1 00:00:01,801 --> 00:00:03,369 MIKE ROWE: Our world, 2 00:00:03,470 --> 00:00:07,373 our solar system, our universe. 3 00:00:07,474 --> 00:00:09,174 None of it would exist without 4 00:00:09,275 --> 00:00:13,078 a ghostly particle called the neutrino. 5 00:00:13,179 --> 00:00:15,247 They can pass right through a wall, 6 00:00:15,348 --> 00:00:17,249 right through a planet, right through a star, 7 00:00:17,350 --> 00:00:18,384 without even noticing. 8 00:00:20,186 --> 00:00:22,888 ROWE: They are our early warning system. 9 00:00:22,989 --> 00:00:24,957 Whenever there's trouble in the universe, 10 00:00:25,058 --> 00:00:28,794 you can expect a flood of neutrinos. 11 00:00:28,895 --> 00:00:32,598 ROWE: Neutrinos trigger star-killing explosions, 12 00:00:32,699 --> 00:00:33,699 supernovas. 13 00:00:35,668 --> 00:00:39,905 Neutrinos can answer so many questions, from why 14 00:00:40,006 --> 00:00:44,309 do we exist to, how was the universe created? 15 00:00:44,411 --> 00:00:47,880 ROWE: These tiny particles saved the infant cosmos 16 00:00:47,981 --> 00:00:49,581 from annihilation. 17 00:00:49,682 --> 00:00:51,617 They cause destruction. 18 00:00:51,718 --> 00:00:53,986 They, you know, sometimes they blow up a star. 19 00:00:54,087 --> 00:00:55,621 But, at the end of the day, 20 00:00:55,722 --> 00:01:00,359 they can be the very reason that we exist at all. 21 00:01:00,460 --> 00:01:03,829 ROWE: Neutrinos are the key to how the universe works. 22 00:01:05,899 --> 00:01:07,066 [electricity buzzing] 23 00:01:08,802 --> 00:01:11,336 [explosion blasts] 24 00:01:19,379 --> 00:01:23,048 ROWE: In the 1960s, our sun appeared to be dying. 25 00:01:25,285 --> 00:01:27,252 FILIPPENKO: There was tantalizing evidence that 26 00:01:27,353 --> 00:01:29,788 our sun might be shutting down. 27 00:01:29,889 --> 00:01:32,257 This question was a biggie for astronomers. 28 00:01:32,358 --> 00:01:35,094 If the sun isn't undergoing nuclear fusion at the rate 29 00:01:35,195 --> 00:01:37,896 we thought it was, then that's a big deal. 30 00:01:40,667 --> 00:01:44,136 ROWE: Was the sun's nuclear core shutting down? 31 00:01:44,237 --> 00:01:46,538 Stars, including our own sun, 32 00:01:46,639 --> 00:01:49,675 are giant nuclear fusion reactors. 33 00:01:49,776 --> 00:01:52,978 ROWE: Inside these fusion reactors, 34 00:01:53,079 --> 00:01:55,714 hydrogen atoms smash together, 35 00:01:57,750 --> 00:02:01,620 producing heat and light in the form of photons. 36 00:02:05,825 --> 00:02:07,392 All the light and all the heat that 37 00:02:07,494 --> 00:02:10,362 we receive on Earth comes from the sun. 38 00:02:10,463 --> 00:02:13,098 If the sun were to suddenly start cooling off, 39 00:02:13,166 --> 00:02:17,102 that would be seriously bad news for us. 40 00:02:19,072 --> 00:02:21,940 ROWE: How do we check if the sun is shutting down? 41 00:02:26,079 --> 00:02:30,082 We have a spacecraft monitoring the solar surface, 42 00:02:30,183 --> 00:02:32,951 but they can't see into the heart of the reactor, 43 00:02:33,052 --> 00:02:34,987 the sun's core. 44 00:02:35,088 --> 00:02:38,123 You can see the surface, and the sun is very bright. 45 00:02:38,224 --> 00:02:40,092 That makes it very easy to study. 46 00:02:40,193 --> 00:02:45,330 Sadly, the core of the sun is under 400,000 miles of sun, 47 00:02:45,431 --> 00:02:48,467 and that makes it pretty hard to look at. 48 00:02:48,568 --> 00:02:52,271 ROWE: Studying the light made in the core doesn't help. 49 00:02:54,040 --> 00:02:57,242 By the time it gets to us, it's old news. 50 00:02:57,343 --> 00:03:00,879 TREMBLAY: Imagine a photon or this particle of light 51 00:03:00,980 --> 00:03:03,248 that's born in the center of a star, 52 00:03:03,349 --> 00:03:06,685 and now imagine that it wants to reach the surface of the star. 53 00:03:06,786 --> 00:03:10,122 It turns out that the star is so dense in the center, 54 00:03:10,223 --> 00:03:13,258 and the star itself is so physically large that it will 55 00:03:13,359 --> 00:03:16,428 take it 30,000 years to escape the core. 56 00:03:19,032 --> 00:03:21,466 MINGARELLI: It's like being at a cocktail party, 57 00:03:21,568 --> 00:03:24,102 where you're trying to leave, and every time that you 58 00:03:24,204 --> 00:03:25,871 make another step towards the door, 59 00:03:25,972 --> 00:03:28,440 another group of people want to talk to you, and you also 60 00:03:28,541 --> 00:03:30,709 want to talk to them, and then it just takes 61 00:03:30,810 --> 00:03:33,879 30,000 years to leave your cocktail party. 62 00:03:33,980 --> 00:03:36,715 ROWE: Any information we get from sunlight 63 00:03:36,816 --> 00:03:38,584 about what's going on in the core 64 00:03:38,685 --> 00:03:41,320 is tens of thousands of years old. 65 00:03:42,689 --> 00:03:46,291 If you want the current events, the news headlines of 66 00:03:46,392 --> 00:03:49,027 what's going on in the sun's core right now, 67 00:03:49,128 --> 00:03:50,929 photons are not the way to do it. 68 00:03:51,030 --> 00:03:52,497 You want neutrinos. 69 00:03:54,467 --> 00:03:57,402 ROWE: So what are these mysterious particles? 70 00:03:57,470 --> 00:04:01,673 Neutrino literally means tiny neutral one, right? 71 00:04:01,741 --> 00:04:04,243 We think they carry no net electrical charge, 72 00:04:04,344 --> 00:04:05,444 and they're really, 73 00:04:05,545 --> 00:04:08,313 really small, so we call them neutrinos. 74 00:04:08,414 --> 00:04:10,649 ROWE: Neutrinos don't like to interact with matter. 75 00:04:12,118 --> 00:04:14,920 They fly through almost everything. 76 00:04:15,021 --> 00:04:19,524 The sun itself is generating enough neutrinos to 77 00:04:19,626 --> 00:04:23,695 send 60 billion of them through your thumbnail 78 00:04:23,796 --> 00:04:27,399 every single second, and you will spend -- 79 00:04:27,500 --> 00:04:28,700 This is the craziest thing -- 80 00:04:28,801 --> 00:04:33,338 You will spend your entire life without feeling 81 00:04:33,439 --> 00:04:34,706 a single one. 82 00:04:36,676 --> 00:04:40,045 ROWE: Neutrinos form during nuclear fusion reactions 83 00:04:40,146 --> 00:04:45,584 inside the core of stars -- Hydrogen atoms collide, 84 00:04:45,685 --> 00:04:50,989 fuse into helium, and release photons of light and neutrinos. 85 00:04:51,090 --> 00:04:53,759 MINGARELLI: In the core of the sun, 86 00:04:53,860 --> 00:04:56,194 nuclear bombs are going off, 87 00:04:56,296 --> 00:05:00,032 and all of these nuclear reactions release neutrinos. 88 00:05:00,133 --> 00:05:02,167 That's about 10 trillion, trillion, 89 00:05:02,268 --> 00:05:07,606 trillion neutrinos being created every second. 90 00:05:07,707 --> 00:05:11,810 ROWE: The trillions of neutrinos shoot out of the core 91 00:05:11,911 --> 00:05:14,880 and up through 323,000 miles 92 00:05:14,981 --> 00:05:17,149 of the sun to the surface. 93 00:05:19,218 --> 00:05:21,053 A neutrino basically doesn't even notice 94 00:05:21,154 --> 00:05:22,187 the sun is there. 95 00:05:22,288 --> 00:05:25,123 It sails out at very close to the speed of light. 96 00:05:26,526 --> 00:05:29,194 If you imagine a gridlocked highway, 97 00:05:29,295 --> 00:05:31,763 the neutrinos would be the motor bikes that are just 98 00:05:31,864 --> 00:05:34,099 zooming through the traffic. 99 00:05:35,501 --> 00:05:37,803 ROWE: The solar neutrinos race towards Earth. 100 00:05:39,472 --> 00:05:41,306 Most pass straight through. 101 00:05:42,675 --> 00:05:45,711 SUTTER: All the neutrinos, the trillions upon 102 00:05:45,812 --> 00:05:48,880 trillions of neutrinos passing through the Earth 103 00:05:48,981 --> 00:05:50,782 every single second, 104 00:05:50,883 --> 00:05:54,286 the entire Earth will only interact 105 00:05:54,387 --> 00:05:58,623 with one neutrino out of 10 billion. 106 00:06:00,193 --> 00:06:02,461 ROWE: Because they pass through anything, 107 00:06:02,562 --> 00:06:05,063 they're hard to detect. 108 00:06:05,164 --> 00:06:09,735 I consider neutrino physicists to be the ghost hunters of 109 00:06:09,836 --> 00:06:11,203 the particle physics realm, 110 00:06:11,304 --> 00:06:14,940 because we study something so elusive, and they're really, 111 00:06:15,041 --> 00:06:17,843 really hard to nail down and study. 112 00:06:21,013 --> 00:06:23,782 ROWE: Hard, but not impossible. 113 00:06:23,883 --> 00:06:26,618 While most neutrinos pass through Earth, 114 00:06:26,719 --> 00:06:31,056 a few collide with atoms in the planet, and we can detect 115 00:06:31,157 --> 00:06:33,225 those collisions. 116 00:06:33,326 --> 00:06:35,193 To spot these tiny impacts, 117 00:06:35,294 --> 00:06:38,363 we built underground neutrino detectors 118 00:06:38,464 --> 00:06:41,032 with giant sensors full of chlorine. 119 00:06:43,002 --> 00:06:46,538 When a neutrino strikes this chlorine atom, 120 00:06:46,639 --> 00:06:48,640 it transforms into argon. 121 00:06:48,741 --> 00:06:51,643 And then we can pick out the argon atoms from 122 00:06:51,744 --> 00:06:53,478 the detector and count them up 123 00:06:53,579 --> 00:06:56,982 to see how many neutrinos actually struck our atoms. 124 00:06:59,185 --> 00:07:02,254 ROWE: The sensors detected neutrinos from the sun, 125 00:07:02,355 --> 00:07:04,656 but the numbers were lower than expected. 126 00:07:05,858 --> 00:07:08,860 Detectors were only detecting about a third of 127 00:07:08,961 --> 00:07:12,364 the number of the neutrinos that their models predicted. 128 00:07:12,465 --> 00:07:15,133 This is called the solar neutrino problem. 129 00:07:15,234 --> 00:07:18,336 That is a big deal -- That either means 130 00:07:18,438 --> 00:07:21,440 we're doing something wrong or our physics is wrong. 131 00:07:21,541 --> 00:07:22,574 Where were the missing 132 00:07:22,675 --> 00:07:24,910 two-thirds of the solar neutrinos? 133 00:07:25,011 --> 00:07:27,312 ROWE: They weren't AWOL. 134 00:07:27,413 --> 00:07:28,780 The detector had missed them, 135 00:07:28,881 --> 00:07:31,516 because neutrinos can change identities. 136 00:07:33,219 --> 00:07:36,354 It turns out neutrinos can change what kind 137 00:07:36,456 --> 00:07:39,424 of neutrino they are as they're flying through space, 138 00:07:39,525 --> 00:07:41,426 and we call this flavor changing. 139 00:07:42,929 --> 00:07:45,397 ROWE: Neutrinos come in three different flavors. 140 00:07:47,233 --> 00:07:49,701 Think of them as different types of playing cards. 141 00:07:52,472 --> 00:07:55,974 The king is the electron neutrino. 142 00:07:56,075 --> 00:07:58,777 The muon neutrino is the queen, 143 00:07:58,878 --> 00:08:01,446 and the jack is the tau neutrino. 144 00:08:01,547 --> 00:08:05,450 The sun produces electron neutrinos, 145 00:08:05,551 --> 00:08:07,185 but by the time they reach Earth, 146 00:08:07,286 --> 00:08:08,687 they could be a different flavor. 147 00:08:10,056 --> 00:08:11,389 As they travel to the Earth, 148 00:08:11,491 --> 00:08:13,792 they constantly wave back and forth, 149 00:08:13,893 --> 00:08:15,460 trading their identities. 150 00:08:15,528 --> 00:08:19,164 So you never know exactly what you're gonna get 151 00:08:19,265 --> 00:08:22,033 until it arrives at the Earth, and we observe it. 152 00:08:22,134 --> 00:08:25,203 It could be... anything. 153 00:08:25,304 --> 00:08:28,907 ROWE: The detectors weren't seeing the different flavors. 154 00:08:29,008 --> 00:08:31,343 But when we fine-tuned the sensors, 155 00:08:31,444 --> 00:08:34,613 we saw all the solar neutrinos. 156 00:08:34,714 --> 00:08:37,382 So there were actually enough neutrinos coming from 157 00:08:37,483 --> 00:08:40,719 the sun, but we were only detecting a third of them. 158 00:08:40,820 --> 00:08:43,922 ROWE: Flavor-changing neutrinos showed the sun was healthy. 159 00:08:45,124 --> 00:08:47,225 The changing identities also answered 160 00:08:47,326 --> 00:08:49,528 an important question about neutrinos. 161 00:08:51,030 --> 00:08:52,297 Do they have mass? 162 00:08:53,633 --> 00:08:57,168 Einstein showed that only particles without mass can 163 00:08:57,270 --> 00:08:59,170 travel at the speed of light, 164 00:08:59,272 --> 00:09:02,841 and these particles don't experience time. 165 00:09:02,942 --> 00:09:05,644 But neutrinos can change their flavor, 166 00:09:05,745 --> 00:09:08,246 so that must happen over time. 167 00:09:08,347 --> 00:09:12,350 And that means neutrinos can't travel at the speed of light, 168 00:09:12,451 --> 00:09:15,353 and so they must have mass. 169 00:09:15,454 --> 00:09:18,356 When scientists first started thinking about neutrinos, 170 00:09:18,457 --> 00:09:20,058 they thought that they were massless, 171 00:09:20,159 --> 00:09:22,661 and if a neutrino has no mass, 172 00:09:22,762 --> 00:09:25,797 then it's bound to be one flavor 173 00:09:25,898 --> 00:09:27,732 or one type of neutrino forever. 174 00:09:30,503 --> 00:09:34,406 ROWE: Experiments proved that neutrinos have mass. 175 00:09:34,507 --> 00:09:38,610 And if they have mass, they must produce gravity, 176 00:09:38,711 --> 00:09:42,280 which means they can influence other things around them. 177 00:09:48,821 --> 00:09:51,923 Neutrinos are also involved in moments of huge 178 00:09:52,024 --> 00:09:54,359 cosmic violence. 179 00:09:54,460 --> 00:09:57,529 Whenever there's trouble in the universe, 180 00:09:57,630 --> 00:10:00,832 you can expect a flood of neutrinos. 181 00:10:03,302 --> 00:10:05,470 ROWE: These floods of neutrinos are the key to 182 00:10:05,571 --> 00:10:07,572 some of the biggest bangs in the cosmos. 183 00:10:08,674 --> 00:10:12,177 And new research suggests that without them, 184 00:10:12,278 --> 00:10:19,317 there would be no solar system, no planets, and no us. 185 00:10:27,159 --> 00:10:29,661 ROWE: Neutrinos are one of the smallest particles in 186 00:10:29,762 --> 00:10:31,696 the cosmos. 187 00:10:31,797 --> 00:10:34,432 However, new research suggests they play 188 00:10:34,533 --> 00:10:37,369 a role in some of the universe's biggest events. 189 00:10:41,040 --> 00:10:43,775 Exploding stars called supernovas. 190 00:10:46,445 --> 00:10:49,848 The deaths of giant stars. 191 00:10:49,949 --> 00:10:53,284 But there is a mystery surrounding 192 00:10:53,386 --> 00:10:54,619 their explosive ends. 193 00:10:54,720 --> 00:11:01,259 Why do these giant stars end their lives so violently? 194 00:11:01,360 --> 00:11:05,096 This is a major puzzle in astrophysics. 195 00:11:05,197 --> 00:11:07,799 ROWE: We got a lead when we detected 196 00:11:07,900 --> 00:11:12,270 a huge flash of light in the large Magellanic Cloud, 197 00:11:12,371 --> 00:11:15,440 a satellite galaxy of the Milky Way. 198 00:11:15,541 --> 00:11:18,109 The light was a supernova explosion. 199 00:11:20,379 --> 00:11:22,080 But three hours before the flash, 200 00:11:22,181 --> 00:11:25,083 astronomers spotted something else 201 00:11:25,184 --> 00:11:28,787 a burst of neutrinos coming from the same region of the sky. 202 00:11:30,690 --> 00:11:32,957 SUTTER: This was the first time we have seen neutrinos 203 00:11:33,059 --> 00:11:35,293 coming from a source other than the sun, 204 00:11:35,394 --> 00:11:38,863 so there must be some sort of connection between neutrinos 205 00:11:38,964 --> 00:11:39,831 and supernovae, 206 00:11:39,932 --> 00:11:42,534 but -- but what is that connection? 207 00:11:42,635 --> 00:11:44,736 ROWE: When a star runs out of fuel, 208 00:11:44,837 --> 00:11:48,440 its core crushes down to a neutron star. 209 00:11:48,541 --> 00:11:51,943 Then the rest of the star collapses inwards, 210 00:11:52,044 --> 00:11:55,346 hits the neutron star, and bounces out, 211 00:11:55,448 --> 00:11:57,248 triggering a supernova. 212 00:11:59,452 --> 00:12:03,221 But computer models of supernovas reveal a problem. 213 00:12:03,322 --> 00:12:06,591 The star doesn't explode. 214 00:12:06,692 --> 00:12:09,994 SUTTER: When we run computer simulations of how supernova 215 00:12:10,096 --> 00:12:13,698 might work, after this bounce, 216 00:12:13,766 --> 00:12:17,469 the explosion stalls, it peters out. 217 00:12:17,570 --> 00:12:20,238 The supernova isn't so super. 218 00:12:20,339 --> 00:12:22,540 It needs another source of energy to 219 00:12:22,641 --> 00:12:26,377 propel it to become an actual explosion. 220 00:12:28,481 --> 00:12:30,448 ROWE: Could the neutrinos that appeared before 221 00:12:30,549 --> 00:12:33,518 the explosion be that energy source? 222 00:12:35,287 --> 00:12:37,021 First, we need to understand 223 00:12:37,123 --> 00:12:40,158 what created the burst of neutrinos. 224 00:12:42,461 --> 00:12:47,265 The core of the star collapses inward and eventually, 225 00:12:47,366 --> 00:12:48,833 the outer layers of the star 226 00:12:48,934 --> 00:12:52,103 fall in toward that star at an appreciable fraction of 227 00:12:52,204 --> 00:12:54,839 the speed of light. 228 00:12:54,940 --> 00:12:56,841 ROWE: As the core rapidly collapses, 229 00:12:56,942 --> 00:13:01,412 the intense pressure squeezes atoms together. 230 00:13:01,514 --> 00:13:03,648 That core of iron gets squeezed down 231 00:13:03,783 --> 00:13:05,216 to become a neutron star. 232 00:13:06,919 --> 00:13:09,521 The electrons and the protons that are part of this core are 233 00:13:09,622 --> 00:13:12,090 under so much pressure that they fuse together to form 234 00:13:12,191 --> 00:13:15,527 neutrons and neutrinos in the process. 235 00:13:15,628 --> 00:13:19,197 ROWE: The neutrinos shoot out from the newly formed 236 00:13:19,298 --> 00:13:21,266 neutron star core, 237 00:13:21,367 --> 00:13:24,869 carrying an enormous amount of energy. 238 00:13:24,970 --> 00:13:28,840 99% of the energy is carried by the neutrinos. 239 00:13:28,941 --> 00:13:31,176 Neutrinos are the main event. 240 00:13:31,277 --> 00:13:33,711 ROWE: Trillions of neutrinos smash into 241 00:13:33,813 --> 00:13:36,080 the remains of the dying star. 242 00:13:36,182 --> 00:13:39,651 And when those neutrinos are flying out of that core region, 243 00:13:39,752 --> 00:13:43,454 a very tiny fraction of them interact with the gas, 244 00:13:43,556 --> 00:13:46,191 and that fraction heats the gas. 245 00:13:48,794 --> 00:13:50,428 Everything that's hanging around 246 00:13:50,529 --> 00:13:53,164 this newborn neutron star 247 00:13:53,265 --> 00:13:56,301 get heated to an unimaginable degree. 248 00:13:56,402 --> 00:14:00,138 ROWE: The heat creates pressures in the surrounding gas. 249 00:14:00,239 --> 00:14:02,740 It builds and builds until it triggers 250 00:14:02,842 --> 00:14:04,242 an enormous shock wave. 251 00:14:04,343 --> 00:14:06,911 [explosion blasts] 252 00:14:07,012 --> 00:14:09,180 And then the actual explosion, 253 00:14:09,315 --> 00:14:11,683 the actual fireworks show, begins. 254 00:14:11,784 --> 00:14:14,018 [explosion blasts] 255 00:14:14,119 --> 00:14:16,421 ROWE: The star explodes 256 00:14:16,522 --> 00:14:19,624 in one of the brightest events in the universe, 257 00:14:19,725 --> 00:14:22,560 powered by neutrinos. 258 00:14:22,661 --> 00:14:25,797 We think that if it weren't for neutrinos, 259 00:14:25,898 --> 00:14:28,399 supernovas might not even exist. 260 00:14:29,869 --> 00:14:32,270 ROWE: And we might not exist either. 261 00:14:32,371 --> 00:14:35,373 Our bodies contain heavy elements, like calcium 262 00:14:35,474 --> 00:14:39,210 in our bones and iron in our blood. 263 00:14:39,311 --> 00:14:42,847 These elements form in supernovas and are 264 00:14:42,948 --> 00:14:46,184 scattered across the cosmos by the blast. 265 00:14:46,285 --> 00:14:51,389 Neutrinos are what kindle the fire 266 00:14:51,490 --> 00:14:54,392 in the forages of these elements. 267 00:14:54,493 --> 00:14:57,161 And without the neutrinos, you don't have the elements. 268 00:14:57,263 --> 00:14:58,329 And without the elements, 269 00:14:58,430 --> 00:15:00,465 you don't have planets like the Earth. 270 00:15:00,566 --> 00:15:03,902 And without planets like the Earth, you don't have life. 271 00:15:04,003 --> 00:15:07,171 There's this common phrase, you know, we are stardust, 272 00:15:07,273 --> 00:15:09,307 which is true, but I like to think 273 00:15:09,408 --> 00:15:12,010 we're more like neutrino dust. 274 00:15:14,446 --> 00:15:17,916 ROWE: Neutrinos reveal how supernovas explode, 275 00:15:18,017 --> 00:15:21,519 and they also warn us when one is about to detonate. 276 00:15:21,620 --> 00:15:23,421 So neutrinos can even be these 277 00:15:23,522 --> 00:15:26,457 ghostly signposts for something very violent 278 00:15:26,558 --> 00:15:27,959 that's happened in the universe, right? 279 00:15:28,060 --> 00:15:30,395 We detect a sudden burst of neutrinos. 280 00:15:30,496 --> 00:15:33,431 It could be that a star has gone supernova somewhere. 281 00:15:35,367 --> 00:15:38,736 ROWE: Neutrino bursts are cosmic watchdogs, 282 00:15:38,837 --> 00:15:40,972 alerting us to danger. 283 00:15:41,073 --> 00:15:44,142 Neutrinos are definitely a sign 284 00:15:44,243 --> 00:15:47,712 that something troubling is happening. 285 00:15:47,813 --> 00:15:50,982 ROWE: And in 2017, a single neutrino 286 00:15:51,083 --> 00:15:54,485 told us about something very troubling, 287 00:15:54,586 --> 00:15:57,221 one of the most intense sources of radiation 288 00:15:57,323 --> 00:16:01,059 in the universe, and it was pointing right at us. 289 00:16:10,803 --> 00:16:13,171 ROWE: Spring 2017. 290 00:16:13,272 --> 00:16:15,506 Scientists at the South Pole are on the lookout 291 00:16:15,607 --> 00:16:16,574 for neutrinos. 292 00:16:18,010 --> 00:16:21,946 These ghostly particles are extremely hard to detect. 293 00:16:23,515 --> 00:16:25,984 Neutrinos are the biggest introverts in the universe. 294 00:16:26,085 --> 00:16:28,786 They just don't like interacting with anything, so if 295 00:16:28,887 --> 00:16:30,154 you want to detect one of these things, 296 00:16:30,255 --> 00:16:31,456 you need a lot of stuff. 297 00:16:31,557 --> 00:16:34,525 You need a lot of atoms in one spot. 298 00:16:34,626 --> 00:16:36,627 ROWE: So scientists built a facility 299 00:16:36,729 --> 00:16:38,796 with lots of available atoms. 300 00:16:38,897 --> 00:16:41,599 It's called IceCube, with neutrino 301 00:16:41,700 --> 00:16:45,770 detectors buried deep beneath sheets of ice. 302 00:16:45,871 --> 00:16:48,673 It turns out that water is a very, 303 00:16:48,774 --> 00:16:52,377 very good detector of neutrinos. 304 00:16:52,478 --> 00:16:54,679 ROWE: To catch neutrinos, you need to build 305 00:16:54,780 --> 00:16:57,782 a very large target for a reasonable cost. 306 00:16:57,883 --> 00:17:02,754 Large areas of ice checks both boxes. 307 00:17:02,855 --> 00:17:06,057 So you need a lot of water that's very, very clean. 308 00:17:06,158 --> 00:17:08,593 What's the cleanest source of water on the planet? 309 00:17:08,660 --> 00:17:11,729 The Antarctic Ice Sheet. 310 00:17:11,830 --> 00:17:14,265 The Antarctic detector IceCube 311 00:17:14,366 --> 00:17:18,803 measures 3,280 feet across. 312 00:17:18,904 --> 00:17:22,407 That's about the length of nine football fields. 313 00:17:22,508 --> 00:17:27,011 It contains 5,000 sensors, surrounded by more water 314 00:17:27,112 --> 00:17:30,048 atoms than there are stars in the universe. 315 00:17:33,018 --> 00:17:35,820 September 22nd, 2017. 316 00:17:37,623 --> 00:17:42,226 IceCube detects a neutrino colliding with a water atom. 317 00:17:42,327 --> 00:17:45,530 When a neutrino hits an ice atom inside of IceCube, 318 00:17:45,631 --> 00:17:47,565 a charged particle flies out, 319 00:17:47,666 --> 00:17:49,867 and it's this charged particle that makes a signal 320 00:17:49,968 --> 00:17:50,968 we can detect. 321 00:17:51,070 --> 00:17:53,971 ROWE: The ejected particle appears to fly out 322 00:17:54,073 --> 00:17:56,207 faster than the speed of light. 323 00:17:56,308 --> 00:17:58,676 At first glance, this looks like it violates 324 00:17:58,777 --> 00:18:00,011 something very, very important 325 00:18:00,112 --> 00:18:03,281 about physics, that nothing can travel faster than light. 326 00:18:03,382 --> 00:18:07,185 But light slows down when traveling through a medium like 327 00:18:07,286 --> 00:18:11,355 air or water, and it is possible 328 00:18:11,457 --> 00:18:15,426 for other things, other particles, to outrun light 329 00:18:15,527 --> 00:18:17,261 in a medium. 330 00:18:17,362 --> 00:18:20,431 ROWE: As it hurtles through the ice, 331 00:18:20,532 --> 00:18:24,168 the particle generates a burst of blue light called 332 00:18:24,269 --> 00:18:26,104 Cherenkov radiation. 333 00:18:26,205 --> 00:18:27,839 It's almost like a sonic boom. 334 00:18:27,940 --> 00:18:29,674 If you travel faster than the speed of sound, 335 00:18:29,775 --> 00:18:32,376 there's a boom, right? - When you hear that boom, 336 00:18:32,478 --> 00:18:35,813 you also see this cone of wind. 337 00:18:35,914 --> 00:18:38,916 It's the same thing with Cherenkov radiation. 338 00:18:39,017 --> 00:18:40,518 You get this cone of light. 339 00:18:42,521 --> 00:18:44,989 ROWE: Neutrinos carry different amounts of energy. 340 00:18:45,090 --> 00:18:49,427 Some, like the 2017 neutrino, 341 00:18:49,528 --> 00:18:51,696 carry quite a punch, 342 00:18:51,797 --> 00:18:55,800 and the energy of the neutrino depends on its source. 343 00:18:55,901 --> 00:19:00,037 High-energy neutrinos come from high-energy events, 344 00:19:00,139 --> 00:19:02,507 so we're looking for stuff blowing up. 345 00:19:02,608 --> 00:19:04,208 We're looking for stuff colliding. 346 00:19:04,309 --> 00:19:06,878 We're looking for stuff colliding and blowing up. 347 00:19:06,979 --> 00:19:08,880 We're looking for awesome things. 348 00:19:08,981 --> 00:19:13,951 ROWE: The blue burst of Cherenkov radiation 349 00:19:14,052 --> 00:19:16,387 gives us a clue about the fearsome origin of 350 00:19:16,488 --> 00:19:18,122 the neutrino. 351 00:19:18,223 --> 00:19:21,859 We can follow the path of that blue light, 352 00:19:21,960 --> 00:19:26,898 and we can look backwards to see where the neutrino came from. 353 00:19:29,201 --> 00:19:30,535 ROWE: We track the neutrino to 354 00:19:30,636 --> 00:19:33,838 a galaxy nearly six billion light-years away. 355 00:19:35,207 --> 00:19:36,841 At its heart sits one of 356 00:19:36,942 --> 00:19:39,744 the most powerful objects in the universe, 357 00:19:44,016 --> 00:19:45,249 a blazar. 358 00:19:46,518 --> 00:19:52,456 A blazar is the biggest, baddest form of feeding 359 00:19:52,558 --> 00:19:55,493 active, supermassive black hole out there, 360 00:19:55,594 --> 00:19:58,629 where material isn't just falling into the black hole, 361 00:19:58,730 --> 00:20:00,865 it's swirling around, creating a high-energy 362 00:20:00,966 --> 00:20:02,867 accretion disk. 363 00:20:02,968 --> 00:20:06,470 ROWE: ROWE: The blazar's accretion disk spins at millions 364 00:20:06,572 --> 00:20:08,973 of miles an hour, 365 00:20:09,074 --> 00:20:11,108 charging particles of gas and dust. 366 00:20:11,210 --> 00:20:14,645 The disk also generates magnetic fields 367 00:20:14,746 --> 00:20:18,182 that twist and tangle as they swirl around the black hole. 368 00:20:20,986 --> 00:20:23,454 Because you have magnetic fields that are 369 00:20:23,555 --> 00:20:24,789 twisted around, 370 00:20:24,890 --> 00:20:27,024 they also generate electric fields. 371 00:20:27,125 --> 00:20:29,760 The electric fields can then accelerate the charged 372 00:20:29,861 --> 00:20:32,129 particles along the magnetic fields 373 00:20:32,231 --> 00:20:35,132 and thus produce a lot of both particles 374 00:20:35,234 --> 00:20:38,102 and radiation coming out along jets. 375 00:20:38,203 --> 00:20:40,304 ROWE: The jets blast out 376 00:20:40,405 --> 00:20:41,939 of the poles of the black hole. 377 00:20:44,910 --> 00:20:50,047 These are the most intense sources of radiation 378 00:20:50,148 --> 00:20:52,383 that the cosmos can ever produce, 379 00:20:52,484 --> 00:20:56,153 and they are pointed right at us from billions of 380 00:20:56,255 --> 00:20:57,355 light-years away. 381 00:20:57,456 --> 00:21:00,825 ROWE: Do the jets create the powerful neutrinos? 382 00:21:02,227 --> 00:21:03,694 It's a bit of a mystery. 383 00:21:03,795 --> 00:21:05,196 For a while, it was thought that 384 00:21:05,297 --> 00:21:07,865 neutrinos are produced directly by the jet. 385 00:21:07,966 --> 00:21:09,667 But now we think that matter, 386 00:21:09,768 --> 00:21:12,570 like protons, come in from the accretion disk, 387 00:21:12,671 --> 00:21:14,071 and they slam into each other, 388 00:21:14,172 --> 00:21:17,275 and that's what produces the neutrinos. 389 00:21:17,376 --> 00:21:19,644 ROWE: Particles racing around the accretion disk 390 00:21:19,745 --> 00:21:22,046 crash into the base of the jet. 391 00:21:22,147 --> 00:21:25,683 The enormous energy there smashes the particles together, 392 00:21:25,784 --> 00:21:27,985 producing neutrinos. 393 00:21:28,086 --> 00:21:30,221 The jets focus the stream of 394 00:21:30,322 --> 00:21:34,392 neutrinos and fire them straight towards Earth. 395 00:21:34,493 --> 00:21:36,294 By just detecting one neutrino, 396 00:21:36,395 --> 00:21:39,230 we get to see a lot of information from 397 00:21:39,331 --> 00:21:42,600 the inner workings of an object outside of our galaxy. 398 00:21:42,701 --> 00:21:44,902 And that's what's really exciting about neutrinos 399 00:21:45,003 --> 00:21:48,439 is that it could peer into the unknown. 400 00:21:48,540 --> 00:21:52,810 ROWE: Now we use neutrinos to probe even further 401 00:21:52,911 --> 00:21:54,312 into the universe, 402 00:21:57,149 --> 00:22:01,285 back towards the first second of the Big Bang 403 00:22:01,386 --> 00:22:04,622 to answer the biggest question of them all -- 404 00:22:04,723 --> 00:22:08,125 How and why do we exist? 405 00:22:20,772 --> 00:22:23,307 ROWE: Neutrinos are key to our understanding 406 00:22:23,408 --> 00:22:25,042 of how the universe works. 407 00:22:26,411 --> 00:22:29,280 They show us that the sun is healthy. 408 00:22:31,483 --> 00:22:35,086 They are the trigger that makes supernovas explode, 409 00:22:35,187 --> 00:22:39,824 and they reveal the location of lethal blazars. 410 00:22:39,925 --> 00:22:42,927 And now they may solve something that still 411 00:22:43,028 --> 00:22:47,598 puzzles physicists -- How we exist. 412 00:22:47,699 --> 00:22:50,935 The fact that our universe appears to be filled 413 00:22:51,036 --> 00:22:53,738 with matter is puzzling. 414 00:22:53,839 --> 00:22:55,740 There should have been equal amounts of matter 415 00:22:55,841 --> 00:22:57,742 and antimatter in the beginning, 416 00:22:57,843 --> 00:22:59,710 and they should have annihilated one another, 417 00:22:59,811 --> 00:23:01,912 producing just pure energy. 418 00:23:02,013 --> 00:23:03,647 So why do we exist? 419 00:23:03,749 --> 00:23:05,716 This is a fundamental question, 420 00:23:05,817 --> 00:23:09,220 because this is a question about why is there something 421 00:23:09,321 --> 00:23:10,821 rather than nothing? 422 00:23:12,290 --> 00:23:15,092 ROWE: To answer that question, we have to 423 00:23:15,193 --> 00:23:16,427 rewind the clock back 424 00:23:16,528 --> 00:23:21,866 nearly 14 billion years to the birth of the universe. 425 00:23:21,967 --> 00:23:25,970 A speck of energy sparks into existence. 426 00:23:26,071 --> 00:23:28,806 This energy cools and forms tiny, 427 00:23:28,907 --> 00:23:33,077 primitive particles of matter, including neutrinos, 428 00:23:33,178 --> 00:23:36,781 the building blocks of everything we see today. 429 00:23:36,882 --> 00:23:40,584 The early universe appears chaotic, 430 00:23:40,685 --> 00:23:43,554 but it quickly establishes some ground rules, 431 00:23:43,655 --> 00:23:45,456 including symmetry. 432 00:23:45,557 --> 00:23:49,627 Our universe is full of symmetries. 433 00:23:49,728 --> 00:23:52,029 There are positive electric charges 434 00:23:52,130 --> 00:23:53,898 and negative electric charges. 435 00:23:53,999 --> 00:23:55,433 There's the yin and the yang. 436 00:23:55,534 --> 00:23:59,837 Well, there's also matter and antimatter. 437 00:23:59,938 --> 00:24:02,873 ROWE: The Big Bang stuck to the rule of symmetry 438 00:24:02,974 --> 00:24:07,211 and made the same amount of both forms of matter. 439 00:24:07,312 --> 00:24:10,681 The mechanisms that we have for creating matter in 440 00:24:10,782 --> 00:24:14,285 the early universe create an equal amount of antimatter. 441 00:24:14,386 --> 00:24:19,190 That symmetry is baked into the laws of physics. 442 00:24:19,291 --> 00:24:21,692 ROWE: The laws of physics also say 443 00:24:21,793 --> 00:24:25,596 that when matter and antimatter meet... 444 00:24:25,697 --> 00:24:27,698 sparks fly. 445 00:24:27,799 --> 00:24:29,867 So matter and antimatter, 446 00:24:29,968 --> 00:24:31,769 when they touch, they annihilate. 447 00:24:31,870 --> 00:24:34,472 They just disappear in a flash of energy. 448 00:24:34,573 --> 00:24:37,408 And as far as we understand, the earliest moments of 449 00:24:37,509 --> 00:24:40,177 the universe, matter and antimatter were created in 450 00:24:40,312 --> 00:24:41,178 equal amounts. 451 00:24:41,279 --> 00:24:44,081 So they should have annihilated, 452 00:24:44,182 --> 00:24:46,984 leaving nothing but energy. 453 00:24:47,085 --> 00:24:51,088 Which means, no matter, no antimatter, no gas, 454 00:24:51,189 --> 00:24:54,358 no dust, no stars, no galaxies, no life, nothing. 455 00:24:54,459 --> 00:24:57,995 Somehow matter won the battle 456 00:24:58,096 --> 00:25:00,331 over antimatter in the early universe. 457 00:25:03,401 --> 00:25:04,568 ROWE: In some ways, 458 00:25:04,669 --> 00:25:06,570 the universe ignored the rule of symmetry. 459 00:25:07,906 --> 00:25:12,409 Something has to drive the universe off balance. 460 00:25:12,511 --> 00:25:14,879 There has to be a violation 461 00:25:14,980 --> 00:25:18,516 of this fundamental balance in our universe. 462 00:25:18,617 --> 00:25:21,585 OLUSEYI: That way, when the matter and antimatter met 463 00:25:21,686 --> 00:25:24,588 and annihilated, because there was more matter, 464 00:25:24,689 --> 00:25:27,558 there would be a residual of leftover matter, 465 00:25:27,659 --> 00:25:31,295 and there would be no antimatter. 466 00:25:31,396 --> 00:25:33,430 ROWE: How did the Big Bang break 467 00:25:33,498 --> 00:25:36,567 the symmetry between matter and antimatter? 468 00:25:36,668 --> 00:25:39,670 So we're looking for any interaction, 469 00:25:39,771 --> 00:25:44,174 any process whatsoever where matter behaves slightly 470 00:25:44,276 --> 00:25:46,043 differently than antimatter. 471 00:25:46,144 --> 00:25:50,714 We're trying to find a flaw in physics. 472 00:25:50,815 --> 00:25:54,451 ROWE: We can't look for that flaw directly, 473 00:25:54,553 --> 00:25:56,453 because we can't see the Big Bang, 474 00:25:56,555 --> 00:25:59,156 but we can recreate it, 475 00:25:59,257 --> 00:26:01,959 and we think neutrinos are involved. 476 00:26:03,228 --> 00:26:05,296 This is incredibly complicated. 477 00:26:05,397 --> 00:26:09,166 I'm -- we are diving deep into the bowels of 478 00:26:09,267 --> 00:26:12,102 fundamental physics, and it is not a pretty sight. 479 00:26:15,106 --> 00:26:17,474 ROWE: Japanese scientists conducted an experiment 480 00:26:17,576 --> 00:26:19,910 called TK2. 481 00:26:20,011 --> 00:26:22,546 They re-created part of the Big Bang by 482 00:26:22,647 --> 00:26:24,281 studying neutrinos 483 00:26:24,382 --> 00:26:28,819 and their symmetrical twin, antineutrinos. 484 00:26:28,920 --> 00:26:32,089 The goal -- to see if antineutrinos change their 485 00:26:32,190 --> 00:26:36,627 identity or flavor at the same rate as regular neutrinos. 486 00:26:37,696 --> 00:26:42,466 Matter and antimatter should behave exactly the same, 487 00:26:42,567 --> 00:26:44,969 but we found something very interesting with 488 00:26:45,070 --> 00:26:46,870 this experiment. 489 00:26:46,972 --> 00:26:49,373 ROWE: The particles broke symmetry. 490 00:26:49,474 --> 00:26:52,876 Neutrinos and antineutrinos changed flavor at 491 00:26:52,978 --> 00:26:54,178 different rates. 492 00:26:55,680 --> 00:26:57,615 This was a clear-cut example 493 00:26:57,716 --> 00:27:01,051 of matter behaving differently than antimatter. 494 00:27:02,320 --> 00:27:04,955 ROWE: And that has revolutionized our understanding 495 00:27:05,056 --> 00:27:07,458 of the formation of particles during the Big Bang. 496 00:27:08,927 --> 00:27:10,894 OLUSEYI: What could have happened in the early universe 497 00:27:10,996 --> 00:27:14,398 is that more of the neutrinos converted into matter 498 00:27:14,499 --> 00:27:18,435 than there were antineutrinos became into antimatter, 499 00:27:18,536 --> 00:27:21,739 and in this way, you end up with a surplus of matter 500 00:27:21,840 --> 00:27:23,040 over antimatter. 501 00:27:28,246 --> 00:27:29,947 ROWE: Even though that surplus was just 502 00:27:30,048 --> 00:27:32,116 one particle in a billion, 503 00:27:32,217 --> 00:27:34,084 it was enough to build the cosmos. 504 00:27:36,121 --> 00:27:38,155 OLUSEYI: So neutrinos in the early universe 505 00:27:38,256 --> 00:27:40,457 could possibly solve the matter, 506 00:27:40,592 --> 00:27:42,660 antimatter asymmetry problem we have. 507 00:27:45,563 --> 00:27:47,498 Yes, they cause destruction. 508 00:27:47,599 --> 00:27:49,767 They -- you know, sometimes they blow up a star, 509 00:27:49,868 --> 00:27:52,870 but, at the end of the day, they did save 510 00:27:52,971 --> 00:27:54,571 the entire universe. 511 00:27:56,508 --> 00:28:00,444 ROWE: Now, scientists hope that neutrinos may solve 512 00:28:00,545 --> 00:28:03,647 one of the biggest mysteries in the cosmos -- 513 00:28:03,748 --> 00:28:06,583 The identity of dark matter. 514 00:28:18,363 --> 00:28:20,831 ROWE: Neutrinos have been around since 515 00:28:20,932 --> 00:28:22,433 the birth of the universe. 516 00:28:22,534 --> 00:28:28,105 They may even be responsible for the formation of matter. 517 00:28:28,206 --> 00:28:30,808 Now we investigate if they play an even 518 00:28:30,909 --> 00:28:34,311 larger role in the development of the universe, 519 00:28:34,412 --> 00:28:37,514 the formation of the cosmic web. 520 00:28:39,984 --> 00:28:43,487 At the very largest scales in our universe, 521 00:28:43,588 --> 00:28:48,058 galaxies are arranged in a very peculiar pattern. 522 00:28:48,159 --> 00:28:51,662 We see long, thin threads of galaxies, 523 00:28:51,763 --> 00:28:55,065 and at the intersections, we see dense clumps of galaxies 524 00:28:55,133 --> 00:28:56,133 called clusters. 525 00:28:56,234 --> 00:28:57,935 In between them, we have these vast 526 00:28:58,036 --> 00:29:01,238 empty regions called the cosmic voids. 527 00:29:01,339 --> 00:29:03,474 ROWE: For a long time, how the cosmic 528 00:29:03,575 --> 00:29:06,343 web formed and held together was a mystery. 529 00:29:06,444 --> 00:29:09,980 One of the real mysteries about our existence is 530 00:29:10,081 --> 00:29:12,983 why the universe was able to hold together at all. 531 00:29:13,084 --> 00:29:15,586 All the matter was simply spread apart 532 00:29:15,687 --> 00:29:18,655 to sparsely to ever form galaxies or stars. 533 00:29:18,757 --> 00:29:21,925 Instead, something helped to hold it together. 534 00:29:22,026 --> 00:29:26,597 We now think the glue binding the cosmic web 535 00:29:26,698 --> 00:29:31,401 is a mysterious substance known as dark matter. 536 00:29:31,503 --> 00:29:34,571 If it wasn't for dark matter in the very early universe, 537 00:29:34,672 --> 00:29:36,740 there might be no structure at all. 538 00:29:39,511 --> 00:29:42,412 ROWE: But what is this architect of the universe, 539 00:29:42,514 --> 00:29:43,413 this dark matter? 540 00:29:44,816 --> 00:29:47,618 ESQUIVE: Dark matter is invisible matter that we can't 541 00:29:47,719 --> 00:29:51,488 see -- so you, me, all of the particles, everything that 542 00:29:51,589 --> 00:29:56,460 we see is actually only 5% of actual matter in the universe. 543 00:29:56,561 --> 00:29:58,428 The rest is dark matter. 544 00:29:59,998 --> 00:30:03,467 TEGMARK: Dark matter is a fancy name 545 00:30:03,568 --> 00:30:05,769 for something we don't understand. 546 00:30:05,870 --> 00:30:07,871 What we do know is that there 547 00:30:07,972 --> 00:30:10,774 is much more stuff than we can see. 548 00:30:10,875 --> 00:30:13,277 But we have no idea what it is. 549 00:30:13,411 --> 00:30:17,281 It's one of the greatest open mysteries in science. 550 00:30:19,083 --> 00:30:21,718 ROWE: Dark matter hardly interacts with anything, 551 00:30:21,820 --> 00:30:26,123 a bit like neutrinos -- Also like neutrinos, 552 00:30:26,224 --> 00:30:30,360 dark matter was abundant and active in the infant universe. 553 00:30:30,461 --> 00:30:34,798 So could neutrinos and dark matter be the same thing? 554 00:30:36,334 --> 00:30:38,335 PLAIT: We don't know what dark matter is, 555 00:30:38,436 --> 00:30:40,637 but we kind of know how it behaves. 556 00:30:40,738 --> 00:30:43,607 And neutrinos sound like a pretty good candidate for it 557 00:30:43,708 --> 00:30:45,576 because, hey, they are dark. 558 00:30:45,677 --> 00:30:47,010 They are everywhere in the universe, 559 00:30:47,111 --> 00:30:49,346 and they do have a little bit of mess. 560 00:30:50,548 --> 00:30:54,651 ROWE: And by little, we do mean little -- neutrinos 561 00:30:54,752 --> 00:30:58,155 weigh around 10 billion, billion, billion 562 00:30:58,256 --> 00:31:02,192 times less than a grain of sand. 563 00:31:02,293 --> 00:31:06,029 But neutrinos are also exquisitely abundant, and so 564 00:31:06,130 --> 00:31:08,565 because they're so abundant, 565 00:31:08,666 --> 00:31:12,970 their individual tiny mass can actually add up to a large 566 00:31:13,071 --> 00:31:16,473 diffuse mass on very large scales. 567 00:31:22,981 --> 00:31:24,014 ROWE: To investigate 568 00:31:24,115 --> 00:31:26,516 if neutrinos and dark matter are the same thing, 569 00:31:26,618 --> 00:31:28,185 we must return to the Big Bang. 570 00:31:30,855 --> 00:31:34,892 As the universe expands and cools, primitive matter forms, 571 00:31:34,993 --> 00:31:39,162 including dark matter and trillions of neutrinos. 572 00:31:39,264 --> 00:31:43,166 The dark matter clumps together, forming regions of 573 00:31:43,268 --> 00:31:47,437 higher gravity, which pulls in regular matter. 574 00:31:47,572 --> 00:31:51,842 THALLER: It formed a structure, a scaffolding, that allowed 575 00:31:51,943 --> 00:31:54,244 regular matter to gravitationally begin to come 576 00:31:54,345 --> 00:31:56,947 together and collapse into galaxies, 577 00:31:57,048 --> 00:31:58,548 stars, and planets. 578 00:31:58,650 --> 00:32:01,985 ROWE: Could the combined mass of neutrinos in the early 579 00:32:02,086 --> 00:32:05,489 cosmos have produced the extra gravity to help 580 00:32:05,590 --> 00:32:07,024 structures form? 581 00:32:09,193 --> 00:32:11,995 Could it be possible that this really is dark matter? 582 00:32:12,063 --> 00:32:13,297 These tiny little particles, 583 00:32:13,398 --> 00:32:15,165 but in abundance across the universe. 584 00:32:15,266 --> 00:32:18,101 And we know more -- 585 00:32:18,202 --> 00:32:21,038 Not all -- we know more about neutrinos than we do 586 00:32:21,139 --> 00:32:22,572 about dark matter, 587 00:32:22,674 --> 00:32:28,111 but there's still a question around whether or not neutrinos 588 00:32:28,212 --> 00:32:31,381 can be a specific type of dark matter. 589 00:32:33,351 --> 00:32:36,286 ROWE: To answer this question, we have to work out what 590 00:32:36,387 --> 00:32:41,491 specific type of dark matter was around in the Big Bang -- 591 00:32:41,592 --> 00:32:43,894 Hot or cold. 592 00:32:43,995 --> 00:32:46,530 THALLER: People talk about hot dark matter 593 00:32:46,631 --> 00:32:47,965 and cold dark matter. 594 00:32:48,066 --> 00:32:49,766 And really, what you're saying is 595 00:32:49,867 --> 00:32:51,702 the speed of the particles themselves. 596 00:32:51,803 --> 00:32:53,737 The cold dark matter is moving slowly, 597 00:32:53,838 --> 00:32:56,340 and the hot dark matter is moving fast. 598 00:32:56,441 --> 00:33:00,410 ROWE: This speed difference is an important clue 599 00:33:00,511 --> 00:33:03,046 to whether neutrinos make up dark matter. 600 00:33:05,016 --> 00:33:06,550 With hot and cold dark matter, 601 00:33:06,651 --> 00:33:08,685 the way they interact with regular matter has 602 00:33:08,786 --> 00:33:10,554 a lot to do with how fast they're going. 603 00:33:10,655 --> 00:33:12,889 So it's a good analogy to think about a river. 604 00:33:12,991 --> 00:33:15,525 With hot dark matter, you'd have a torrent. 605 00:33:15,626 --> 00:33:17,294 Basically, it's going so fast, 606 00:33:17,395 --> 00:33:18,929 it doesn't actually connect with anything. 607 00:33:19,030 --> 00:33:20,397 It just goes right on past. 608 00:33:20,498 --> 00:33:22,699 So there's no chance to form that larger structure. 609 00:33:24,369 --> 00:33:26,970 If you have relatively slow-moving dark matter, 610 00:33:27,071 --> 00:33:29,740 cold dark matter, think about a slow-moving river. 611 00:33:29,841 --> 00:33:32,275 A slow-moving river begins to deposit silt. 612 00:33:34,078 --> 00:33:35,946 ROWE: Think of that silt as the billions 613 00:33:36,047 --> 00:33:39,483 of galaxies that make up the cosmic web. 614 00:33:39,584 --> 00:33:42,019 BULLOCK: We observed that galaxies formed very early in 615 00:33:42,120 --> 00:33:43,587 the universe, and this is good 616 00:33:43,688 --> 00:33:45,622 for cold dark matter, but it doesn't work for 617 00:33:45,723 --> 00:33:46,656 hot dark matter. 618 00:33:46,758 --> 00:33:47,858 So we think cold dark matter is 619 00:33:47,959 --> 00:33:49,926 really dominating structure formation 620 00:33:50,028 --> 00:33:52,262 in the early universe. 621 00:33:52,363 --> 00:33:56,533 ROWE: But cold and slow does not describe neutrinos. 622 00:33:56,634 --> 00:33:59,736 They move very fast, close to the speed of light. 623 00:33:59,837 --> 00:34:02,773 This is a problem with neutrinos, 624 00:34:02,874 --> 00:34:04,975 because neutrinos would be hot dark matter. 625 00:34:05,076 --> 00:34:09,112 ROWE: That rules out neutrinos as cold dark matter. 626 00:34:11,416 --> 00:34:13,683 The idea that neutrinos are dark matter 627 00:34:13,785 --> 00:34:17,020 hit another setback when we weighed the universe. 628 00:34:19,490 --> 00:34:22,225 If you add up the total mass of all the neutrinos in 629 00:34:22,326 --> 00:34:24,127 the universe, it would wind up 630 00:34:24,228 --> 00:34:27,664 being about a half a percent to 1.5% of 631 00:34:27,765 --> 00:34:29,666 the total mass of dark matter. 632 00:34:31,402 --> 00:34:33,170 TEGMARK: Neutrinos were a good candidates for 633 00:34:33,271 --> 00:34:36,373 dark matter because they exist, 634 00:34:36,474 --> 00:34:38,875 and they're very shy, just like the dark matter 635 00:34:38,976 --> 00:34:40,677 particles are. 636 00:34:40,778 --> 00:34:43,780 But then we were able to measure more accurately how 637 00:34:43,881 --> 00:34:47,484 much dark matter there is and how much neutrinos there are, 638 00:34:47,585 --> 00:34:49,853 and there's just way less neutrinos 639 00:34:49,954 --> 00:34:51,088 than there is dark matter. 640 00:34:52,757 --> 00:34:54,658 ROWE: It sounds like game over, 641 00:34:54,759 --> 00:34:57,961 but the neutrino hunters aren't giving up. 642 00:34:58,062 --> 00:35:01,331 The search is on for a mysterious new kind 643 00:35:01,432 --> 00:35:02,599 of neutrino, 644 00:35:02,700 --> 00:35:05,435 one that could solve the riddle 645 00:35:05,536 --> 00:35:06,937 of dark matter. 646 00:35:17,982 --> 00:35:20,550 ROWE: Neutrinos played a huge role in shaping 647 00:35:20,651 --> 00:35:22,352 the early universe. 648 00:35:24,322 --> 00:35:28,725 They helped matter defeat antimatter, 649 00:35:28,826 --> 00:35:31,128 and the cosmos develop structure. 650 00:35:32,597 --> 00:35:36,566 This led us to wonder if neutrinos might be dark matter. 651 00:35:38,970 --> 00:35:41,438 But when we weighed the universe, 652 00:35:41,539 --> 00:35:43,240 the numbers didn't add up. 653 00:35:43,341 --> 00:35:46,977 Neutrinos do have mass, and there are a lot of them 654 00:35:47,111 --> 00:35:49,079 out there, so it might be some tiny, 655 00:35:49,180 --> 00:35:52,048 tiny fraction of dark matter is made up of neutrinos. 656 00:35:52,150 --> 00:35:54,084 But we know that these things do 657 00:35:54,185 --> 00:35:55,919 not make up the bulk of dark matter. 658 00:35:56,020 --> 00:35:58,155 It must be something else. 659 00:35:58,256 --> 00:36:01,424 ROWE: So neutrino scientists hunt for a different 660 00:36:01,526 --> 00:36:03,293 contender for dark matter, 661 00:36:03,361 --> 00:36:06,830 a completely new kind of neutrino. 662 00:36:06,931 --> 00:36:10,567 We know about three flavors of neutrinos -- 663 00:36:10,668 --> 00:36:13,103 The electron neutrino, 664 00:36:13,204 --> 00:36:15,972 the muon neutrino, and the tau neutrino. 665 00:36:16,073 --> 00:36:19,943 But there could be a hidden fourth flavor of 666 00:36:20,044 --> 00:36:23,813 neutrino that could solve the riddle of dark matter. 667 00:36:27,685 --> 00:36:30,787 We call this a sterile neutrino. 668 00:36:30,888 --> 00:36:32,923 ROWE: So-called because they interact 669 00:36:33,024 --> 00:36:36,593 even less than regular neutrinos. 670 00:36:36,694 --> 00:36:41,031 A particle so tiny, so hard to detect could actually turn out 671 00:36:41,132 --> 00:36:43,567 to have lots of the secrets wrapped up inside it 672 00:36:43,668 --> 00:36:45,168 as to how the universe works. 673 00:36:47,838 --> 00:36:49,739 ROWE: The first step to find out if 674 00:36:49,840 --> 00:36:52,342 sterile neutrinos are dark matter 675 00:36:52,443 --> 00:36:54,544 is to prove they exist, 676 00:36:54,645 --> 00:36:56,479 and that's tough. 677 00:36:56,581 --> 00:37:00,417 Even though sterile neutrinos are almost impossible 678 00:37:00,518 --> 00:37:03,486 to detect, we can still hunt for them. 679 00:37:03,588 --> 00:37:06,156 Back in the day, neutrinos were also said 680 00:37:06,257 --> 00:37:09,526 to be difficult to detect. 681 00:37:09,627 --> 00:37:11,995 THALLER: Trying to find dark matter, trying to find 682 00:37:12,096 --> 00:37:13,330 these sterile neutrinos, 683 00:37:13,431 --> 00:37:15,632 it's almost like using one invisible, 684 00:37:15,733 --> 00:37:18,301 undetectable thing to find another, using a ghost 685 00:37:18,402 --> 00:37:19,970 to find a goblin. 686 00:37:20,071 --> 00:37:23,473 ESQUIVE: We are definitely pushing the limits of science. 687 00:37:23,574 --> 00:37:28,378 ROWE: A team at Fermilab has an ingenious idea. 688 00:37:28,479 --> 00:37:33,049 They can't spot sterile neutrinos directly, because 689 00:37:33,150 --> 00:37:36,586 they don't interact with atoms in the detectors. 690 00:37:36,687 --> 00:37:38,688 So they're looking for neutrinos as 691 00:37:38,789 --> 00:37:42,492 they change flavor into sterile neutrinos. 692 00:37:42,593 --> 00:37:46,162 We know that normally, neutrinos change type as they 693 00:37:46,264 --> 00:37:47,464 move through space, 694 00:37:47,565 --> 00:37:50,400 but they have to move far enough before that change happens. 695 00:37:52,436 --> 00:37:54,537 ROWE: So tracking neutrinos over a short 696 00:37:54,639 --> 00:37:57,274 distance shouldn't show any flavor changing. 697 00:37:59,443 --> 00:38:01,011 BULLOCK: In this experiment, they've constructed 698 00:38:01,112 --> 00:38:03,246 only a half-mile-long path. 699 00:38:03,347 --> 00:38:04,814 It's not enough time from the neutrinos 700 00:38:04,915 --> 00:38:06,916 to change flavor in the normal way. 701 00:38:07,018 --> 00:38:10,520 If they do see something, if they see something change, 702 00:38:10,621 --> 00:38:13,790 this could be some interesting aspect, perhaps evidence 703 00:38:13,891 --> 00:38:15,091 for sterile neutrinos. 704 00:38:15,192 --> 00:38:17,627 So is it possible that, over short distances, 705 00:38:17,728 --> 00:38:19,529 regular neutrinos can oscillate into this 706 00:38:19,630 --> 00:38:20,730 sterile neutrino? 707 00:38:26,370 --> 00:38:28,838 ROWE: The team shoots beams of muon flavor 708 00:38:28,939 --> 00:38:31,007 neutrinos along the detector. 709 00:38:33,644 --> 00:38:36,313 In theory, they won't have time to change flavor. 710 00:38:40,985 --> 00:38:44,054 We can see whether or not these muon neutrinos 711 00:38:44,155 --> 00:38:48,491 morphed into a different type of neutrino. 712 00:38:48,592 --> 00:38:51,361 They shouldn't change, but if they do, 713 00:38:51,462 --> 00:38:54,497 that points us towards sterile neutrinos. 714 00:38:56,901 --> 00:38:58,335 ROWE: The team compare the number 715 00:38:58,436 --> 00:39:01,871 of muon neutrinos reaching the detectors 716 00:39:01,972 --> 00:39:04,908 to those fired along the beam. 717 00:39:05,009 --> 00:39:09,512 Fewer muon neutrinos hit the detectors. 718 00:39:09,613 --> 00:39:12,716 Some neutrinos had changed flavor. 719 00:39:14,719 --> 00:39:17,787 So we are seeing that oscillation of 720 00:39:17,888 --> 00:39:21,224 neutrinos changing from one type to another. 721 00:39:21,325 --> 00:39:26,496 We had an idea of how many we should have seen, 722 00:39:26,597 --> 00:39:27,897 but we're seeing more, 723 00:39:27,998 --> 00:39:32,469 and that could be sterile neutrinos. 724 00:39:32,570 --> 00:39:35,205 ROWE: If sterile neutrinos do exist, 725 00:39:35,306 --> 00:39:38,975 would they be dark matter? 726 00:39:39,076 --> 00:39:40,944 Right now, we don't know the mass 727 00:39:41,045 --> 00:39:42,912 of the sterile neutrino, 728 00:39:43,013 --> 00:39:48,118 but if it's heavy enough, it could be a contender. 729 00:39:48,219 --> 00:39:53,123 If it exists, it's prevalent enough to account 730 00:39:53,224 --> 00:39:56,359 for all the dark matter in the universe. 731 00:39:56,460 --> 00:39:59,062 ROWE: Fermilab's results haven't been verified by 732 00:39:59,163 --> 00:40:01,464 other scientists. 733 00:40:01,565 --> 00:40:03,633 So it's too soon to say 734 00:40:03,734 --> 00:40:06,369 definitively that sterile neutrinos are real 735 00:40:08,038 --> 00:40:10,540 or that they make up dark matter. 736 00:40:10,641 --> 00:40:12,942 ESQUIVE: Dark matter is probably one of 737 00:40:13,043 --> 00:40:15,345 the biggest questions of our time. 738 00:40:15,446 --> 00:40:17,313 And the fact that Fermilab 739 00:40:17,415 --> 00:40:21,651 may be one of the places to answer that question, 740 00:40:21,752 --> 00:40:25,522 and the fact that I am working here is really fantastic, 741 00:40:25,623 --> 00:40:27,924 because we're attempting the impossible. 742 00:40:30,694 --> 00:40:34,164 ROWE: We have to wait to see if the impossible is possible. 743 00:40:37,301 --> 00:40:39,636 We know neutrinos have played a vital 744 00:40:39,737 --> 00:40:41,838 role in the history of our universe, 745 00:40:43,507 --> 00:40:47,877 and even now, they refresh it by powering supernovas. 746 00:40:51,115 --> 00:40:55,718 Without them, our sun, our world, 747 00:40:57,188 --> 00:41:00,089 and even our bodies would not have formed. 748 00:41:01,459 --> 00:41:05,295 Neutrinos are pesky little particles, super elusive, 749 00:41:05,396 --> 00:41:08,565 difficult to study, but when you can catch them, 750 00:41:08,666 --> 00:41:12,535 they offer secrets to the universe. 751 00:41:12,636 --> 00:41:15,839 TREMBLAY: A story of neutrinos has been really interesting. 752 00:41:15,940 --> 00:41:16,973 It's like reading a book, 753 00:41:17,074 --> 00:41:18,374 and you think you're on the last page, and then 754 00:41:18,476 --> 00:41:21,544 you turn it, and then suddenly there's 100 new pages. 755 00:41:21,645 --> 00:41:24,681 Neutrinos are teaching us that the universe is, 756 00:41:24,782 --> 00:41:27,650 in many ways, subtle and hard to figure out. 757 00:41:27,751 --> 00:41:29,786 And the more we learn about these things, 758 00:41:29,887 --> 00:41:32,222 the more we learn about the universe. 759 00:41:32,323 --> 00:41:35,592 Neutrinos are the universe's great escape artists, 760 00:41:35,693 --> 00:41:37,293 the Houdini of particles. 761 00:41:37,394 --> 00:41:39,429 In fact, they may have helped us to 762 00:41:39,530 --> 00:41:42,532 escape the Big Bang and end up existing. 763 00:41:42,633 --> 00:41:46,002 At the end of the day, they're what saves us. 764 00:41:46,103 --> 00:41:49,806 The more we understand these elusive particles, 765 00:41:49,907 --> 00:41:55,111 the more we can gain insight into how the universe works, 766 00:41:55,212 --> 00:41:57,580 so it's really cool.