory of oscillations, 775 00:42:50,266 --> 00:42:54,700 {\an1}sadly, the discovery came too late. 776 00:42:54,733 --> 00:42:56,433 CLOSE: Nobel Prizes aren't everything, 777 00:42:56,466 --> 00:42:59,366 {\an1}but by the time the oscillations had been sorted out 778 00:42:59,400 --> 00:43:01,866 {\an1}and the whole thing finally understood, 779 00:43:01,900 --> 00:43:04,400 {\an1}Pontecorvo was dead. 780 00:43:04,433 --> 00:43:08,433 {\an1}So that's the final tragedy of his life. 781 00:43:13,366 --> 00:43:17,566 NARRATOR: After almost 100 years of research and discovery, 782 00:43:17,600 --> 00:43:20,400 {\an1}today, neutrino physicists face 783 00:43:20,433 --> 00:43:23,133 {\an1}perhaps their biggest puzzle yet. 784 00:43:23,166 --> 00:43:26,433 {\an1}The Standard Model's equations, 785 00:43:26,466 --> 00:43:29,266 {\an1}which are so precise for other particles, 786 00:43:29,300 --> 00:43:35,200 {\an1}cannot explain why neutrinos have mass or why they oscillate. 787 00:43:35,233 --> 00:43:37,400 {\an1}It's a sign that our understanding of matter 788 00:43:37,433 --> 00:43:39,966 {\an1}is still incomplete. 789 00:43:40,000 --> 00:43:42,166 ♪ ♪ 790 00:43:42,200 --> 00:43:44,700 {\an1}Today, neutrino experiments are in overdrive, 791 00:43:44,733 --> 00:43:46,500 {\an1}hunting for clues. 792 00:43:46,533 --> 00:43:48,566 KAISER: We're in the midst of, really, 793 00:43:48,600 --> 00:43:51,333 {\an1}a neutrino bonanza... I mean, they're just, they're popping up 794 00:43:51,366 --> 00:43:54,000 {\an1}all over the field of physics. 795 00:43:54,033 --> 00:43:55,833 ♪ ♪ 796 00:43:55,866 --> 00:43:57,166 NARRATOR: At the South Pole, 797 00:43:57,200 --> 00:43:59,200 {\an1}scientists have built 798 00:43:59,233 --> 00:44:03,800 {\an1}the largest neutrino detector on the planet. 799 00:44:03,833 --> 00:44:06,966 {\an1}It's made of more than 5,000 sensors drilled into 800 00:44:07,000 --> 00:44:10,866 {\an1}a cubic kilometer of Antarctic ice. 801 00:44:10,900 --> 00:44:13,366 {\an1}It's known as IceCube. 802 00:44:13,400 --> 00:44:15,966 ♪ ♪ 803 00:44:16,000 --> 00:44:17,400 KAEL HANSON: IceCube is in this, 804 00:44:17,433 --> 00:44:18,809 {\an1}this huge field around me... I'm sitting, 805 00:44:18,833 --> 00:44:22,666 {\an1}kind of standing in the middle of IceCube. 806 00:44:22,700 --> 00:44:24,200 {\an7}It's kind of amazing to think 807 00:44:24,233 --> 00:44:26,866 {\an7}that we were able to haul something like 808 00:44:26,900 --> 00:44:27,866 {\an7}five million pounds of cargo 809 00:44:27,900 --> 00:44:30,166 {\an1}down to the South Pole... this is 810 00:44:30,200 --> 00:44:32,566 instrumentation, cables, drill equipment, 811 00:44:32,600 --> 00:44:34,600 fuel... 812 00:44:34,633 --> 00:44:39,666 NARRATOR: As well as probing neutrino oscillations, 813 00:44:39,700 --> 00:44:42,066 {\an1}IceCube acts like a neutrino telescope, 814 00:44:42,100 --> 00:44:43,600 {\an1}catching cosmic neutrinos 815 00:44:43,633 --> 00:44:47,200 from billions of light years away. 816 00:44:47,233 --> 00:44:48,633 {\an1}This is the universe that has really 817 00:44:48,666 --> 00:44:51,366 {\an1}only been opened to our eyes for the last 50 years. 818 00:44:51,400 --> 00:44:52,833 {\an8}♪ ♪ 819 00:44:52,866 --> 00:44:56,066 {\an7}There's all kinds of discoveries that are waiting out there. 820 00:44:57,166 --> 00:45:00,266 {\an8}NARRATOR: With new experiments like IceCube, 821 00:45:00,300 --> 00:45:04,000 {\an1}scientists believe that neutrinos may reveal discoveries 822 00:45:04,033 --> 00:45:06,833 {\an1}beyond the Standard Model. 823 00:45:08,433 --> 00:45:09,933 {\an1}Neutrinos could even help unlock 824 00:45:09,966 --> 00:45:13,666 {\an1}one of the biggest mysteries in physics today. 825 00:45:13,700 --> 00:45:15,333 ♪ ♪ 826 00:45:15,366 --> 00:45:18,366 {\an1}It seems that most of what our universe is made of 827 00:45:18,400 --> 00:45:21,633 is missing. 828 00:45:22,933 --> 00:45:25,533 PEREZ: The whole quest of particle physics 829 00:45:25,566 --> 00:45:29,400 {\an1}is to explain the matter contents of the universe. 830 00:45:29,433 --> 00:45:34,366 {\an1}And we seem to be doing this phenomenally good job. 831 00:45:34,400 --> 00:45:36,109 {\an1}You crank through the math of the Standard Model, 832 00:45:36,133 --> 00:45:38,900 {\an1}and everything makes sense. 833 00:45:38,933 --> 00:45:43,100 {\an7}And yet it only describes some very small fraction 834 00:45:43,133 --> 00:45:45,066 {\an7}of what the universe is made out of. 835 00:45:47,166 --> 00:45:50,433 NARRATOR: Looking into space, 836 00:45:50,466 --> 00:45:53,433 {\an1}cosmologists can see the gravitational influence 837 00:45:53,466 --> 00:45:57,933 {\an1}of a material that binds entire galaxies together, 838 00:45:57,966 --> 00:46:03,166 {\an1}but that is completely invisible to their detectors. 839 00:46:03,200 --> 00:46:07,200 Scientists call this material dark matter, 840 00:46:07,233 --> 00:46:12,766 {\an1}because nothing in the Standard Model can describe what it is. 841 00:46:12,800 --> 00:46:14,700 {\an1}And yet, it seems to be 842 00:46:14,733 --> 00:46:19,533 {\an1}what most of the matter in the universe is made of. 843 00:46:19,566 --> 00:46:23,033 CLOSE: The Standard Model is very good at describing 844 00:46:23,066 --> 00:46:26,566 {\an7}about five percent of the universe. 845 00:46:26,600 --> 00:46:29,333 {\an7}95% of the stuff is an utter, complete mystery, 846 00:46:29,366 --> 00:46:32,600 {\an1}made of dark stuff, whether it's dark matter or dark energy. 847 00:46:32,633 --> 00:46:36,066 {\an1}And what either of those are, we don't know. 848 00:46:36,100 --> 00:46:38,266 {\an1}All we really know about dark matter 849 00:46:38,300 --> 00:46:39,833 {\an1}is that it creates gravity, 850 00:46:39,866 --> 00:46:42,966 {\an1}but it's not interacting with the instruments 851 00:46:43,000 --> 00:46:46,800 {\an1}that we have used to observe the universe. 852 00:46:46,833 --> 00:46:49,033 KAISER: Whatever is filling space, 853 00:46:49,066 --> 00:46:50,966 {\an7}much more of it than the ordinary matter 854 00:46:51,000 --> 00:46:53,900 {\an8}that makes up us and our planet and our stars, 855 00:46:53,933 --> 00:46:56,666 {\an8}it's some other, other kind of particle. 856 00:46:56,700 --> 00:47:00,700 NARRATOR: Whatever dark matter particles are, 857 00:47:00,733 --> 00:47:06,333 {\an1}scientists must look beyond the Standard Model to find them. 858 00:47:06,366 --> 00:47:10,566 {\an1}Neutrinos might be the key. 859 00:47:10,600 --> 00:47:16,300 ♪ ♪ 860 00:47:16,333 --> 00:47:19,466 {\an1}At Fermilab, for over 20 years, 861 00:47:19,500 --> 00:47:21,700 {\an1}scientists have been investigating 862 00:47:21,733 --> 00:47:23,733 {\an1}neutrino oscillations. 863 00:47:23,766 --> 00:47:25,900 {\an1}What they've found 864 00:47:25,933 --> 00:47:28,100 doesn't add up. 865 00:47:28,133 --> 00:47:31,400 ZELLER: The first observation that something was amiss 866 00:47:31,433 --> 00:47:34,966 {\an1}was in the late 1990s. 867 00:47:35,000 --> 00:47:38,366 {\an7}Something we don't quite understand is going on. 868 00:47:38,400 --> 00:47:40,800 {\an8}♪ ♪ 869 00:47:40,833 --> 00:47:44,600 {\an8}NARRATOR: At Fermilab, scientists fired a beam of neutrinos 870 00:47:44,633 --> 00:47:48,600 {\an7}just 500 yards to their detector. 871 00:47:48,633 --> 00:47:50,466 {\an7}Neutrinos oscillate too slowly 872 00:47:50,500 --> 00:47:52,200 {\an7}for the detector to see them change 873 00:47:52,233 --> 00:47:54,800 {\an7}over such a short distance... 874 00:47:54,833 --> 00:47:58,700 {\an7}at least according to theory. 875 00:47:58,733 --> 00:48:01,566 {\an7}But the detectors saw an increase in one type 876 00:48:01,600 --> 00:48:05,033 {\an8}of neutrinos. 877 00:48:05,066 --> 00:48:07,166 {\an7}Neutrinos seem to oscillate faster 878 00:48:07,200 --> 00:48:11,066 {\an7}than is theoretically possible. 879 00:48:11,100 --> 00:48:12,833 {\an8}KARAGIORGI: The strange thing 880 00:48:12,866 --> 00:48:18,733 {\an7}that we're seeing is that neutrinos seem to be 881 00:48:18,766 --> 00:48:22,166 {\an7}changing from one type to the other 882 00:48:22,200 --> 00:48:24,466 {\an7}much faster than expected. 883 00:48:24,500 --> 00:48:27,833 {\an1}In order for that to happen, 884 00:48:27,866 --> 00:48:29,866 {\an1}we think it's possible 885 00:48:29,900 --> 00:48:33,633 {\an1}that there are extra neutrinos out there. 886 00:48:33,666 --> 00:48:36,166 NARRATOR: In addition to the three flavors of neutrino 887 00:48:36,200 --> 00:48:40,266 {\an1}that the Standard Model describes, 888 00:48:40,300 --> 00:48:44,066 {\an1}there could be a fourth neutrino that affects them, 889 00:48:44,100 --> 00:48:47,933 {\an1}making them oscillate faster. 890 00:48:47,966 --> 00:48:52,300 {\an1}Scientists call it a sterile neutrino, 891 00:48:52,333 --> 00:48:55,866 {\an1}and it's never been directly detected. 892 00:48:57,866 --> 00:49:00,400 PEREZ: So we call it a sterile neutrino, 893 00:49:00,433 --> 00:49:04,800 {\an1}in essence, just because it interacts even less 894 00:49:04,833 --> 00:49:07,233 {\an1}with other particles than the regular neutrinos do. 895 00:49:07,266 --> 00:49:09,100 ♪ ♪ 896 00:49:09,133 --> 00:49:13,733 NARRATOR: A sterile neutrino would be the ultimate ghost particle. 897 00:49:13,766 --> 00:49:16,866 {\an1}It would never collide with atoms in our world. 898 00:49:16,900 --> 00:49:19,633 {\an1}No detector could ever see it. 899 00:49:19,666 --> 00:49:21,766 {\an1}But it may reveal itself 900 00:49:21,800 --> 00:49:26,066 {\an1}through its effects on the neutrinos we can see. 901 00:49:26,100 --> 00:49:30,866 KARAGIORGI: The only way that we can tell they exist 902 00:49:30,900 --> 00:49:34,900 {\an1}is through their effects on neutrino oscillation. 903 00:49:34,933 --> 00:49:38,600 NARRATOR: If sterile neutrinos exist, 904 00:49:38,633 --> 00:49:42,033 {\an1}it would break the neat symmetry of the Standard Model 905 00:49:42,066 --> 00:49:46,166 {\an1}that organizes particles in groups of three. 906 00:49:46,200 --> 00:49:48,366 {\an1}What if there's a fourth kind of neutrino, 907 00:49:48,400 --> 00:49:49,600 {\an1}a so-called sterile neutrino? 908 00:49:49,633 --> 00:49:53,133 {\an1}Well, where would you put that on our map? 909 00:49:53,166 --> 00:49:55,566 {\an1}There's no room to kind of shoehorn in, 910 00:49:55,600 --> 00:49:58,333 {\an1}to squeeze in a fourth neutrino. 911 00:49:58,366 --> 00:50:02,633 {\an1}So I think there really is a lot riding on this. 912 00:50:02,666 --> 00:50:07,533 NARRATOR: If they're real, sterile neutrinos would have mass, 913 00:50:07,566 --> 00:50:09,933 {\an1}but not interact with our detectors... 914 00:50:09,966 --> 00:50:13,166 {\an1}just like dark matter. 915 00:50:13,200 --> 00:50:17,933 {\an1}They could be the first particle of dark matter ever discovered, 916 00:50:17,966 --> 00:50:21,533 {\an1}and through their effects on the neutrinos we can see, 917 00:50:21,566 --> 00:50:26,700 {\an1}they could give scientists a window into another world. 918 00:50:26,733 --> 00:50:29,600 KAISER: The neutrino might be a kind of link, 919 00:50:29,633 --> 00:50:31,800 {\an1}almost a kind of messenger or portal 920 00:50:31,833 --> 00:50:35,133 {\an1}to this whole other possible kind of stuff out there. 921 00:50:38,866 --> 00:50:44,900 NARRATOR: At Fermilab, scientists are edging towards the truth. 922 00:50:44,933 --> 00:50:47,433 ZELLER: I think we're getting a lot closer. 923 00:50:47,466 --> 00:50:49,733 {\an1}Neutrino physicists are incredibly patient. 924 00:50:49,766 --> 00:50:52,500 {\an1}It takes a long time for us to collect our data, 925 00:50:52,533 --> 00:50:55,433 {\an1}and we really want to be sure in what we're seeing before 926 00:50:55,466 --> 00:50:59,400 {\an1}we potentially make a very important discovery. 927 00:50:59,433 --> 00:51:01,366 {\an1}We're trying to answer 928 00:51:01,400 --> 00:51:03,533 {\an1}some of the biggest questions in physics. 929 00:51:03,566 --> 00:51:05,533 {\an1}I think it's really unique that neutrinos 930 00:51:05,566 --> 00:51:08,300 {\an1}may hold all the answers. 931 00:51:08,333 --> 00:51:10,266 NARRATOR: What began as a hypothetical particle 932 00:51:10,300 --> 00:51:13,700 {\an1}that no one thought possible to detect 933 00:51:13,733 --> 00:51:15,733 {\an1}could now be a key that unlocks 934 00:51:15,766 --> 00:51:20,400 {\an1}what most of our universe is made of and how it works. 935 00:51:23,000 --> 00:51:24,233 KAISER: Every time we look up, 936 00:51:24,266 --> 00:51:27,066 {\an1}there seem to be these very curious neutrinos. 937 00:51:27,100 --> 00:51:28,466 {\an1}They are constantly bedeviling 938 00:51:28,500 --> 00:51:31,300 {\an1}our mental maps of how we carve up nature 939 00:51:31,333 --> 00:51:32,833 {\an1}and try to dig in and study it. 940 00:51:32,866 --> 00:51:35,200 And that's just amazingly exciting. 941 00:51:35,233 --> 00:51:38,200 {\an1}So they've gone from, "Maybe they exist, maybe they don't, 942 00:51:38,233 --> 00:51:39,900 {\an1}we might never know," 943 00:51:39,933 --> 00:51:43,833 {\an1}to being our surest ticket to the next step. 944 00:51:43,866 --> 00:51:45,833 KARAGIORGI: History has shown that 945 00:51:45,866 --> 00:51:48,733 {\an1}with every little bit of progress, 946 00:51:48,766 --> 00:51:53,433 {\an1}we've learned huge, surprising things about our cosmos. 947 00:51:53,466 --> 00:51:55,600 {\an1}To me, that's really exciting. 948 00:51:55,633 --> 00:52:00,500 {\an1}And I'm curious to know, where else could we go? 949 00:52:00,533 --> 00:52:02,800 NARRATOR: Wherever we go, 950 00:52:02,833 --> 00:52:06,866 {\an1}neutrinos could be our guide. 951 00:52:13,966 --> 00:52:21,966 {\an8}♪ ♪ 952 00:52:28,900 --> 00:52:33,966 {\an8}♪ ♪ 953 00:52:34,000 --> 00:52:35,642 {\an8}ALOK PATEL: Discover the science behind the news 954 00:52:35,666 --> 00:52:37,533 {\an7}with the "NOVA Now" podcast. 955 00:52:37,566 --> 00:52:41,066 {\an7}Listen at pbs.org/novanowpodcast 956 00:52:41,100 --> 00:52:44,266 {\an7}or wherever you find your favorite podcasts. 957 00:52:44,300 --> 00:52:48,233 {\an8}ANNOUNCER: To order this program on DVD, visit ShopPBS 958 00:52:48,266 --> 00:52:51,233 {\an7}or call 1-800-PLAY-PBS. 959 00:52:51,266 --> 00:52:54,133 {\an7}Episodes of "NOVA" are available with Passport. 960 00:52:54,166 --> 00:52:57,933 {\an7}"NOVA" is also available on Amazon Prime Video. 961 00:52:57,966 --> 00:53:02,233 {\an8}♪ ♪ 962 00:53:12,033 --> 00:53:15,666 {\an8}♪ ♪