1 00:00:21,752 --> 00:00:24,921 The dream of becoming a citizen of the cosmos 2 00:00:24,955 --> 00:00:29,191 was born here, more than two millennia ago, 3 00:00:29,226 --> 00:00:31,427 in the city of Alexandria, 4 00:00:31,461 --> 00:00:34,697 named after and conceived by its dead conqueror, 5 00:00:34,731 --> 00:00:37,366 Alexander the Great. 6 00:00:37,401 --> 00:00:39,402 The Ptolemys, the Greek kings 7 00:00:39,436 --> 00:00:42,405 who inherited the Egyptian portion of Alexander's empire, 8 00:00:42,439 --> 00:00:44,473 built this library 9 00:00:44,508 --> 00:00:47,476 and its associated research institution. 10 00:00:47,511 --> 00:00:49,111 Rarely, if ever, 11 00:00:49,146 --> 00:00:50,813 before or since, 12 00:00:50,847 --> 00:00:53,416 has there been a government that was willing to spend 13 00:00:53,450 --> 00:00:55,651 so much of its gross national product 14 00:00:55,686 --> 00:00:58,154 on the acquisition of knowledge. 15 00:00:58,188 --> 00:00:59,589 And it paid off. 16 00:00:59,623 --> 00:01:01,090 Big time. 17 00:01:01,124 --> 00:01:04,327 Every ship entering Alexandria's harbor 18 00:01:04,361 --> 00:01:07,229 was searched-- not for contraband, 19 00:01:07,264 --> 00:01:09,999 but for books that might be copied and stored here, 20 00:01:10,033 --> 00:01:13,836 in what was then the greatest library on Earth. 21 00:01:19,876 --> 00:01:23,412 Here, Eratosthenes, one of the chief librarians, 22 00:01:23,447 --> 00:01:26,582 accurately calculated the size of the Earth 23 00:01:26,617 --> 00:01:28,618 and invented geography. 24 00:01:30,220 --> 00:01:32,221 Pythagoras. 25 00:01:33,557 --> 00:01:36,092 Hypatia. 26 00:01:36,126 --> 00:01:38,895 Euclid. 27 00:01:42,532 --> 00:01:46,135 Euclid set forth the precepts of geometry 28 00:01:46,169 --> 00:01:50,172 in a textbook that remained in use for 2,300 years. 29 00:01:57,147 --> 00:01:59,115 The Old Testament Bible comes down to us 30 00:01:59,149 --> 00:02:01,517 mainly from the Greek translations made here. 31 00:02:01,551 --> 00:02:04,620 The original manuscripts of the masterpieces 32 00:02:04,655 --> 00:02:07,123 of Greek comedy and drama, poetry, 33 00:02:07,157 --> 00:02:09,525 science, engineering, 34 00:02:09,559 --> 00:02:12,295 medicine and history-- 35 00:02:12,329 --> 00:02:14,163 the total work product 36 00:02:14,197 --> 00:02:17,199 of the awakening of ancient civilization-- were kept here. 37 00:02:17,234 --> 00:02:20,636 Estimates vary on the total number of scrolls. 38 00:02:20,671 --> 00:02:23,806 They range from 500,000 39 00:02:23,840 --> 00:02:26,409 to nearly a million. 40 00:02:26,443 --> 00:02:28,377 And all of it... 41 00:02:28,412 --> 00:02:29,845 all of this 42 00:02:29,880 --> 00:02:32,148 is but a tiny fraction of the information 43 00:02:32,182 --> 00:02:36,219 that you have at your fingertips at this very moment. 44 00:02:38,522 --> 00:02:41,824 The collective knowledge of our species, 45 00:02:41,858 --> 00:02:45,094 our own electronic Library of Alexandria, 46 00:02:45,128 --> 00:02:48,197 may be accessed by anyone who has a device 47 00:02:48,232 --> 00:02:51,100 and the interest and the freedom to do so. 48 00:02:51,134 --> 00:02:53,569 This was not true in Alexandria, 49 00:02:53,604 --> 00:02:55,671 where knowledge belonged to the elite. 50 00:02:55,706 --> 00:02:58,341 So in the fourth century AD, 51 00:02:58,375 --> 00:03:01,177 when the mob came to destroy the library 52 00:03:01,211 --> 00:03:03,613 and the genius of classical civilization, 53 00:03:03,647 --> 00:03:07,083 there were not enough people to defend it. 54 00:03:07,117 --> 00:03:11,387 What will happen the next time the mob comes? 55 00:04:42,980 --> 00:04:46,401 Sync and corrections by n17t01 www.addic7ed.com 56 00:04:51,421 --> 00:04:53,723 We've come a long way together, 57 00:04:53,757 --> 00:04:57,026 traveling from deep inside the heart of an atom 58 00:04:57,060 --> 00:04:59,729 clear out to the cosmic horizon, 59 00:04:59,763 --> 00:05:03,566 and from the beginning of time to the distant future. 60 00:05:03,600 --> 00:05:06,669 I think we're ready to perform an experiment. 61 00:05:32,729 --> 00:05:35,998 It's not the kind of experiment that requires a laboratory. 62 00:05:36,033 --> 00:05:38,067 You can do it in your head. 63 00:05:38,101 --> 00:05:40,636 It's called a "thought experiment." 64 00:05:41,705 --> 00:05:43,539 Pick a star-- 65 00:05:43,574 --> 00:05:46,375 any one of the hundreds of billions of stars 66 00:05:46,410 --> 00:05:48,678 in our Milky Way Galaxy, 67 00:05:48,712 --> 00:05:50,513 which is just one galaxy 68 00:05:50,547 --> 00:05:53,749 out of the hundred billion in the known universe. 69 00:05:53,784 --> 00:05:55,384 How about that star? 70 00:05:55,419 --> 00:05:57,186 Or that one? 71 00:05:57,221 --> 00:05:59,288 Okay, this one. 72 00:05:59,323 --> 00:06:02,625 It's orbited by dozens of planets and moons. 73 00:06:02,659 --> 00:06:04,694 Suppose, on one of them, 74 00:06:04,728 --> 00:06:06,762 there lives an intelligent species, 75 00:06:06,797 --> 00:06:11,100 one the ten million life-forms on that planet, 76 00:06:11,134 --> 00:06:13,236 and there's a subgroup of that species 77 00:06:13,270 --> 00:06:16,038 who believe they have it all figured out-- 78 00:06:16,073 --> 00:06:19,208 their world is the center of the universe, 79 00:06:19,243 --> 00:06:21,911 a universe made for them, 80 00:06:21,945 --> 00:06:25,248 and that they know everything they need to know about it-- 81 00:06:25,282 --> 00:06:28,217 their knowledge is complete. 82 00:06:28,252 --> 00:06:31,654 How seriously would you take their claim? 83 00:06:36,827 --> 00:06:38,828 Our ancestors believed 84 00:06:38,862 --> 00:06:40,963 the universe was made for them. 85 00:06:40,998 --> 00:06:43,733 It was natural to assume that we were at the center. 86 00:06:43,767 --> 00:06:46,102 After all, it looks like the Sun and stars 87 00:06:46,136 --> 00:06:48,170 all revolve around us. 88 00:06:48,205 --> 00:06:51,240 We still speak of the Sun "rising." 89 00:06:51,275 --> 00:06:54,110 The architecture of our language, myths and dreams 90 00:06:54,144 --> 00:06:57,446 comes from that prescientific age. 91 00:06:57,481 --> 00:07:00,816 This is our planet as it was known then, 92 00:07:00,851 --> 00:07:03,486 just before Columbus set sail. 93 00:07:03,520 --> 00:07:06,255 This first globe of the Earth was cutting-edge 94 00:07:06,290 --> 00:07:09,458 when Martin Behaim made it in 1492. 95 00:07:09,493 --> 00:07:11,994 Like everyone else, he believed 96 00:07:12,029 --> 00:07:15,631 that the jigsaw puzzle of geography was complete. 97 00:07:15,666 --> 00:07:17,500 There were three continents... 98 00:07:17,534 --> 00:07:19,769 Europe, Africa and Asia, 99 00:07:19,803 --> 00:07:22,471 and only the great world ocean in between. 100 00:07:22,506 --> 00:07:27,343 Behaim had no clue that North and South America even existed. 101 00:07:29,246 --> 00:07:32,648 It's easy to feel smug, right? 102 00:07:32,683 --> 00:07:37,453 Well, the fact is Martin Behaim knew infinitely more 103 00:07:37,487 --> 00:07:39,555 about his world, the Earth, 104 00:07:39,590 --> 00:07:43,726 than we know about ours, the universe. 105 00:07:43,760 --> 00:07:45,795 A recent lesson in humility 106 00:07:45,829 --> 00:07:48,998 will serve to illustrate. 107 00:07:49,032 --> 00:07:53,669 In 1912, Victor Hess made a series of voyages 108 00:07:53,704 --> 00:07:55,538 into the sky above Austria, 109 00:07:55,572 --> 00:07:58,708 and found the thing that scientists love best... 110 00:08:00,377 --> 00:08:03,012 a mystery that defied understanding 111 00:08:03,046 --> 00:08:05,948 in terms of conventional scientific wisdom. 112 00:08:07,417 --> 00:08:09,218 And even today, 113 00:08:09,253 --> 00:08:12,521 a century later, we are still searching 114 00:08:12,556 --> 00:08:15,458 for a complete explanation of what Hess found. 115 00:08:17,261 --> 00:08:20,329 A new kind of energy had recently been discovered, 116 00:08:20,364 --> 00:08:21,731 radioactivity. 117 00:08:21,765 --> 00:08:23,199 It was given off 118 00:08:23,233 --> 00:08:25,334 by certain elements, like radium. 119 00:08:25,369 --> 00:08:28,037 But it was also found in the air, 120 00:08:28,071 --> 00:08:30,840 even far away from radioactive rocks. 121 00:08:30,874 --> 00:08:33,509 It was everywhere. 122 00:08:33,543 --> 00:08:36,078 Where did this strange energy come from? 123 00:08:36,113 --> 00:08:38,414 No one knew. 124 00:08:38,448 --> 00:08:42,084 Hess suspected that it might come from above the Earth. 125 00:08:42,119 --> 00:08:45,054 To test his hypothesis, he carried radiation detectors 126 00:08:45,088 --> 00:08:46,722 high into the sky. 127 00:08:46,757 --> 00:08:50,393 During a risky ascent in a hydrogen balloon, 128 00:08:50,427 --> 00:08:53,396 he attained an altitude of more than three miles. 129 00:08:55,232 --> 00:08:57,133 When he reached the thin, cold, 130 00:08:57,167 --> 00:08:59,168 upper half of the atmosphere... 131 00:09:09,580 --> 00:09:11,280 he found that the radiation 132 00:09:11,315 --> 00:09:14,450 was more than twice as strong as on the ground. 133 00:09:14,484 --> 00:09:17,987 The radiation must be coming from above. 134 00:09:18,021 --> 00:09:21,123 That's why its intensity was weaker on the ground-- 135 00:09:21,158 --> 00:09:23,926 the Earth's atmosphere was absorbing most of it. 136 00:09:23,961 --> 00:09:28,397 Some thought that the radiation might come from the Sun. 137 00:09:28,432 --> 00:09:32,068 To test that idea, Hess timed one of his ascents 138 00:09:32,102 --> 00:09:34,303 to coincide with a solar eclipse. 139 00:09:41,812 --> 00:09:44,780 But the eclipse had no effect on the radiation. 140 00:09:44,815 --> 00:09:47,950 Hess also found that the radiation 141 00:09:47,985 --> 00:09:51,420 was just as strong at night as in daylight. 142 00:09:51,455 --> 00:09:54,090 It was coming from above, but not from the Sun. 143 00:09:54,124 --> 00:09:56,959 What Hess did not know 144 00:09:56,994 --> 00:10:00,129 was that the solar wind doesn't move that quickly. 145 00:10:00,163 --> 00:10:02,431 And so, for the wrong reason, 146 00:10:02,466 --> 00:10:05,635 he came to the right conclusion. 147 00:10:05,669 --> 00:10:08,704 Hess had discovered cosmic rays-- 148 00:10:08,739 --> 00:10:12,541 showers of subatomic particles that crisscross the universe 149 00:10:12,576 --> 00:10:14,944 at literally the speed of light. 150 00:10:14,978 --> 00:10:17,546 Without the shielding effect of the Earth's atmosphere, 151 00:10:17,581 --> 00:10:19,548 they would be lethal. 152 00:10:19,583 --> 00:10:22,318 Some cosmic rays can carry as much energy 153 00:10:22,352 --> 00:10:24,687 as a bullet fired from a rifle. 154 00:10:24,721 --> 00:10:27,823 It would take decades to trace those cosmic rays 155 00:10:27,858 --> 00:10:31,994 back to a death of unimaginable violence. 156 00:10:45,082 --> 00:10:47,083 The cosmic rays that Victor Hess detected 157 00:10:47,117 --> 00:10:50,920 in the skies above Austria posed a mystery to scientists. 158 00:10:52,322 --> 00:10:54,590 Radioactivity in minerals on Earth-- 159 00:10:54,625 --> 00:10:56,959 like uranium ore-- 160 00:10:56,994 --> 00:10:59,495 comes from the disintegration of atoms. 161 00:10:59,529 --> 00:11:02,665 But cosmic rays were of a different nature. 162 00:11:02,699 --> 00:11:05,134 They were far more powerful 163 00:11:05,168 --> 00:11:07,637 than anything known in Hess's world. 164 00:11:07,671 --> 00:11:09,805 Scientists wondered for two decades 165 00:11:09,840 --> 00:11:12,475 what could possibly produce cosmic rays. 166 00:11:12,509 --> 00:11:16,078 Enter Fritz Zwicky, 167 00:11:16,113 --> 00:11:20,182 the most brilliant man you've never heard of. 168 00:11:20,217 --> 00:11:23,252 In 1933, he and a colleague discovered 169 00:11:23,287 --> 00:11:26,022 that some stars flare up to become as bright 170 00:11:26,056 --> 00:11:28,257 as their entire galaxy for a few weeks, 171 00:11:28,292 --> 00:11:30,693 before fading out again. 172 00:11:30,727 --> 00:11:32,194 Fritz Zwicky was the first person 173 00:11:32,229 --> 00:11:34,764 to understand what just happened. 174 00:11:34,798 --> 00:11:37,767 He correctly surmised that this is the way 175 00:11:37,801 --> 00:11:39,969 a massive star dies-- it blows 176 00:11:40,003 --> 00:11:42,038 its guts out into space. 177 00:11:42,072 --> 00:11:45,841 He called this kind of stellar death a "supernova"... 178 00:11:50,848 --> 00:11:53,516 and predicted that the dying star would shrink 179 00:11:53,550 --> 00:11:56,319 from about a million miles across to only ten. 180 00:11:57,088 --> 00:11:59,922 This corpse would be so dense 181 00:11:59,957 --> 00:12:02,425 that a single grain of it would weigh as much 182 00:12:02,459 --> 00:12:04,160 as the Great Pyramid in Egypt. 183 00:12:04,194 --> 00:12:06,262 It would consist almost entirely 184 00:12:06,296 --> 00:12:08,598 of subatomic particles called neutrons, 185 00:12:08,632 --> 00:12:11,968 so he named these bizarre objects "neutron stars." 186 00:12:12,002 --> 00:12:15,304 And 35 years after Zwicky predicted their existence, 187 00:12:15,339 --> 00:12:17,206 astronomers began to find them. 188 00:12:17,241 --> 00:12:19,142 We call them "pulsars" 189 00:12:19,176 --> 00:12:22,779 when they spin rapidly and emit regular pulses of radio energy. 190 00:12:22,813 --> 00:12:25,615 Supernovas and neutron stars could account 191 00:12:25,649 --> 00:12:27,784 for a wide range of cosmic rays, 192 00:12:27,818 --> 00:12:30,319 but not the most energetic ones. 193 00:12:30,354 --> 00:12:32,021 Nothing yet known to science 194 00:12:32,055 --> 00:12:34,624 can explain them, and we're fine with that. 195 00:12:34,658 --> 00:12:36,993 It's one of the things I love about science, 196 00:12:37,027 --> 00:12:39,629 we don't have to pretend we have all the answers. 197 00:12:39,663 --> 00:12:42,198 Zwicky also came up with the idea 198 00:12:42,232 --> 00:12:45,168 that the gravity of a galaxy warps the fabric 199 00:12:45,202 --> 00:12:47,303 of space around it, to act like a lens. 200 00:12:47,337 --> 00:12:49,572 This distorts and magnifies light 201 00:12:49,606 --> 00:12:52,008 from any other galaxy lying directly behind it. 202 00:12:52,042 --> 00:12:55,645 So, astronomers on Earth would see multiple images 203 00:12:55,679 --> 00:12:57,914 of that same distant galaxy, 204 00:12:57,948 --> 00:13:00,917 deformed, as in a funhouse mirror. 205 00:13:00,951 --> 00:13:06,622 40 years after this prediction, we started finding them, too. 206 00:13:08,392 --> 00:13:12,395 And Zwicky made yet another discovery back in the 1930s. 207 00:13:12,429 --> 00:13:15,932 While studying the Coma Cluster of galaxies, 208 00:13:15,966 --> 00:13:20,436 he noticed something funny about the way they moved. 209 00:13:20,471 --> 00:13:23,673 The galaxies were going way too fast, 210 00:13:23,707 --> 00:13:26,843 so fast that they should've been flying apart from each other, 211 00:13:26,877 --> 00:13:29,278 because all the stars in all those galaxies 212 00:13:29,313 --> 00:13:32,715 had far too little gravity to hold the cluster together. 213 00:13:32,750 --> 00:13:34,517 Zwicky thought 214 00:13:34,551 --> 00:13:37,120 that something else must be binding them to each other. 215 00:13:37,154 --> 00:13:39,956 That mysterious missing component 216 00:13:39,990 --> 00:13:43,259 would have to weigh something like 50 times as much 217 00:13:43,293 --> 00:13:44,894 as the stars themselves. 218 00:13:44,928 --> 00:13:48,197 But no one paid much attention to this wild notion. 219 00:13:48,232 --> 00:13:51,401 Just another one of Zwicky's crazy ideas. 220 00:13:51,435 --> 00:13:53,069 In our solar system, 221 00:13:53,103 --> 00:13:55,638 the innermost planet, Mercury, moves much faster 222 00:13:55,672 --> 00:13:57,640 than the outermost one, Neptune. 223 00:13:57,674 --> 00:13:59,475 And that makes sense, right? 224 00:13:59,510 --> 00:14:02,879 The harder you push or pull on something, the faster it goes. 225 00:14:02,913 --> 00:14:06,215 The Sun's gravity weakens with increasing distance, 226 00:14:06,250 --> 00:14:10,620 so, the planets that are farther from the Sun move more slowly. 227 00:14:10,654 --> 00:14:14,323 Everyone expected that the outermost stars in a galaxy 228 00:14:14,358 --> 00:14:16,993 would act the same way. 229 00:14:17,027 --> 00:14:20,496 Most of the stars are concentrated towards the center, 230 00:14:20,531 --> 00:14:23,800 so, their collective gravity pulls on the other stars 231 00:14:23,834 --> 00:14:27,003 the same way the Sun pulls on the planets. 232 00:14:27,037 --> 00:14:29,439 But in the 1970s, when astronomer 233 00:14:29,473 --> 00:14:31,841 Vera Rubin studied the Andromeda Galaxy, 234 00:14:31,875 --> 00:14:36,412 she discovered that the outer stars obeyed no such rule. 235 00:14:36,447 --> 00:14:39,015 Unlike the outer planets of the solar system, 236 00:14:39,049 --> 00:14:42,752 the outer stars in the galaxy were all going at the same speed 237 00:14:42,786 --> 00:14:45,088 as the stars that were closer in, 238 00:14:45,122 --> 00:14:47,757 and they were moving way faster than expected. 239 00:14:47,791 --> 00:14:50,159 "That's funny," Vera thought. 240 00:14:50,194 --> 00:14:54,297 "There must be something weird about the Andromeda Galaxy." 241 00:14:54,331 --> 00:14:55,598 So she looked at another galaxy. 242 00:14:55,632 --> 00:14:57,834 Same story. 243 00:14:57,868 --> 00:14:59,202 And another. 244 00:14:59,236 --> 00:15:01,704 Vera studied 60 galaxies 245 00:15:01,739 --> 00:15:03,706 and found that all of them 246 00:15:03,741 --> 00:15:05,441 seemed to be violating the Law of Gravity, 247 00:15:05,476 --> 00:15:07,443 a core principle of physics. 248 00:15:07,478 --> 00:15:10,813 After some initial healthy skepticism, 249 00:15:10,848 --> 00:15:12,849 her colleagues looked for themselves, 250 00:15:12,883 --> 00:15:16,152 and found that Vera was right. 251 00:15:16,186 --> 00:15:19,322 It's not that Isaac Newton had gotten the Law of Gravity wrong; 252 00:15:19,356 --> 00:15:21,891 Vera Rubin had discovered that the gravity 253 00:15:21,925 --> 00:15:23,993 of something massive and invisible 254 00:15:24,027 --> 00:15:28,231 was forcing the stars to go fast. 255 00:15:28,265 --> 00:15:33,736 And then, someone remembered crazy old Fritz Zwicky, 256 00:15:33,771 --> 00:15:36,706 and the unknown source of gravity 257 00:15:36,740 --> 00:15:38,241 in the galaxy clusters 258 00:15:38,275 --> 00:15:42,345 that he called "dark matter," back in 1933. 259 00:15:48,585 --> 00:15:50,820 Vera Rubin had verified the existence 260 00:15:50,854 --> 00:15:53,489 of a new, much larger cosmos. 261 00:15:53,524 --> 00:15:55,758 And just like the one we thought we knew, 262 00:15:55,793 --> 00:15:57,827 it was filled with mystery. 263 00:15:57,861 --> 00:16:01,330 Dark matter is completely unobservable, 264 00:16:01,365 --> 00:16:03,099 except for its gravitational effect, 265 00:16:03,133 --> 00:16:07,837 which makes visible stars and galaxies move faster. 266 00:16:07,871 --> 00:16:11,574 Its nature is another deep mystery. 267 00:16:11,608 --> 00:16:14,710 Rubin had provided the evidence for an invisible universe 268 00:16:14,745 --> 00:16:19,515 nearly ten times more massive than the one we thought we knew. 269 00:16:19,550 --> 00:16:21,584 It was as if we had been standing on the seashore 270 00:16:21,618 --> 00:16:23,686 at night, mistakenly believing 271 00:16:23,720 --> 00:16:26,956 that the froth on the waves was all there was to the ocean. 272 00:16:26,990 --> 00:16:30,593 Vera Rubin looked at the stars and realized 273 00:16:30,627 --> 00:16:33,196 they were merely the foam on the waves, 274 00:16:33,230 --> 00:16:38,367 and that the greatest part of the ocean remained unknown. 275 00:16:38,402 --> 00:16:39,702 But wait. 276 00:16:39,737 --> 00:16:41,738 It gets crazier. 277 00:16:46,813 --> 00:16:48,814 Our Milky Way Galaxy, 278 00:16:48,849 --> 00:16:50,950 a few hundred billion stars, 279 00:16:50,984 --> 00:16:53,452 plus the clouds of gas and dust, 280 00:16:53,487 --> 00:16:56,489 the stuff of once and future stars-- 281 00:16:56,523 --> 00:17:01,460 and about a hundred billion other galaxies-- all of that, 282 00:17:01,495 --> 00:17:04,730 including those uncounted billions of trillions 283 00:17:04,764 --> 00:17:07,800 of planets, moons, and comets-- 284 00:17:07,834 --> 00:17:11,904 amounts to only five percent of what is actually there. 285 00:17:11,938 --> 00:17:14,073 Because there's a bigger unsolved mystery 286 00:17:14,107 --> 00:17:16,108 than dark matter-- 287 00:17:16,142 --> 00:17:20,279 dark energy, which makes up even more of the cosmos 288 00:17:20,313 --> 00:17:22,414 and drives its expansion. 289 00:17:22,449 --> 00:17:25,117 And it was Fritz Zwicky's supernovas 290 00:17:25,152 --> 00:17:28,721 that lit the way to the revelation of its existence. 291 00:17:30,757 --> 00:17:35,328 In one scenario, a star consumes all of its nuclear fuel... 292 00:17:38,899 --> 00:17:40,366 then cools, 293 00:17:40,400 --> 00:17:43,102 and suddenly collapses under its own gravity. 294 00:17:43,137 --> 00:17:46,806 The star rebounds in a massive explosion, 295 00:17:46,840 --> 00:17:50,743 leaving behind a neutron star or a black hole. 296 00:17:52,346 --> 00:17:54,781 Since the mass of the original star 297 00:17:54,815 --> 00:17:56,582 can fall within a wide range, 298 00:17:56,617 --> 00:17:58,151 its peak brightness as a supernova 299 00:17:58,185 --> 00:17:59,986 can also vary widely. 300 00:18:00,020 --> 00:18:02,722 So, you can't tell how far away it is 301 00:18:02,756 --> 00:18:04,624 just from how bright it looks. 302 00:18:04,658 --> 00:18:08,127 A relatively nearby supernova might appear just as bright 303 00:18:08,162 --> 00:18:11,297 as one that was more powerful, but farther away. 304 00:18:11,331 --> 00:18:14,233 But there's another kind of supernova 305 00:18:14,268 --> 00:18:16,302 that comes in only one strength. 306 00:18:16,336 --> 00:18:18,638 It marks the violent grand finale 307 00:18:18,672 --> 00:18:23,309 of a tango danced by a giant star and a dwarf. 308 00:18:25,179 --> 00:18:27,980 As the two stars orbit closely around each other, 309 00:18:28,015 --> 00:18:30,450 the giant sheds its outer layers of gas 310 00:18:30,484 --> 00:18:32,452 onto the dwarf. 311 00:18:32,486 --> 00:18:34,954 When the added weight becomes too much 312 00:18:34,988 --> 00:18:36,789 for it to bear, 313 00:18:36,824 --> 00:18:41,360 the dwarf detonates like a stupendous thermonuclear bomb. 314 00:18:41,395 --> 00:18:43,463 For a few weeks, 315 00:18:43,497 --> 00:18:46,098 the brilliance of such a supernova rivals 316 00:18:46,133 --> 00:18:49,435 the combined light of all the stars in its galaxy. 317 00:18:49,470 --> 00:18:51,270 This kind of supernova 318 00:18:51,305 --> 00:18:53,806 always has the same maximum power output, 319 00:18:53,841 --> 00:18:57,610 about five billion times brighter than our Sun. 320 00:18:57,644 --> 00:18:59,378 With big telescopes, 321 00:18:59,413 --> 00:19:01,814 we can see them in galaxies very far away, 322 00:19:01,849 --> 00:19:05,351 out toward the edge of the observable universe. 323 00:19:14,094 --> 00:19:17,196 Because all such supernovas have the same wattage, 324 00:19:17,231 --> 00:19:20,166 they're ideal tools for measuring distances 325 00:19:20,200 --> 00:19:22,401 to the farthest reaches of the universe. 326 00:19:22,436 --> 00:19:25,338 We call them "standard candles." 327 00:19:30,010 --> 00:19:35,648 In 1929, Edwin Hubble discovered that the universe is expanding. 328 00:19:35,682 --> 00:19:38,918 The distant galaxies are drifting away from one another. 329 00:19:38,952 --> 00:19:40,586 Later, we learned 330 00:19:40,621 --> 00:19:43,556 that the expansion began some 14 billion years ago 331 00:19:43,590 --> 00:19:48,060 with the explosive birth of the universe-- the big bang. 332 00:19:49,730 --> 00:19:51,764 Everybody assumed 333 00:19:51,799 --> 00:19:54,167 that the rate of expansion would be slowing down, 334 00:19:54,201 --> 00:19:55,935 due to the mutual pull of gravity 335 00:19:55,969 --> 00:19:57,703 between all the parts of the universe. 336 00:19:57,738 --> 00:19:59,572 If there is enough dark matter, 337 00:19:59,606 --> 00:20:02,909 its gravity would eventually bring the expansion to a stop, 338 00:20:02,943 --> 00:20:05,745 and the universe would then fall back on itself. 339 00:20:05,779 --> 00:20:08,948 In that case, everything would eventually collapse 340 00:20:08,982 --> 00:20:10,750 in a big crunch. 341 00:20:10,784 --> 00:20:14,120 On the other hand, if the universe had less dark matter, 342 00:20:14,154 --> 00:20:16,222 the expansion would continue forever, 343 00:20:16,256 --> 00:20:19,125 just getting slower and slower. 344 00:20:19,159 --> 00:20:21,727 Two competing teams of astronomers 345 00:20:21,762 --> 00:20:25,031 were observing those supernovas in distant galaxies. 346 00:20:25,065 --> 00:20:26,699 It turned out to be another one 347 00:20:26,733 --> 00:20:29,135 of those "that's funny" moments. 348 00:20:29,169 --> 00:20:32,472 In 1998, both teams independently came 349 00:20:32,506 --> 00:20:34,140 to the same conclusion. 350 00:20:34,174 --> 00:20:36,542 The expansion isn't slowing down at all... 351 00:20:36,577 --> 00:20:38,311 it's speeding up. 352 00:20:38,345 --> 00:20:41,948 This means the universe will continue to expand forever. 353 00:20:41,982 --> 00:20:45,751 There seems to be a mysterious force in the universe, 354 00:20:45,786 --> 00:20:48,754 one that overwhelms gravity on the grandest scale 355 00:20:48,789 --> 00:20:51,324 to push the cosmos apart. 356 00:20:51,358 --> 00:20:54,060 Most of the energy of the universe is bound up 357 00:20:54,094 --> 00:20:55,561 in this unknown force. 358 00:20:55,596 --> 00:20:57,964 We call it "dark energy," 359 00:20:57,998 --> 00:21:00,666 but that name, like "dark matter," 360 00:21:00,701 --> 00:21:04,137 is merely a code word for our ignorance. 361 00:21:04,171 --> 00:21:06,506 It's okay not to know all the answers. 362 00:21:06,540 --> 00:21:08,508 It's better to admit our ignorance 363 00:21:08,542 --> 00:21:10,910 than to believe answers that might be wrong. 364 00:21:10,944 --> 00:21:14,413 Pretending to know everything closes the door 365 00:21:14,448 --> 00:21:16,449 to finding out what's really there. 366 00:21:17,484 --> 00:21:19,986 Tonight, our ships sail 367 00:21:20,020 --> 00:21:23,456 into even more exotic waters. 368 00:21:23,490 --> 00:21:25,491 Come with me. 369 00:21:35,877 --> 00:21:38,145 Only two of our ships have ventured 370 00:21:38,179 --> 00:21:41,648 into the great dark ocean of interstellar space. 371 00:21:41,682 --> 00:21:45,519 The longest odyssey in all of history 372 00:21:45,553 --> 00:21:48,822 was launched back in 1977-- 373 00:21:48,856 --> 00:21:52,559 NASA's Voyager 1 and 2. 374 00:21:52,593 --> 00:21:56,897 The Voyagers move about 40,000 miles an hour. 375 00:21:56,931 --> 00:21:59,733 They gave us our first close-up look 376 00:21:59,767 --> 00:22:01,801 at Jupiter's Great Red Spot, 377 00:22:01,836 --> 00:22:05,705 a hurricane three times the size of Earth 378 00:22:05,740 --> 00:22:09,676 and one that's been raging since at least 1644, 379 00:22:09,710 --> 00:22:11,411 when it was first observed. 380 00:22:11,445 --> 00:22:15,215 For all we know, it could be thousands of years old. 381 00:22:15,249 --> 00:22:18,552 The Voyagers discovered the first active volcano 382 00:22:18,586 --> 00:22:22,522 on another world, on Jupiter's moon Io... 383 00:22:24,091 --> 00:22:26,226 and an ocean beneath the icy surface 384 00:22:26,260 --> 00:22:27,861 of the moon Europa... 385 00:22:31,566 --> 00:22:35,101 with at least twice as much water as we have here on Earth. 386 00:22:38,206 --> 00:22:41,575 The Voyagers dared to fly across Saturn's rings 387 00:22:41,609 --> 00:22:45,946 and revealed that they were made of hundreds of thin bands 388 00:22:45,980 --> 00:22:47,781 of orbiting snowballs. 389 00:22:51,152 --> 00:22:53,787 On Saturn's giant moon Titan, 390 00:22:53,821 --> 00:22:57,958 Voyager detected an atmosphere four times denser than Earth's. 391 00:22:57,992 --> 00:23:02,729 That hinted at the existence of hydrocarbon seas on Titan, 392 00:23:02,763 --> 00:23:04,431 which we later confirmed. 393 00:23:06,400 --> 00:23:08,368 Voyager 2 gave us our first portrait 394 00:23:08,402 --> 00:23:12,205 of the outermost planet, Neptune... 395 00:23:12,240 --> 00:23:16,109 where the winds roar at 1,000 miles per hour... 396 00:23:17,945 --> 00:23:20,814 and its moon Triton, where geysers 397 00:23:20,848 --> 00:23:25,352 of boiling nitrogen shoot five miles high. 398 00:23:28,489 --> 00:23:31,291 Voyager successfully completed its mission of discovery 399 00:23:31,325 --> 00:23:33,126 to the outer planets, 400 00:23:33,161 --> 00:23:35,128 but its odyssey into the darkness 401 00:23:35,163 --> 00:23:37,164 was just beginning. 402 00:23:39,467 --> 00:23:41,168 35 years after its launch, 403 00:23:41,202 --> 00:23:43,837 Voyager 1 became the first of our spacecraft 404 00:23:43,871 --> 00:23:46,740 to enter an uncharted realm. 405 00:23:46,774 --> 00:23:49,976 The Sun is constantly shooting out streams 406 00:23:50,011 --> 00:23:52,312 of charged particles in all directions, 407 00:23:52,346 --> 00:23:54,648 moving at a million miles an hour. 408 00:23:54,682 --> 00:23:59,419 This solar wind blows a vast magnetic bubble, 409 00:23:59,453 --> 00:24:03,323 our heliosphere, that extends beyond the outer planets. 410 00:24:03,357 --> 00:24:08,695 It pushes out against the thin gas of interstellar space. 411 00:24:08,729 --> 00:24:13,300 There's a border where one ends and the other begins. 412 00:24:13,334 --> 00:24:15,468 Voyager 1 reported back to Earth 413 00:24:15,503 --> 00:24:18,004 that its detectors were being pummeled 414 00:24:18,039 --> 00:24:19,539 by more and more cosmic rays. 415 00:24:19,574 --> 00:24:21,975 Until then, we didn't know 416 00:24:22,009 --> 00:24:24,277 where the interstellar ocean began. 417 00:24:24,612 --> 00:24:26,846 Voyager 1 pressed on 418 00:24:26,881 --> 00:24:30,083 past a boundary we had never crossed before. 419 00:24:30,117 --> 00:24:33,953 The heliosphere shields us from most of the deadly cosmic rays. 420 00:24:33,988 --> 00:24:36,122 When stormy solar winds blow, 421 00:24:36,157 --> 00:24:38,358 this zone of protection grows; 422 00:24:38,392 --> 00:24:40,493 in calm solar weather, it shrinks. 423 00:24:40,528 --> 00:24:43,796 When a star goes supernova in our galactic neighborhood... 424 00:24:45,633 --> 00:24:47,867 the debris from the exploded star 425 00:24:47,902 --> 00:24:50,870 pushes the heliosphere back towards the Sun. 426 00:24:50,905 --> 00:24:52,505 If it's strong enough to push it 427 00:24:52,540 --> 00:24:54,440 all the way back to Earth's orbit, 428 00:24:54,475 --> 00:24:59,512 our planet gets a radioactive bath of supernova debris. 429 00:24:59,547 --> 00:25:02,348 Luckily, this doesn't happen very often. 430 00:25:02,383 --> 00:25:05,785 The last one was perhaps two million years ago. 431 00:25:05,820 --> 00:25:08,955 A neighboring star explodes a million years 432 00:25:08,989 --> 00:25:12,625 before there's even such a thing as the human species. 433 00:25:12,660 --> 00:25:15,128 How can we possibly know this? 434 00:25:15,162 --> 00:25:17,964 Because the dying star left its traces 435 00:25:17,998 --> 00:25:20,467 miles below the surface of the ocean. 436 00:25:20,501 --> 00:25:25,205 Manganese nodules, small rocks like this one, 437 00:25:25,239 --> 00:25:29,709 are scattered over much of the deep sea floor. 438 00:25:29,743 --> 00:25:32,545 They grow very slowly. 439 00:25:32,580 --> 00:25:37,317 I'm talking a millimeter in a million years, 440 00:25:37,351 --> 00:25:41,321 layer upon layer. 441 00:25:41,355 --> 00:25:44,390 These nodules grow in partnership with bacteria 442 00:25:44,425 --> 00:25:49,262 by taking up minerals dissolved in the seawater. 443 00:25:49,296 --> 00:25:53,266 A supernova produces a radioactive form of iron, 444 00:25:53,300 --> 00:25:57,337 unlike anything made by natural processes on Earth. 445 00:25:57,371 --> 00:25:59,739 Researchers found telltale traces of this iron 446 00:25:59,773 --> 00:26:01,741 in a thin layer below the surface 447 00:26:01,775 --> 00:26:03,743 of the manganese nodules. 448 00:26:03,777 --> 00:26:06,579 They used the known rate of growth of the nodules 449 00:26:06,614 --> 00:26:08,915 to date that layer and to connect it 450 00:26:08,949 --> 00:26:12,952 to the fate of a star that perished eons ago. 451 00:26:14,588 --> 00:26:17,457 The difference between seeing nothing but a pebble 452 00:26:17,491 --> 00:26:21,895 and reading the history of the cosmos inscribed inside it 453 00:26:21,929 --> 00:26:24,330 is science. 454 00:26:26,500 --> 00:26:30,470 The interstellar ocean is dark and deep. 455 00:26:30,504 --> 00:26:34,474 Out here, the Sun is just the brightest star in the sky. 456 00:26:34,508 --> 00:26:37,911 Yet the Voyagers maintain their regular communications 457 00:26:37,945 --> 00:26:40,079 with NASA's Jet Propulsion Laboratory, 458 00:26:40,114 --> 00:26:42,715 talking back and forth across the light-hours 459 00:26:42,750 --> 00:26:45,618 that separate these ships from their home port. 460 00:26:45,653 --> 00:26:49,455 No other objects touched by human hands 461 00:26:49,490 --> 00:26:52,826 have ever ventured this far from home. 462 00:26:59,133 --> 00:27:00,600 Even after they lose their ability 463 00:27:00,634 --> 00:27:02,669 to respond to our command, 464 00:27:02,703 --> 00:27:04,904 the last and, by far, the longest phase 465 00:27:04,939 --> 00:27:06,940 of the Voyager mission will begin. 466 00:27:16,450 --> 00:27:19,619 Back in 1979, when both Voyagers rounded Jupiter, 467 00:27:19,653 --> 00:27:22,054 its massive gravity acted as a slingshot, 468 00:27:22,089 --> 00:27:24,056 flinging them out of the solar system 469 00:27:24,091 --> 00:27:26,259 to travel among the stars of our galaxy 470 00:27:26,293 --> 00:27:28,728 for a billion years. 471 00:27:28,762 --> 00:27:31,397 Carl Sagan recognized that the Voyager mission 472 00:27:31,431 --> 00:27:35,468 offered two free tickets to something approaching eternity. 473 00:27:35,502 --> 00:27:38,471 He assembled a small team to create a message 474 00:27:38,505 --> 00:27:41,474 to any civilization that might, one day, 475 00:27:41,508 --> 00:27:44,844 encounter the derelict spacecraft. 476 00:27:48,115 --> 00:27:50,282 26 centuries ago, 477 00:27:50,317 --> 00:27:53,119 the Assyrian king Esarhaddon wrote... 478 00:27:53,153 --> 00:27:55,888 "I had monuments made of bronze 479 00:27:55,922 --> 00:27:58,557 and inscriptions of baked clay. 480 00:27:58,592 --> 00:28:01,961 I left them in the foundations for future times." 481 00:28:01,995 --> 00:28:06,298 These hieroglyphics continue that ancient tradition. 482 00:28:06,333 --> 00:28:09,301 They are inscribed on the cover of a message designed 483 00:28:09,336 --> 00:28:13,806 to be read by the beings of other worlds and times. 484 00:28:15,475 --> 00:28:17,009 What could we possibly have in common 485 00:28:17,043 --> 00:28:18,477 with an alien civilization 486 00:28:18,512 --> 00:28:21,180 with its own separate evolutionary history 487 00:28:21,214 --> 00:28:23,449 and one so far advanced beyond us 488 00:28:23,483 --> 00:28:27,686 that they can patrol interstellar space? 489 00:28:27,721 --> 00:28:29,255 One thing at least, 490 00:28:29,289 --> 00:28:31,023 a universal language... 491 00:28:31,057 --> 00:28:32,858 science. 492 00:28:32,893 --> 00:28:35,861 It's hard to break the bonds of gravity. 493 00:28:35,896 --> 00:28:38,130 You can only sail the cosmic seas 494 00:28:38,165 --> 00:28:41,500 if you speak mathematics and physics. 495 00:28:41,535 --> 00:28:44,603 Hydrogen is the most common element in the universe. 496 00:28:44,638 --> 00:28:46,605 The electron in a hydrogen atom 497 00:28:46,640 --> 00:28:48,441 flips the direction of its spin 498 00:28:48,475 --> 00:28:50,443 at a constant rate, or frequency. 499 00:28:50,477 --> 00:28:54,446 Hydrogen atoms are like tiny natural clocks-- 500 00:28:54,481 --> 00:28:57,283 tick... tock. 501 00:28:57,317 --> 00:28:59,485 Now we have a unit of time in common 502 00:28:59,519 --> 00:29:01,120 with the extraterrestrials. 503 00:29:01,154 --> 00:29:03,022 This will come in handy when we get to the next level 504 00:29:03,056 --> 00:29:04,623 of the message. 505 00:29:04,658 --> 00:29:07,226 Here's our return address in space. 506 00:29:07,260 --> 00:29:10,062 Pulsars are rapidly-spinning neutron stars 507 00:29:10,097 --> 00:29:13,199 that give off regular bursts of radio waves. 508 00:29:13,233 --> 00:29:15,901 You can set your clock by them. 509 00:29:15,936 --> 00:29:18,504 The Sun is at the center of this diagram, 510 00:29:18,538 --> 00:29:22,174 and the lines point to the 14 nearest pulsars. 511 00:29:22,209 --> 00:29:26,011 A simple code labels each pulsar with its unique frequency, 512 00:29:26,046 --> 00:29:28,180 using the ticktock of the hydrogen atom 513 00:29:28,215 --> 00:29:29,915 as the unit of time. 514 00:29:29,950 --> 00:29:32,751 So alien astronomers could use this diagram 515 00:29:32,786 --> 00:29:35,354 to locate the home star of the Voyager spacecraft 516 00:29:35,388 --> 00:29:36,989 in our galaxy. 517 00:29:37,023 --> 00:29:38,591 They could also tell 518 00:29:38,625 --> 00:29:40,659 how long ago the spacecraft was launched. 519 00:29:40,694 --> 00:29:42,862 And that's important because the Voyager record 520 00:29:42,896 --> 00:29:46,899 has a projected shelf life of 1,000 million years. 521 00:29:49,936 --> 00:29:51,537 Become an extraterrestrial archaeologist 522 00:29:51,571 --> 00:29:53,539 for a few moments. 523 00:29:53,573 --> 00:29:56,742 An artifact has been fished out of the interstellar ocean. 524 00:29:57,911 --> 00:29:59,411 It was made by beings 525 00:29:59,446 --> 00:30:01,580 that lived about a billion years ago. 526 00:30:01,615 --> 00:30:03,783 What would you make of them and their world? 527 00:30:04,985 --> 00:30:08,687 They've sent us their music 528 00:30:08,722 --> 00:30:12,792 and greetings in 59 human languages. 529 00:30:15,696 --> 00:30:18,864 Hello from the children of planet Earth. 530 00:30:18,866 --> 00:30:19,965 And one whale language. 531 00:30:21,969 --> 00:30:25,337 And a sound essay that includes a Saturn V rocket launch. 532 00:30:27,175 --> 00:30:30,009 A mother's first words to her newborn baby. 533 00:30:31,512 --> 00:30:32,812 Oh, come on now. 534 00:30:32,814 --> 00:30:34,980 Be a good boy... be a good boy. 535 00:30:34,982 --> 00:30:37,783 The brain waves of a young woman newly fallen in love. 536 00:30:40,788 --> 00:30:44,323 And the sound of a pulsar. 537 00:30:48,462 --> 00:30:53,466 All of that will live for a billion years. 538 00:31:05,846 --> 00:31:09,281 How long is a billion years? 539 00:31:18,246 --> 00:31:21,345 If you can press all the time since the Big Bang, 540 00:31:21,482 --> 00:31:23,450 the explosive birth of the universe, 541 00:31:23,484 --> 00:31:25,452 into a single Earth year, 542 00:31:25,486 --> 00:31:28,788 a billion years is about one month of that year. 543 00:31:28,823 --> 00:31:33,160 What was happening on Earth a billion years ago? 544 00:31:36,197 --> 00:31:39,199 Most of Earth's land was amassed into a supercontinent 545 00:31:39,233 --> 00:31:40,901 called Rodinia. 546 00:31:40,935 --> 00:31:44,571 It was a barren desert-- no animals, no plants. 547 00:31:44,605 --> 00:31:46,540 A billion years ago, there wasn't enough oxygen 548 00:31:46,574 --> 00:31:48,975 in our atmosphere to form an ozone layer, 549 00:31:49,010 --> 00:31:51,645 and without it, ultraviolet radiation 550 00:31:51,679 --> 00:31:54,381 prevented life from colonizing the land. 551 00:31:54,415 --> 00:31:56,917 Rodinia probably looked more like Mars 552 00:31:56,951 --> 00:31:59,653 than present-day Earth. 553 00:31:59,687 --> 00:32:02,389 The giant world ocean produced huge rainstorms 554 00:32:02,423 --> 00:32:04,925 causing flooding and erosion. 555 00:32:04,959 --> 00:32:08,061 Glaciers formed, and their slow but relentless movements 556 00:32:08,095 --> 00:32:11,264 carved the land into new shapes. 557 00:32:11,299 --> 00:32:14,668 Single-celled organisms dominated the oceans, 558 00:32:14,702 --> 00:32:18,271 but some existed in colonies called "microbial mats," 559 00:32:18,306 --> 00:32:23,009 and the first multicellular organisms would soon evolve. 560 00:32:23,044 --> 00:32:25,011 And a billion years from now, 561 00:32:25,046 --> 00:32:27,013 what will Earth be like... 562 00:32:27,048 --> 00:32:29,683 long after our cities, the Egyptian pyramids, 563 00:32:29,717 --> 00:32:32,853 the Rocky Mountains have all been eroded to dust? 564 00:32:32,887 --> 00:32:35,455 There are few things we can say with confidence 565 00:32:35,490 --> 00:32:38,492 about such a far distant time. 566 00:32:42,163 --> 00:32:44,631 The only thing we can say for sure 567 00:32:44,665 --> 00:32:48,468 is that Earth as we know it will be so changed 568 00:32:48,503 --> 00:32:51,805 that we would scarcely recognize it as home. 569 00:32:51,839 --> 00:32:55,375 But even a thousand million years from now, 570 00:32:55,409 --> 00:32:58,912 something of who we were and the music that we made 571 00:32:58,946 --> 00:33:01,214 in that long-ago spring... 572 00:33:01,249 --> 00:33:03,250 will live on. 573 00:33:08,122 --> 00:33:11,291 In that distant future, our Sun will have completed 574 00:33:11,325 --> 00:33:14,661 another four orbits around the center of the galaxy... 575 00:33:22,970 --> 00:33:27,474 ...and the Voyagers will have ventured far from the Sun. 576 00:33:30,812 --> 00:33:33,513 Carl Sagan was a member of Voyager's imaging team, 577 00:33:33,548 --> 00:33:37,751 and it was his idea that Voyager take one last picture. 578 00:33:37,785 --> 00:33:39,786 A generation before, 579 00:33:39,821 --> 00:33:42,656 an astronaut on the last Apollo flight to the Moon 580 00:33:42,690 --> 00:33:45,325 had taken a picture of the whole Earth-- 581 00:33:45,359 --> 00:33:48,528 the planet as a world without borders. 582 00:33:48,563 --> 00:33:51,932 It became an icon of a new consciousness. 583 00:33:51,966 --> 00:33:56,103 Carl realized the next step in this process. 584 00:33:56,137 --> 00:33:59,172 He convinced NASA to turn the Voyager 1 camera 585 00:33:59,207 --> 00:34:02,976 back towards Earth when the spacecraft went beyond Neptune 586 00:34:03,010 --> 00:34:06,646 for one last look homeward at what he called... 587 00:34:06,681 --> 00:34:09,683 the pale blue dot. 588 00:34:15,490 --> 00:34:17,057 That's here. 589 00:34:17,091 --> 00:34:18,625 That's home. 590 00:34:18,659 --> 00:34:20,227 That's us. 591 00:34:20,261 --> 00:34:22,963 On it, everyone you love, 592 00:34:22,997 --> 00:34:24,998 everyone you know, 593 00:34:25,032 --> 00:34:27,000 everyone you ever heard of, 594 00:34:27,034 --> 00:34:29,669 every human being who ever was, 595 00:34:29,704 --> 00:34:32,072 lived out their lives. 596 00:34:32,106 --> 00:34:35,842 The aggregate of our joy and suffering, 597 00:34:35,877 --> 00:34:38,678 thousands of confident religions, ideologies, 598 00:34:38,713 --> 00:34:40,680 and economic doctrines, 599 00:34:40,715 --> 00:34:42,849 every hunter and forager, 600 00:34:42,884 --> 00:34:44,851 every hero and coward, 601 00:34:44,886 --> 00:34:48,321 every creator and destroyer of civilization, 602 00:34:48,356 --> 00:34:50,323 every king and peasant, 603 00:34:50,358 --> 00:34:52,359 every young couple in love, 604 00:34:52,393 --> 00:34:55,662 every mother and father, hopeful child, 605 00:34:55,696 --> 00:34:57,931 inventor and explorer, 606 00:34:57,965 --> 00:35:01,601 every teacher of morals, every corrupt politician, 607 00:35:01,636 --> 00:35:05,705 every superstar, every supreme leader, 608 00:35:05,740 --> 00:35:09,576 every saint and sinner in the history of our species, 609 00:35:09,610 --> 00:35:11,578 lived there... 610 00:35:11,612 --> 00:35:13,413 on a mote of dust 611 00:35:13,448 --> 00:35:16,683 suspended... in a sunbeam. 612 00:35:16,717 --> 00:35:20,921 The Earth is a very small stage 613 00:35:20,955 --> 00:35:24,758 in a vast, cosmic arena. 614 00:35:24,792 --> 00:35:28,028 Think of the rivers of blood 615 00:35:28,062 --> 00:35:31,765 spilled by all those generals and emperors 616 00:35:31,799 --> 00:35:34,234 so that in glory and triumph 617 00:35:34,268 --> 00:35:37,204 they could become the momentary masters 618 00:35:37,238 --> 00:35:40,474 of a fraction... of a dot. 619 00:35:40,508 --> 00:35:43,610 Think of the endless cruelties visited 620 00:35:43,644 --> 00:35:47,147 by the inhabitants of one corner of this pixel 621 00:35:47,149 --> 00:35:50,050 on the scarcely distinguishable inhabitants 622 00:35:50,084 --> 00:35:52,152 of some other corner. 623 00:35:52,186 --> 00:35:55,222 How frequent their misunderstandings, 624 00:35:55,256 --> 00:35:58,225 how eager they are to kill one another, 625 00:35:58,259 --> 00:36:01,561 how fervent their hatreds. 626 00:36:01,596 --> 00:36:04,097 Our posturings, 627 00:36:04,132 --> 00:36:07,167 our imagined self-importance, 628 00:36:07,201 --> 00:36:09,770 the delusion that we have some privileged position 629 00:36:09,804 --> 00:36:11,405 in the universe, 630 00:36:11,439 --> 00:36:16,109 are challenged by this point of pale light. 631 00:36:16,144 --> 00:36:18,145 Our planet... 632 00:36:18,179 --> 00:36:21,181 is a lonely speck in the great, 633 00:36:21,215 --> 00:36:24,284 enveloping cosmic dark. 634 00:36:24,318 --> 00:36:27,120 In our obscurity, 635 00:36:27,155 --> 00:36:29,923 in all this vastness, 636 00:36:29,957 --> 00:36:33,660 there is no hint that help will come from elsewhere 637 00:36:33,694 --> 00:36:36,630 to save us from ourselves. 638 00:36:36,664 --> 00:36:40,867 The Earth is the only world known so far to harbor life. 639 00:36:40,902 --> 00:36:42,869 There is nowhere else, 640 00:36:42,904 --> 00:36:44,871 at least in the near future, 641 00:36:44,906 --> 00:36:47,707 to which our species could migrate. 642 00:36:47,742 --> 00:36:50,477 Visit, yes. 643 00:36:50,511 --> 00:36:52,713 Settle, not yet. 644 00:36:52,747 --> 00:36:54,781 Like it or not, 645 00:36:54,816 --> 00:36:59,352 for the moment, the Earth is where we make our stand. 646 00:36:59,387 --> 00:37:02,689 It has been said that astronomy is a humbling 647 00:37:02,724 --> 00:37:06,827 and character-building experience. 648 00:37:06,861 --> 00:37:09,329 There is perhaps no better demonstration 649 00:37:09,364 --> 00:37:12,199 of the folly of human conceits 650 00:37:12,233 --> 00:37:15,736 than this distant image. 651 00:37:15,770 --> 00:37:19,339 To me, it underscores our responsibility 652 00:37:19,374 --> 00:37:22,042 to deal more kindly with one another 653 00:37:22,076 --> 00:37:24,544 and to preserve and cherish 654 00:37:24,579 --> 00:37:26,713 the pale blue dot, 655 00:37:26,748 --> 00:37:29,883 the only home we've ever known. 656 00:37:36,791 --> 00:37:38,759 How did we, 657 00:37:38,793 --> 00:37:41,161 tiny creatures living on that speck of dust, 658 00:37:41,195 --> 00:37:44,498 ever manage to figure out how to send spacecraft 659 00:37:44,532 --> 00:37:47,334 out among the stars of the Milky Way? 660 00:37:47,368 --> 00:37:51,738 Only a few centuries ago, a mere second of cosmic time, 661 00:37:51,773 --> 00:37:54,374 we knew nothing of where or when we were. 662 00:37:54,409 --> 00:37:57,177 Oblivious to the rest of the cosmos, 663 00:37:57,211 --> 00:37:59,780 we inhabited a kind of prison-- 664 00:37:59,814 --> 00:38:01,782 a tiny universe 665 00:38:01,816 --> 00:38:05,318 bounded by a nutshell. 666 00:38:07,422 --> 00:38:11,692 How did we escape from the prison? 667 00:38:11,726 --> 00:38:15,629 It was the work of generations of searchers 668 00:38:15,663 --> 00:38:19,499 who took five simple rules to heart. 669 00:38:23,404 --> 00:38:25,372 Question authority. 670 00:38:25,406 --> 00:38:28,709 No idea is true just because someone says so, 671 00:38:28,743 --> 00:38:30,410 including me. 672 00:38:31,412 --> 00:38:33,246 Think for yourself. 673 00:38:36,684 --> 00:38:38,652 Question yourself. 674 00:38:41,022 --> 00:38:43,657 Don't believe anything just because you want to. 675 00:38:43,691 --> 00:38:48,462 Believing something doesn't make it so. 676 00:38:48,496 --> 00:38:50,464 Test ideas by the evidence gained 677 00:38:50,498 --> 00:38:54,167 from observation and experiment. 678 00:38:54,202 --> 00:38:57,571 If a favorite idea fails a well-designed test, 679 00:38:57,605 --> 00:38:59,773 it's wrong! 680 00:38:59,807 --> 00:39:01,408 Get over it. 681 00:39:01,442 --> 00:39:06,279 Follow the evidence, wherever it leads. 682 00:39:07,882 --> 00:39:11,284 If you have no evidence, reserve judgment. 683 00:39:11,319 --> 00:39:14,354 And perhaps the most important rule of all... 684 00:39:14,389 --> 00:39:16,123 Remember, you could be wrong. 685 00:39:16,157 --> 00:39:18,859 Even the best scientists 686 00:39:18,893 --> 00:39:20,794 have been wrong about some things. 687 00:39:20,828 --> 00:39:23,530 Newton, Einstein, 688 00:39:23,564 --> 00:39:26,299 and every other great scientist in history, 689 00:39:26,334 --> 00:39:28,802 they all made mistakes. 690 00:39:28,836 --> 00:39:32,005 Of course they did-- they were human. 691 00:39:32,040 --> 00:39:36,343 Science is a way to keep from fooling ourselves... 692 00:39:36,377 --> 00:39:39,146 and each other. 693 00:39:39,180 --> 00:39:41,515 Have scientists known sin? 694 00:39:44,686 --> 00:39:46,153 Of course. 695 00:39:46,187 --> 00:39:48,522 We have misused science, just as we have 696 00:39:48,556 --> 00:39:51,792 every other tool at our disposal, 697 00:39:51,826 --> 00:39:53,994 and that's why we can't afford 698 00:39:54,028 --> 00:39:57,965 to leave it in the hands of a powerful few. 699 00:39:57,999 --> 00:40:00,901 The more science belongs to all of us, 700 00:40:00,935 --> 00:40:04,004 the less likely it is to be misused. 701 00:40:06,240 --> 00:40:08,875 These values undermine the appeals 702 00:40:08,910 --> 00:40:11,178 of fanaticism and ignorance 703 00:40:11,212 --> 00:40:13,080 and, after all, 704 00:40:13,114 --> 00:40:15,549 the universe is mostly dark, 705 00:40:15,583 --> 00:40:18,819 dotted by islands of light. 706 00:40:18,853 --> 00:40:22,489 Learning the age of the Earth or the distance to the stars 707 00:40:22,523 --> 00:40:24,491 or how life evolves-- 708 00:40:24,525 --> 00:40:26,760 what difference does that make? 709 00:40:26,794 --> 00:40:29,496 Well, part of it depends on how big a universe 710 00:40:29,530 --> 00:40:31,398 you're willing to live in. 711 00:40:31,432 --> 00:40:33,400 Some of us like it small. 712 00:40:33,434 --> 00:40:35,035 That's fine. 713 00:40:35,069 --> 00:40:36,436 Understandable. 714 00:40:36,471 --> 00:40:38,171 But I like it big. 715 00:40:38,206 --> 00:40:41,341 And when I take all of this into my heart and my mind, 716 00:40:41,376 --> 00:40:43,277 I'm uplifted by it. 717 00:40:43,311 --> 00:40:47,080 And when I have that feeling, I want to know that it's real, 718 00:40:47,115 --> 00:40:50,584 that it's not just something happening inside my own head, 719 00:40:50,618 --> 00:40:52,753 because it matters what's true, 720 00:40:52,787 --> 00:40:55,222 and our imagination is nothing 721 00:40:55,256 --> 00:40:58,625 compared with Nature's awesome reality. 722 00:41:02,764 --> 00:41:06,767 I want to know what's in those dark places, 723 00:41:06,801 --> 00:41:10,504 and what happened before the Big Bang. 724 00:41:12,573 --> 00:41:15,409 I want to know what lies beyond the cosmic horizon, 725 00:41:15,443 --> 00:41:18,312 and how life began. 726 00:41:18,346 --> 00:41:20,814 Are there other places in the cosmos 727 00:41:20,848 --> 00:41:24,818 where matter and energy have become alive... 728 00:41:24,852 --> 00:41:27,287 and aware? 729 00:41:34,329 --> 00:41:36,497 I want to know my ancestors-- 730 00:41:36,531 --> 00:41:37,998 all of them. 731 00:41:38,033 --> 00:41:40,267 I want to be a good, strong link 732 00:41:40,302 --> 00:41:42,169 in the chain of generations. 733 00:41:42,203 --> 00:41:44,004 I want to protect my children 734 00:41:44,039 --> 00:41:47,174 and the children of ages to come. 735 00:41:47,208 --> 00:41:50,377 We, who embody the local eyes and ears 736 00:41:50,412 --> 00:41:53,113 and thoughts and feelings of the cosmos, 737 00:41:53,148 --> 00:41:56,383 we've begun to learn the story of our origins-- 738 00:41:56,418 --> 00:42:00,287 star stuff contemplating the evolution of matter, 739 00:42:00,322 --> 00:42:03,991 tracing that long path by which it arrived at consciousness. 740 00:42:04,025 --> 00:42:06,994 We and the other living things on this planet 741 00:42:07,028 --> 00:42:09,196 carry a legacy of cosmic evolution 742 00:42:09,230 --> 00:42:11,632 spanning billions of years. 743 00:42:11,666 --> 00:42:13,867 If we take that knowledge to heart, 744 00:42:13,902 --> 00:42:17,404 if we come to know and love nature as it really is, 745 00:42:17,439 --> 00:42:20,474 then we will surely be remembered by our descendants 746 00:42:20,508 --> 00:42:23,577 as good, strong links in the chain of life. 747 00:42:23,611 --> 00:42:26,914 And our children will continue this sacred searching, 748 00:42:26,948 --> 00:42:30,684 seeing for us as we have seen for those who came before, 749 00:42:30,719 --> 00:42:33,988 discovering wonders yet undreamt of... 750 00:42:34,022 --> 00:42:36,223 in the cosmos.