1 00:00:23,309 --> 00:00:26,478 The dream of becoming a citizen of the cosmos 2 00:00:26,512 --> 00:00:30,748 was born here, more than two millennia ago, 3 00:00:30,783 --> 00:00:32,984 in the city of Alexandria, 4 00:00:33,018 --> 00:00:36,254 named after and conceived by its dead conqueror, 5 00:00:36,288 --> 00:00:38,923 Alexander the Great. 6 00:00:38,958 --> 00:00:40,959 The Ptolemys, the Greek kings 7 00:00:40,993 --> 00:00:43,962 who inherited the Egyptian portion of Alexander's empire, 8 00:00:43,996 --> 00:00:46,030 built this library 9 00:00:46,065 --> 00:00:49,033 and its associated research institution. 10 00:00:48,768 --> 00:00:50,368 Rarely, if ever, 11 00:00:50,403 --> 00:00:52,070 before or since, 12 00:00:52,104 --> 00:00:54,673 has there been a government that was willing to spend 13 00:00:54,707 --> 00:00:56,908 so much of its gross national product 14 00:00:56,943 --> 00:00:59,411 on the acquisition of knowledge. 15 00:00:59,445 --> 00:01:00,846 And it paid off. 16 00:01:00,880 --> 00:01:02,347 Big time. 17 00:01:02,381 --> 00:01:05,584 Every ship entering Alexandria's harbor 18 00:01:05,618 --> 00:01:08,486 was searched-- not for contraband, 19 00:01:08,521 --> 00:01:11,256 but for books that might be copied and stored here, 20 00:01:11,290 --> 00:01:15,093 in what was then the greatest library on Earth. 21 00:01:21,133 --> 00:01:24,669 Here, Eratosthenes, one of the chief librarians, 22 00:01:24,704 --> 00:01:27,839 accurately calculated the size of the Earth 23 00:01:27,874 --> 00:01:29,875 and invented geography. 24 00:01:31,477 --> 00:01:33,478 Pythagoras. 25 00:01:34,814 --> 00:01:37,349 Hypatia. 26 00:01:37,383 --> 00:01:40,152 Euclid. 27 00:01:43,789 --> 00:01:47,392 Euclid set forth the precepts of geometry 28 00:01:47,426 --> 00:01:51,429 in a textbook that remained in use for 2,300 years. 29 00:01:58,404 --> 00:02:00,372 The Old Testament Bible comes down to us 30 00:02:00,406 --> 00:02:02,774 mainly from the Greek translations made here. 31 00:02:02,808 --> 00:02:05,877 The original manuscripts of the masterpieces 32 00:02:05,912 --> 00:02:08,380 of Greek comedy and drama, poetry, 33 00:02:08,414 --> 00:02:10,782 science, engineering, 34 00:02:10,816 --> 00:02:13,552 medicine and history-- 35 00:02:13,586 --> 00:02:15,420 the total work product 36 00:02:15,454 --> 00:02:18,456 of the awakening of ancient civilization-- were kept here. 37 00:02:18,491 --> 00:02:21,893 Estimates vary on the total number of scrolls. 38 00:02:21,928 --> 00:02:25,063 They range from 500,000 39 00:02:25,097 --> 00:02:27,666 to nearly a million. 40 00:02:27,700 --> 00:02:29,634 And all of it... 41 00:02:29,669 --> 00:02:31,102 all of this 42 00:02:31,137 --> 00:02:33,405 is but a tiny fraction of the information 43 00:02:33,439 --> 00:02:37,476 that you have at your fingertips at this very moment. 44 00:02:39,779 --> 00:02:43,081 The collective knowledge of our species, 45 00:02:43,115 --> 00:02:46,351 our own electronic Library of Alexandria, 46 00:02:46,385 --> 00:02:49,454 may be accessed by anyone who has a device 47 00:02:49,489 --> 00:02:52,357 and the interest and the freedom to do so. 48 00:02:52,391 --> 00:02:54,826 This was not true in Alexandria, 49 00:02:54,861 --> 00:02:56,928 where knowledge belonged to the elite. 50 00:02:56,963 --> 00:02:59,598 So in the fourth century AD, 51 00:02:59,632 --> 00:03:02,434 when the mob came to destroy the library 52 00:03:02,468 --> 00:03:04,870 and the genius of classical civilization, 53 00:03:04,904 --> 00:03:08,340 there were not enough people to defend it. 54 00:03:08,374 --> 00:03:12,644 What will happen the next time the mob comes? 55 00:04:47,301 --> 00:04:48,801 "Unafraid of the Dark" 56 00:04:52,678 --> 00:04:54,980 We've come a long way together, 57 00:04:55,014 --> 00:04:58,283 traveling from deep inside the heart of an atom 58 00:04:58,317 --> 00:05:00,986 clear out to the cosmic horizon, 59 00:05:01,020 --> 00:05:04,823 and from the beginning of time to the distant future. 60 00:05:04,857 --> 00:05:07,926 I think we're ready to perform an experiment. 61 00:05:33,986 --> 00:05:37,255 It's not the kind of experiment that requires a laboratory. 62 00:05:37,290 --> 00:05:39,324 You can do it in your head. 63 00:05:39,358 --> 00:05:41,893 It's called a "thought experiment." 64 00:05:42,962 --> 00:05:44,796 Pick a star-- 65 00:05:44,831 --> 00:05:47,632 any one of the hundreds of billions of stars 66 00:05:47,667 --> 00:05:49,935 in our Milky Way Galaxy, 67 00:05:49,969 --> 00:05:51,770 which is just one galaxy 68 00:05:51,804 --> 00:05:55,006 out of the hundred billion in the known universe. 69 00:05:55,041 --> 00:05:56,641 How about that star? 70 00:05:56,676 --> 00:05:58,443 Or that one? 71 00:05:58,478 --> 00:06:00,545 Okay, this one. 72 00:06:00,580 --> 00:06:03,882 It's orbited by dozens of planets and moons. 73 00:06:03,916 --> 00:06:05,951 Suppose, on one of them, 74 00:06:05,985 --> 00:06:08,019 there lives an intelligent species, 75 00:06:08,054 --> 00:06:12,357 one the ten million life-forms on that planet, 76 00:06:12,391 --> 00:06:14,493 and there's a subgroup of that species 77 00:06:14,527 --> 00:06:17,295 who believe they have it all figured out-- 78 00:06:17,330 --> 00:06:20,465 their world is the center of the universe, 79 00:06:20,500 --> 00:06:23,168 a universe made for them, 80 00:06:23,202 --> 00:06:26,505 and that they know everything they need to know about it-- 81 00:06:26,539 --> 00:06:29,474 their knowledge is complete. 82 00:06:29,509 --> 00:06:32,911 How seriously would you take their claim? 83 00:06:38,084 --> 00:06:40,085 Our ancestors believed 84 00:06:40,119 --> 00:06:42,220 the universe was made for them. 85 00:06:42,255 --> 00:06:44,990 It was natural to assume that we were at the center. 86 00:06:45,024 --> 00:06:47,359 After all, it looks like the Sun and stars 87 00:06:47,393 --> 00:06:49,427 all revolve around us. 88 00:06:49,462 --> 00:06:52,497 We still speak of the Sun "rising." 89 00:06:52,532 --> 00:06:55,367 The architecture of our language, myths and dreams 90 00:06:55,401 --> 00:06:58,703 comes from that prescientific age. 91 00:06:58,738 --> 00:07:02,073 This is our planet as it was known then, 92 00:07:02,108 --> 00:07:04,743 just before Columbus set sail. 93 00:07:04,777 --> 00:07:07,512 This first globe of the Earth was cutting-edge 94 00:07:07,547 --> 00:07:10,715 when Martin Behaim made it in 1492. 95 00:07:10,750 --> 00:07:13,251 Like everyone else, he believed 96 00:07:13,286 --> 00:07:16,888 that the jigsaw puzzle of geography was complete. 97 00:07:16,923 --> 00:07:18,757 There were three continents... 98 00:07:18,791 --> 00:07:21,026 Europe, Africa and Asia, 99 00:07:21,060 --> 00:07:23,728 and only the great world ocean in between. 100 00:07:23,763 --> 00:07:28,600 Behaim had no clue that North and South America even existed. 101 00:07:30,503 --> 00:07:33,905 It's easy to feel smug, right? 102 00:07:33,940 --> 00:07:38,710 Well, the fact is Martin Behaim knew infinitely more 103 00:07:38,744 --> 00:07:40,812 about his world, the Earth, 104 00:07:40,847 --> 00:07:44,983 than we know about ours, the universe. 105 00:07:45,017 --> 00:07:47,052 A recent lesson in humility 106 00:07:47,086 --> 00:07:50,255 will serve to illustrate. 107 00:07:50,289 --> 00:07:54,926 In 1912, Victor Hess made a series of voyages 108 00:07:54,961 --> 00:07:56,795 into the sky above Austria, 109 00:07:56,829 --> 00:07:59,965 and found the thing that scientists love best... 110 00:08:01,634 --> 00:08:04,269 a mystery that defied understanding 111 00:08:04,303 --> 00:08:07,205 in terms of conventional scientific wisdom. 112 00:08:08,674 --> 00:08:10,475 And even today, 113 00:08:10,510 --> 00:08:13,778 a century later, we are still searching 114 00:08:13,813 --> 00:08:16,715 for a complete explanation of what Hess found. 115 00:08:18,518 --> 00:08:21,586 A new kind of energy had recently been discovered, 116 00:08:21,621 --> 00:08:22,988 radioactivity. 117 00:08:23,022 --> 00:08:24,456 It was given off 118 00:08:24,490 --> 00:08:26,591 by certain elements, like radium. 119 00:08:26,626 --> 00:08:29,294 But it was also found in the air, 120 00:08:29,328 --> 00:08:32,097 even far away from radioactive rocks. 121 00:08:32,131 --> 00:08:34,766 It was everywhere. 122 00:08:34,800 --> 00:08:37,335 Where did this strange energy come from? 123 00:08:37,370 --> 00:08:39,671 No one knew. 124 00:08:39,705 --> 00:08:43,341 Hess suspected that it might come from above the Earth. 125 00:08:43,376 --> 00:08:46,311 To test his hypothesis, he carried radiation detectors 126 00:08:46,345 --> 00:08:47,979 high into the sky. 127 00:08:48,014 --> 00:08:51,650 During a risky ascent in a hydrogen balloon, 128 00:08:51,684 --> 00:08:54,653 he attained an altitude of more than three miles. 129 00:08:56,489 --> 00:08:58,390 When he reached the thin, cold, 130 00:08:58,424 --> 00:09:00,425 upper half of the atmosphere... 131 00:09:10,837 --> 00:09:12,537 he found that the radiation 132 00:09:12,572 --> 00:09:15,707 was more than twice as strong as on the ground. 133 00:09:15,741 --> 00:09:19,244 The radiation must be coming from above. 134 00:09:19,278 --> 00:09:22,380 That's why its intensity was weaker on the ground-- 135 00:09:22,415 --> 00:09:25,183 the Earth's atmosphere was absorbing most of it. 136 00:09:25,218 --> 00:09:29,654 Some thought that the radiation might come from the Sun. 137 00:09:29,689 --> 00:09:33,325 To test that idea, Hess timed one of his ascents 138 00:09:33,359 --> 00:09:35,560 to coincide with a solar eclipse. 139 00:09:43,069 --> 00:09:46,037 But the eclipse had no effect on the radiation. 140 00:09:46,072 --> 00:09:49,207 Hess also found that the radiation 141 00:09:49,242 --> 00:09:52,677 was just as strong at night as in daylight. 142 00:09:52,712 --> 00:09:55,347 It was coming from above, but not from the Sun. 143 00:09:55,381 --> 00:09:58,216 What Hess did not know 144 00:09:58,251 --> 00:10:01,386 was that the solar wind doesn't move that quickly. 145 00:10:01,420 --> 00:10:03,688 And so, for the wrong reason, 146 00:10:03,723 --> 00:10:06,892 he came to the right conclusion. 147 00:10:06,926 --> 00:10:09,961 Hess had discovered cosmic rays-- 148 00:10:09,996 --> 00:10:13,798 showers of subatomic particles that crisscross the universe 149 00:10:13,833 --> 00:10:16,201 at literally the speed of light. 150 00:10:16,235 --> 00:10:18,803 Without the shielding effect of the Earth's atmosphere, 151 00:10:18,838 --> 00:10:20,805 they would be lethal. 152 00:10:20,840 --> 00:10:23,575 Some cosmic rays can carry as much energy 153 00:10:23,609 --> 00:10:25,944 as a bullet fired from a rifle. 154 00:10:25,978 --> 00:10:29,080 It would take decades to trace those cosmic rays 155 00:10:29,115 --> 00:10:33,251 back to a death of unimaginable violence. 156 00:10:46,339 --> 00:10:48,340 The cosmic rays that Victor Hess detected 157 00:10:48,374 --> 00:10:52,177 in the skies above Austria posed a mystery to scientists. 158 00:10:53,579 --> 00:10:55,847 Radioactivity in minerals on Earth-- 159 00:10:55,882 --> 00:10:58,216 like uranium ore-- 160 00:10:58,251 --> 00:11:00,752 comes from the disintegration of atoms. 161 00:11:00,786 --> 00:11:03,922 But cosmic rays were of a different nature. 162 00:11:03,956 --> 00:11:06,391 They were far more powerful 163 00:11:06,425 --> 00:11:08,894 than anything known in Hess's world. 164 00:11:08,928 --> 00:11:11,062 Scientists wondered for two decades 165 00:11:11,097 --> 00:11:13,732 what could possibly produce cosmic rays. 166 00:11:13,766 --> 00:11:17,335 Enter Fritz Zwicky, 167 00:11:17,370 --> 00:11:21,439 the most brilliant man you've never heard of. 168 00:11:21,474 --> 00:11:24,509 In 1933, he and a colleague discovered 169 00:11:24,544 --> 00:11:27,279 that some stars flare up to become as bright 170 00:11:27,313 --> 00:11:29,514 as their entire galaxy for a few weeks, 171 00:11:29,549 --> 00:11:31,950 before fading out again. 172 00:11:31,984 --> 00:11:33,451 Fritz Zwicky was the first person 173 00:11:33,486 --> 00:11:36,021 to understand what just happened. 174 00:11:36,055 --> 00:11:39,024 He correctly surmised that this is the way 175 00:11:39,058 --> 00:11:41,226 a massive star dies-- it blows 176 00:11:41,260 --> 00:11:43,295 its guts out into space. 177 00:11:43,329 --> 00:11:47,098 He called this kind of stellar death a "supernova"... 178 00:11:52,105 --> 00:11:54,773 and predicted that the dying star would shrink 179 00:11:54,807 --> 00:11:57,576 from about a million miles across to only ten. 180 00:11:58,345 --> 00:12:01,179 This corpse would be so dense 181 00:12:01,214 --> 00:12:03,682 that a single grain of it would weigh as much 182 00:12:03,716 --> 00:12:05,417 as the Great Pyramid in Egypt. 183 00:12:05,451 --> 00:12:07,519 It would consist almost entirely 184 00:12:07,553 --> 00:12:09,855 of subatomic particles called neutrons, 185 00:12:09,889 --> 00:12:13,225 so he named these bizarre objects "neutron stars." 186 00:12:13,259 --> 00:12:16,561 And 35 years after Zwicky predicted their existence, 187 00:12:16,596 --> 00:12:18,463 astronomers began to find them. 188 00:12:18,498 --> 00:12:20,399 We call them "pulsars" 189 00:12:20,433 --> 00:12:24,036 when they spin rapidly and emit regular pulses of radio energy. 190 00:12:24,070 --> 00:12:26,872 Supernovas and neutron stars could account 191 00:12:26,906 --> 00:12:29,041 for a wide range of cosmic rays, 192 00:12:29,075 --> 00:12:31,576 but not the most energetic ones. 193 00:12:31,611 --> 00:12:33,278 Nothing yet known to science 194 00:12:33,312 --> 00:12:35,881 can explain them, and we're fine with that. 195 00:12:35,915 --> 00:12:38,250 It's one of the things I love about science, 196 00:12:38,284 --> 00:12:40,886 we don't have to pretend we have all the answers. 197 00:12:40,920 --> 00:12:43,455 Zwicky also came up with the idea 198 00:12:43,489 --> 00:12:46,425 that the gravity of a galaxy warps the fabric 199 00:12:46,459 --> 00:12:48,560 of space around it, to act like a lens. 200 00:12:48,594 --> 00:12:50,829 This distorts and magnifies light 201 00:12:50,863 --> 00:12:53,265 from any other galaxy lying directly behind it. 202 00:12:53,299 --> 00:12:56,902 So, astronomers on Earth would see multiple images 203 00:12:56,936 --> 00:12:59,171 of that same distant galaxy, 204 00:12:59,205 --> 00:13:02,174 deformed, as in a funhouse mirror. 205 00:13:02,208 --> 00:13:07,879 40 years after this prediction, we started finding them, too. 206 00:13:09,649 --> 00:13:13,652 And Zwicky made yet another discovery back in the 1930s. 207 00:13:13,686 --> 00:13:17,189 While studying the Coma Cluster of galaxies, 208 00:13:17,223 --> 00:13:21,693 he noticed something funny about the way they moved. 209 00:13:21,728 --> 00:13:24,930 The galaxies were going way too fast, 210 00:13:24,964 --> 00:13:28,100 so fast that they should've been flying apart from each other, 211 00:13:28,134 --> 00:13:30,535 because all the stars in all those galaxies 212 00:13:30,570 --> 00:13:33,972 had far too little gravity to hold the cluster together. 213 00:13:34,007 --> 00:13:35,774 Zwicky thought 214 00:13:35,808 --> 00:13:38,377 that something else must be binding them to each other. 215 00:13:38,411 --> 00:13:41,213 That mysterious missing component 216 00:13:41,247 --> 00:13:44,516 would have to weigh something like 50 times as much 217 00:13:44,550 --> 00:13:46,151 as the stars themselves. 218 00:13:46,185 --> 00:13:49,454 But no one paid much attention to this wild notion. 219 00:13:49,489 --> 00:13:52,658 Just another one of Zwicky's crazy ideas. 220 00:13:52,692 --> 00:13:54,326 In our solar system, 221 00:13:54,360 --> 00:13:56,895 the innermost planet, Mercury, moves much faster 222 00:13:56,929 --> 00:13:58,897 than the outermost one, Neptune. 223 00:13:58,931 --> 00:14:00,732 And that makes sense, right? 224 00:14:00,767 --> 00:14:04,136 The harder you push or pull on something, the faster it goes. 225 00:14:04,170 --> 00:14:07,472 The Sun's gravity weakens with increasing distance, 226 00:14:07,507 --> 00:14:11,877 so, the planets that are farther from the Sun move more slowly. 227 00:14:11,911 --> 00:14:15,580 Everyone expected that the outermost stars in a galaxy 228 00:14:15,615 --> 00:14:18,250 would act the same way. 229 00:14:18,284 --> 00:14:21,753 Most of the stars are concentrated towards the center, 230 00:14:21,788 --> 00:14:25,057 so, their collective gravity pulls on the other stars 231 00:14:25,091 --> 00:14:28,260 the same way the Sun pulls on the planets. 232 00:14:28,294 --> 00:14:30,696 But in the 1970s, when astronomer 233 00:14:30,730 --> 00:14:33,098 Vera Rubin studied the Andromeda Galaxy, 234 00:14:33,132 --> 00:14:37,669 she discovered that the outer stars obeyed no such rule. 235 00:14:37,704 --> 00:14:40,272 Unlike the outer planets of the solar system, 236 00:14:40,306 --> 00:14:44,009 the outer stars in the galaxy were all going at the same speed 237 00:14:44,043 --> 00:14:46,345 as the stars that were closer in, 238 00:14:46,379 --> 00:14:49,014 and they were moving way faster than expected. 239 00:14:49,048 --> 00:14:51,416 "That's funny," Vera thought. 240 00:14:51,451 --> 00:14:55,554 "There must be something weird about the Andromeda Galaxy." 241 00:14:55,588 --> 00:14:56,855 So she looked at another galaxy. 242 00:14:56,889 --> 00:14:59,091 Same story. 243 00:14:59,125 --> 00:15:00,459 And another. 244 00:15:00,493 --> 00:15:02,961 Vera studied 60 galaxies 245 00:15:02,996 --> 00:15:04,963 and found that all of them 246 00:15:04,998 --> 00:15:06,698 seemed to be violating the Law of Gravity, 247 00:15:06,733 --> 00:15:08,700 a core principle of physics. 248 00:15:08,735 --> 00:15:12,070 After some initial healthy skepticism, 249 00:15:12,105 --> 00:15:14,106 her colleagues looked for themselves, 250 00:15:14,140 --> 00:15:17,409 and found that Vera was right. 251 00:15:17,443 --> 00:15:20,579 It's not that Isaac Newton had gotten the Law of Gravity wrong; 252 00:15:20,613 --> 00:15:23,148 Vera Rubin had discovered that the gravity 253 00:15:23,182 --> 00:15:25,250 of something massive and invisible 254 00:15:25,284 --> 00:15:29,488 was forcing the stars to go fast. 255 00:15:29,522 --> 00:15:34,993 And then, someone remembered crazy old Fritz Zwicky, 256 00:15:35,028 --> 00:15:37,963 and the unknown source of gravity 257 00:15:37,997 --> 00:15:39,498 in the galaxy clusters 258 00:15:39,532 --> 00:15:43,602 that he called "dark matter," back in 1933. 259 00:15:49,842 --> 00:15:52,077 Vera Rubin had verified the existence 260 00:15:52,111 --> 00:15:54,746 of a new, much larger cosmos. 261 00:15:54,781 --> 00:15:57,015 And just like the one we thought we knew, 262 00:15:57,050 --> 00:15:59,084 it was filled with mystery. 263 00:15:59,118 --> 00:16:02,587 Dark matter is completely unobservable, 264 00:16:02,622 --> 00:16:04,356 except for its gravitational effect, 265 00:16:04,390 --> 00:16:09,094 which makes visible stars and galaxies move faster. 266 00:16:09,128 --> 00:16:12,831 Its nature is another deep mystery. 267 00:16:12,865 --> 00:16:15,967 Rubin had provided the evidence for an invisible universe 268 00:16:16,002 --> 00:16:20,772 nearly ten times more massive than the one we thought we knew. 269 00:16:20,807 --> 00:16:22,841 It was as if we had been standing on the seashore 270 00:16:22,875 --> 00:16:24,943 at night, mistakenly believing 271 00:16:24,977 --> 00:16:28,213 that the froth on the waves was all there was to the ocean. 272 00:16:28,247 --> 00:16:31,850 Vera Rubin looked at the stars and realized 273 00:16:31,884 --> 00:16:34,453 they were merely the foam on the waves, 274 00:16:34,487 --> 00:16:39,624 and that the greatest part of the ocean remained unknown. 275 00:16:39,659 --> 00:16:40,959 But wait. 276 00:16:40,994 --> 00:16:42,995 It gets crazier. 277 00:16:48,070 --> 00:16:50,071 Our Milky Way Galaxy, 278 00:16:50,106 --> 00:16:52,207 a few hundred billion stars, 279 00:16:52,241 --> 00:16:54,709 plus the clouds of gas and dust, 280 00:16:54,744 --> 00:16:57,746 the stuff of once and future stars-- 281 00:16:57,780 --> 00:17:02,717 and about a hundred billion other galaxies-- all of that, 282 00:17:02,752 --> 00:17:05,987 including those uncounted billions of trillions 283 00:17:06,021 --> 00:17:09,057 of planets, moons, and comets-- 284 00:17:09,091 --> 00:17:13,161 amounts to only five percent of what is actually there. 285 00:17:13,195 --> 00:17:15,330 Because there's a bigger unsolved mystery 286 00:17:15,364 --> 00:17:17,365 than dark matter-- 287 00:17:17,399 --> 00:17:21,536 dark energy, which makes up even more of the cosmos 288 00:17:21,570 --> 00:17:23,671 and drives its expansion. 289 00:17:23,706 --> 00:17:26,374 And it was Fritz Zwicky's supernovas 290 00:17:26,409 --> 00:17:29,978 that lit the way to the revelation of its existence. 291 00:17:32,014 --> 00:17:36,585 In one scenario, a star consumes all of its nuclear fuel... 292 00:17:40,156 --> 00:17:41,623 then cools, 293 00:17:41,657 --> 00:17:44,359 and suddenly collapses under its own gravity. 294 00:17:44,394 --> 00:17:48,063 The star rebounds in a massive explosion, 295 00:17:48,097 --> 00:17:52,000 leaving behind a neutron star or a black hole. 296 00:17:53,603 --> 00:17:56,038 Since the mass of the original star 297 00:17:56,072 --> 00:17:57,839 can fall within a wide range, 298 00:17:57,874 --> 00:17:59,408 its peak brightness as a supernova 299 00:17:59,442 --> 00:18:01,243 can also vary widely. 300 00:18:01,277 --> 00:18:03,979 So, you can't tell how far away it is 301 00:18:04,013 --> 00:18:05,881 just from how bright it looks. 302 00:18:05,915 --> 00:18:09,384 A relatively nearby supernova might appear just as bright 303 00:18:09,419 --> 00:18:12,554 as one that was more powerful, but farther away. 304 00:18:12,588 --> 00:18:15,490 But there's another kind of supernova 305 00:18:15,525 --> 00:18:17,559 that comes in only one strength. 306 00:18:17,593 --> 00:18:19,895 It marks the violent grand finale 307 00:18:19,929 --> 00:18:24,566 of a tango danced by a giant star and a dwarf. 308 00:18:26,436 --> 00:18:29,237 As the two stars orbit closely around each other, 309 00:18:29,272 --> 00:18:31,707 the giant sheds its outer layers of gas 310 00:18:31,741 --> 00:18:33,709 onto the dwarf. 311 00:18:33,743 --> 00:18:36,211 When the added weight becomes too much 312 00:18:36,245 --> 00:18:38,046 for it to bear, 313 00:18:38,081 --> 00:18:42,617 the dwarf detonates like a stupendous thermonuclear bomb. 314 00:18:42,652 --> 00:18:44,720 For a few weeks, 315 00:18:44,754 --> 00:18:47,355 the brilliance of such a supernova rivals 316 00:18:47,390 --> 00:18:50,692 the combined light of all the stars in its galaxy. 317 00:18:50,727 --> 00:18:52,527 This kind of supernova 318 00:18:52,562 --> 00:18:55,063 always has the same maximum power output, 319 00:18:55,098 --> 00:18:58,867 about five billion times brighter than our Sun. 320 00:18:58,901 --> 00:19:00,635 With big telescopes, 321 00:19:00,670 --> 00:19:03,071 we can see them in galaxies very far away, 322 00:19:03,106 --> 00:19:06,608 out toward the edge of the observable universe. 323 00:19:15,351 --> 00:19:18,453 Because all such supernovas have the same wattage, 324 00:19:18,488 --> 00:19:21,423 they're ideal tools for measuring distances 325 00:19:21,457 --> 00:19:23,658 to the farthest reaches of the universe. 326 00:19:23,693 --> 00:19:26,595 We call them "standard candles." 327 00:19:31,267 --> 00:19:36,905 In 1929, Edwin Hubble discovered that the universe is expanding. 328 00:19:36,939 --> 00:19:40,175 The distant galaxies are drifting away from one another. 329 00:19:40,209 --> 00:19:41,843 Later, we learned 330 00:19:41,878 --> 00:19:44,813 that the expansion began some 14 billion years ago 331 00:19:44,847 --> 00:19:49,317 with the explosive birth of the universe-- the big bang. 332 00:19:50,987 --> 00:19:53,021 Everybody assumed 333 00:19:53,056 --> 00:19:55,424 that the rate of expansion would be slowing down, 334 00:19:55,458 --> 00:19:57,192 due to the mutual pull of gravity 335 00:19:57,226 --> 00:19:58,960 between all the parts of the universe. 336 00:19:58,995 --> 00:20:00,829 If there is enough dark matter, 337 00:20:00,863 --> 00:20:04,166 its gravity would eventually bring the expansion to a stop, 338 00:20:04,200 --> 00:20:07,002 and the universe would then fall back on itself. 339 00:20:07,036 --> 00:20:10,205 In that case, everything would eventually collapse 340 00:20:10,239 --> 00:20:12,007 in a big crunch. 341 00:20:12,041 --> 00:20:15,377 On the other hand, if the universe had less dark matter, 342 00:20:15,411 --> 00:20:17,479 the expansion would continue forever, 343 00:20:17,513 --> 00:20:20,382 just getting slower and slower. 344 00:20:20,416 --> 00:20:22,984 Two competing teams of astronomers 345 00:20:23,019 --> 00:20:26,288 were observing those supernovas in distant galaxies. 346 00:20:26,322 --> 00:20:27,956 It turned out to be another one 347 00:20:27,990 --> 00:20:30,392 of those "that's funny" moments. 348 00:20:30,426 --> 00:20:33,729 In 1998, both teams independently came 349 00:20:33,763 --> 00:20:35,397 to the same conclusion. 350 00:20:35,431 --> 00:20:37,799 The expansion isn't slowing down at all... 351 00:20:37,834 --> 00:20:39,568 it's speeding up. 352 00:20:39,602 --> 00:20:43,205 This means the universe will continue to expand forever. 353 00:20:43,239 --> 00:20:47,008 There seems to be a mysterious force in the universe, 354 00:20:47,043 --> 00:20:50,011 one that overwhelms gravity on the grandest scale 355 00:20:50,046 --> 00:20:52,581 to push the cosmos apart. 356 00:20:52,615 --> 00:20:55,317 Most of the energy of the universe is bound up 357 00:20:55,351 --> 00:20:56,818 in this unknown force. 358 00:20:56,853 --> 00:20:59,221 We call it "dark energy," 359 00:20:59,255 --> 00:21:01,923 but that name, like "dark matter," 360 00:21:01,958 --> 00:21:05,394 is merely a code word for our ignorance. 361 00:21:05,428 --> 00:21:07,763 It's okay not to know all the answers. 362 00:21:07,797 --> 00:21:09,765 It's better to admit our ignorance 363 00:21:09,799 --> 00:21:12,167 than to believe answers that might be wrong. 364 00:21:12,201 --> 00:21:15,670 Pretending to know everything closes the door 365 00:21:15,705 --> 00:21:17,706 to finding out what's really there. 366 00:21:18,741 --> 00:21:21,243 Tonight, our ships sail 367 00:21:21,277 --> 00:21:24,713 into even more exotic waters. 368 00:21:24,747 --> 00:21:26,748 Come with me. 369 00:21:37,134 --> 00:21:39,402 Only two of our ships have ventured 370 00:21:39,436 --> 00:21:42,905 into the great dark ocean of interstellar space. 371 00:21:42,939 --> 00:21:46,776 The longest odyssey in all of history 372 00:21:46,810 --> 00:21:50,079 was launched back in 1977-- 373 00:21:50,113 --> 00:21:53,816 NASA's Voyager 1 and 2. 374 00:21:53,850 --> 00:21:58,154 The Voyagers move about 40,000 miles an hour. 375 00:21:58,188 --> 00:22:00,990 They gave us our first close-up look 376 00:22:01,024 --> 00:22:03,058 at Jupiter's Great Red Spot, 377 00:22:03,093 --> 00:22:06,962 a hurricane three times the size of Earth 378 00:22:06,997 --> 00:22:10,933 and one that's been raging since at least 1644, 379 00:22:10,967 --> 00:22:12,668 when it was first observed. 380 00:22:12,702 --> 00:22:16,472 For all we know, it could be thousands of years old. 381 00:22:16,506 --> 00:22:19,809 The Voyagers discovered the first active volcano 382 00:22:19,843 --> 00:22:23,779 on another world, on Jupiter's moon Io... 383 00:22:25,348 --> 00:22:27,483 and an ocean beneath the icy surface 384 00:22:27,517 --> 00:22:29,118 of the moon Europa... 385 00:22:32,823 --> 00:22:36,358 with at least twice as much water as we have here on Earth. 386 00:22:39,463 --> 00:22:42,832 The Voyagers dared to fly across Saturn's rings 387 00:22:42,866 --> 00:22:47,203 and revealed that they were made of hundreds of thin bands 388 00:22:47,237 --> 00:22:49,038 of orbiting snowballs. 389 00:22:52,409 --> 00:22:55,044 On Saturn's giant moon Titan, 390 00:22:55,078 --> 00:22:59,215 Voyager detected an atmosphere four times denser than Earth's. 391 00:22:59,249 --> 00:23:03,986 That hinted at the existence of hydrocarbon seas on Titan, 392 00:23:04,020 --> 00:23:05,688 which we later confirmed. 393 00:23:07,657 --> 00:23:09,625 Voyager 2 gave us our first portrait 394 00:23:09,659 --> 00:23:13,462 of the outermost planet, Neptune... 395 00:23:13,497 --> 00:23:17,366 where the winds roar at 1,000 miles per hour... 396 00:23:19,202 --> 00:23:22,071 and its moon Triton, where geysers 397 00:23:22,105 --> 00:23:26,609 of boiling nitrogen shoot five miles high. 398 00:23:29,746 --> 00:23:32,548 Voyager successfully completed its mission of discovery 399 00:23:32,582 --> 00:23:34,383 to the outer planets, 400 00:23:34,418 --> 00:23:36,385 but its odyssey into the darkness 401 00:23:36,420 --> 00:23:38,421 was just beginning. 402 00:23:40,724 --> 00:23:42,425 35 years after its launch, 403 00:23:42,459 --> 00:23:45,094 Voyager 1 became the first of our spacecraft 404 00:23:45,128 --> 00:23:47,997 to enter an uncharted realm. 405 00:23:48,031 --> 00:23:51,233 The Sun is constantly shooting out streams 406 00:23:51,268 --> 00:23:53,569 of charged particles in all directions, 407 00:23:53,603 --> 00:23:55,905 moving at a million miles an hour. 408 00:23:55,939 --> 00:24:00,676 This solar wind blows a vast magnetic bubble, 409 00:24:00,710 --> 00:24:04,580 our heliosphere, that extends beyond the outer planets. 410 00:24:04,614 --> 00:24:09,952 It pushes out against the thin gas of interstellar space. 411 00:24:09,986 --> 00:24:14,557 There's a border where one ends and the other begins. 412 00:24:14,591 --> 00:24:16,725 Voyager 1 reported back to Earth 413 00:24:16,760 --> 00:24:19,261 that its detectors were being pummeled 414 00:24:19,296 --> 00:24:20,796 by more and more cosmic rays. 415 00:24:20,831 --> 00:24:23,232 Until then, we didn't know 416 00:24:23,266 --> 00:24:25,534 where the interstellar ocean began. 417 00:24:25,869 --> 00:24:28,103 Voyager 1 pressed on 418 00:24:28,138 --> 00:24:31,340 past a boundary we had never crossed before. 419 00:24:31,374 --> 00:24:35,210 The heliosphere shields us from most of the deadly cosmic rays. 420 00:24:35,245 --> 00:24:37,379 When stormy solar winds blow, 421 00:24:37,414 --> 00:24:39,615 this zone of protection grows; 422 00:24:39,649 --> 00:24:41,750 in calm solar weather, it shrinks. 423 00:24:41,785 --> 00:24:45,053 When a star goes supernova in our galactic neighborhood... 424 00:24:46,890 --> 00:24:49,124 the debris from the exploded star 425 00:24:49,159 --> 00:24:52,127 pushes the heliosphere back towards the Sun. 426 00:24:52,162 --> 00:24:53,762 If it's strong enough to push it 427 00:24:53,797 --> 00:24:55,697 all the way back to Earth's orbit, 428 00:24:55,732 --> 00:25:00,769 our planet gets a radioactive bath of supernova debris. 429 00:25:00,804 --> 00:25:03,605 Luckily, this doesn't happen very often. 430 00:25:03,640 --> 00:25:07,042 The last one was perhaps two million years ago. 431 00:25:07,077 --> 00:25:10,212 A neighboring star explodes a million years 432 00:25:10,246 --> 00:25:13,882 before there's even such a thing as the human species. 433 00:25:13,917 --> 00:25:16,385 How can we possibly know this? 434 00:25:16,419 --> 00:25:19,221 Because the dying star left its traces 435 00:25:19,255 --> 00:25:21,724 miles below the surface of the ocean. 436 00:25:21,758 --> 00:25:26,462 Manganese nodules, small rocks like this one, 437 00:25:26,496 --> 00:25:30,966 are scattered over much of the deep sea floor. 438 00:25:31,000 --> 00:25:33,802 They grow very slowly. 439 00:25:33,837 --> 00:25:38,574 I'm talking a millimeter in a million years, 440 00:25:38,608 --> 00:25:42,578 layer upon layer. 441 00:25:42,612 --> 00:25:45,647 These nodules grow in partnership with bacteria 442 00:25:45,682 --> 00:25:50,519 by taking up minerals dissolved in the seawater. 443 00:25:50,553 --> 00:25:54,523 A supernova produces a radioactive form of iron, 444 00:25:54,557 --> 00:25:58,594 unlike anything made by natural processes on Earth. 445 00:25:58,628 --> 00:26:00,996 Researchers found telltale traces of this iron 446 00:26:01,030 --> 00:26:02,998 in a thin layer below the surface 447 00:26:03,032 --> 00:26:05,000 of the manganese nodules. 448 00:26:05,034 --> 00:26:07,836 They used the known rate of growth of the nodules 449 00:26:07,871 --> 00:26:10,172 to date that layer and to connect it 450 00:26:10,206 --> 00:26:14,209 to the fate of a star that perished eons ago. 451 00:26:15,845 --> 00:26:18,714 The difference between seeing nothing but a pebble 452 00:26:18,748 --> 00:26:23,152 and reading the history of the cosmos inscribed inside it 453 00:26:23,186 --> 00:26:25,587 is science. 454 00:26:27,757 --> 00:26:31,727 The interstellar ocean is dark and deep. 455 00:26:31,761 --> 00:26:35,731 Out here, the Sun is just the brightest star in the sky. 456 00:26:35,765 --> 00:26:39,168 Yet the Voyagers maintain their regular communications 457 00:26:39,202 --> 00:26:41,336 with NASA's Jet Propulsion Laboratory, 458 00:26:41,371 --> 00:26:43,972 talking back and forth across the light-hours 459 00:26:44,007 --> 00:26:46,875 that separate these ships from their home port. 460 00:26:46,910 --> 00:26:50,712 No other objects touched by human hands 461 00:26:50,747 --> 00:26:54,083 have ever ventured this far from home. 462 00:27:00,390 --> 00:27:01,857 Even after they lose their ability 463 00:27:01,891 --> 00:27:03,926 to respond to our command, 464 00:27:03,960 --> 00:27:06,161 the last and, by far, the longest phase 465 00:27:06,196 --> 00:27:08,197 of the Voyager mission will begin. 466 00:27:17,707 --> 00:27:20,876 Back in 1979, when both Voyagers rounded Jupiter, 467 00:27:20,910 --> 00:27:23,311 its massive gravity acted as a slingshot, 468 00:27:23,346 --> 00:27:25,313 flinging them out of the solar system 469 00:27:25,348 --> 00:27:27,516 to travel among the stars of our galaxy 470 00:27:27,550 --> 00:27:29,985 for a billion years. 471 00:27:30,019 --> 00:27:32,654 Carl Sagan recognized that the Voyager mission 472 00:27:32,688 --> 00:27:36,725 offered two free tickets to something approaching eternity. 473 00:27:36,759 --> 00:27:39,728 He assembled a small team to create a message 474 00:27:39,762 --> 00:27:42,731 to any civilization that might, one day, 475 00:27:42,765 --> 00:27:46,101 encounter the derelict spacecraft. 476 00:27:49,372 --> 00:27:51,539 26 centuries ago, 477 00:27:51,574 --> 00:27:54,376 the Assyrian king Esarhaddon wrote... 478 00:27:54,410 --> 00:27:57,145 "I had monuments made of bronze 479 00:27:57,179 --> 00:27:59,814 and inscriptions of baked clay. 480 00:27:59,849 --> 00:28:03,218 I left them in the foundations for future times." 481 00:28:03,252 --> 00:28:07,555 These hieroglyphics continue that ancient tradition. 482 00:28:07,590 --> 00:28:10,558 They are inscribed on the cover of a message designed 483 00:28:10,593 --> 00:28:15,063 to be read by the beings of other worlds and times. 484 00:28:16,732 --> 00:28:18,266 What could we possibly have in common 485 00:28:18,300 --> 00:28:19,734 with an alien civilization 486 00:28:19,769 --> 00:28:22,437 with its own separate evolutionary history 487 00:28:22,471 --> 00:28:24,706 and one so far advanced beyond us 488 00:28:24,740 --> 00:28:28,943 that they can patrol interstellar space? 489 00:28:28,978 --> 00:28:30,512 One thing at least, 490 00:28:30,546 --> 00:28:32,280 a universal language... 491 00:28:32,314 --> 00:28:34,115 science. 492 00:28:34,150 --> 00:28:37,118 It's hard to break the bonds of gravity. 493 00:28:37,153 --> 00:28:39,387 You can only sail the cosmic seas 494 00:28:39,422 --> 00:28:42,757 if you speak mathematics and physics. 495 00:28:42,792 --> 00:28:45,860 Hydrogen is the most common element in the universe. 496 00:28:45,895 --> 00:28:47,862 The electron in a hydrogen atom 497 00:28:47,897 --> 00:28:49,698 flips the direction of its spin 498 00:28:49,732 --> 00:28:51,700 at a constant rate, or frequency. 499 00:28:51,734 --> 00:28:55,703 Hydrogen atoms are like tiny natural clocks-- 500 00:28:55,738 --> 00:28:58,540 tick... tock. 501 00:28:58,574 --> 00:29:00,742 Now we have a unit of time in common 502 00:29:00,776 --> 00:29:02,377 with the extraterrestrials. 503 00:29:02,411 --> 00:29:04,279 This will come in handy when we get to the next level 504 00:29:04,313 --> 00:29:05,880 of the message. 505 00:29:05,915 --> 00:29:08,483 Here's our return address in space. 506 00:29:08,517 --> 00:29:11,319 Pulsars are rapidly-spinning neutron stars 507 00:29:11,354 --> 00:29:14,456 that give off regular bursts of radio waves. 508 00:29:14,490 --> 00:29:17,158 You can set your clock by them. 509 00:29:17,193 --> 00:29:19,761 The Sun is at the center of this diagram, 510 00:29:19,795 --> 00:29:23,431 and the lines point to the 14 nearest pulsars. 511 00:29:23,466 --> 00:29:27,268 A simple code labels each pulsar with its unique frequency, 512 00:29:27,303 --> 00:29:29,437 using the ticktock of the hydrogen atom 513 00:29:29,472 --> 00:29:31,172 as the unit of time. 514 00:29:31,207 --> 00:29:34,008 So alien astronomers could use this diagram 515 00:29:34,043 --> 00:29:36,611 to locate the home star of the Voyager spacecraft 516 00:29:36,645 --> 00:29:38,246 in our galaxy. 517 00:29:38,280 --> 00:29:39,848 They could also tell 518 00:29:39,882 --> 00:29:41,916 how long ago the spacecraft was launched. 519 00:29:41,951 --> 00:29:44,119 And that's important because the Voyager record 520 00:29:44,153 --> 00:29:48,156 has a projected shelf life of 1,000 million years. 521 00:29:51,193 --> 00:29:52,794 Become an extraterrestrial archaeologist 522 00:29:52,828 --> 00:29:54,796 for a few moments. 523 00:29:54,830 --> 00:29:57,999 An artifact has been fished out of the interstellar ocean. 524 00:29:59,168 --> 00:30:00,668 It was made by beings 525 00:30:00,703 --> 00:30:02,837 that lived about a billion years ago. 526 00:30:02,872 --> 00:30:05,040 What would you make of them and their world? 527 00:30:06,242 --> 00:30:09,944 They've sent us their music 528 00:30:09,979 --> 00:30:14,049 and greetings in 59 human languages. 529 00:30:16,953 --> 00:30:20,121 Hello from the children of planet Earth. 530 00:30:20,123 --> 00:30:21,222 And one whale language. 531 00:30:23,226 --> 00:30:26,594 And a sound essay that includes a Saturn V rocket launch. 532 00:30:28,432 --> 00:30:31,266 A mother's first words to her newborn baby. 533 00:30:32,769 --> 00:30:34,069 Oh, come on now. 534 00:30:34,071 --> 00:30:36,237 Be a good boy... be a good boy. 535 00:30:36,239 --> 00:30:39,040 The brain waves of a young woman newly fallen in love. 536 00:30:42,045 --> 00:30:45,580 And the sound of a pulsar. 537 00:30:49,719 --> 00:30:54,723 All of that will live for a billion years. 538 00:31:07,103 --> 00:31:10,538 How long is a billion years? 539 00:31:19,503 --> 00:31:22,602 If you can press all the time since the Big Bang, 540 00:31:22,739 --> 00:31:24,707 the explosive birth of the universe, 541 00:31:24,741 --> 00:31:26,709 into a single Earth year, 542 00:31:26,743 --> 00:31:30,045 a billion years is about one month of that year. 543 00:31:30,080 --> 00:31:34,417 What was happening on Earth a billion years ago? 544 00:31:37,454 --> 00:31:40,456 Most of Earth's land was amassed into a supercontinent 545 00:31:40,490 --> 00:31:42,158 called Rodinia. 546 00:31:42,192 --> 00:31:45,828 It was a barren desert-- no animals, no plants. 547 00:31:45,862 --> 00:31:47,797 A billion years ago, there wasn't enough oxygen 548 00:31:47,831 --> 00:31:50,232 in our atmosphere to form an ozone layer, 549 00:31:50,267 --> 00:31:52,902 and without it, ultraviolet radiation 550 00:31:52,936 --> 00:31:55,638 prevented life from colonizing the land. 551 00:31:55,672 --> 00:31:58,174 Rodinia probably looked more like Mars 552 00:31:58,208 --> 00:32:00,910 than present-day Earth. 553 00:32:00,944 --> 00:32:03,646 The giant world ocean produced huge rainstorms 554 00:32:03,680 --> 00:32:06,182 causing flooding and erosion. 555 00:32:06,216 --> 00:32:09,318 Glaciers formed, and their slow but relentless movements 556 00:32:09,352 --> 00:32:12,521 carved the land into new shapes. 557 00:32:12,556 --> 00:32:15,925 Single-celled organisms dominated the oceans, 558 00:32:15,959 --> 00:32:19,528 but some existed in colonies called "microbial mats," 559 00:32:19,563 --> 00:32:24,266 and the first multicellular organisms would soon evolve. 560 00:32:24,301 --> 00:32:26,268 And a billion years from now, 561 00:32:26,303 --> 00:32:28,270 what will Earth be like... 562 00:32:28,305 --> 00:32:30,940 long after our cities, the Egyptian pyramids, 563 00:32:30,974 --> 00:32:34,110 the Rocky Mountains have all been eroded to dust? 564 00:32:34,144 --> 00:32:36,712 There are few things we can say with confidence 565 00:32:36,747 --> 00:32:39,749 about such a far distant time. 566 00:32:43,420 --> 00:32:45,888 The only thing we can say for sure 567 00:32:45,922 --> 00:32:49,725 is that Earth as we know it will be so changed 568 00:32:49,760 --> 00:32:53,062 that we would scarcely recognize it as home. 569 00:32:53,096 --> 00:32:56,632 But even a thousand million years from now, 570 00:32:56,666 --> 00:33:00,169 something of who we were and the music that we made 571 00:33:00,203 --> 00:33:02,471 in that long-ago spring... 572 00:33:02,506 --> 00:33:04,507 will live on. 573 00:33:09,379 --> 00:33:12,548 In that distant future, our Sun will have completed 574 00:33:12,582 --> 00:33:15,918 another four orbits around the center of the galaxy... 575 00:33:24,227 --> 00:33:28,731 ...and the Voyagers will have ventured far from the Sun. 576 00:33:32,069 --> 00:33:34,770 Carl Sagan was a member of Voyager's imaging team, 577 00:33:34,805 --> 00:33:39,008 and it was his idea that Voyager take one last picture. 578 00:33:39,042 --> 00:33:41,043 A generation before, 579 00:33:41,078 --> 00:33:43,913 an astronaut on the last Apollo flight to the Moon 580 00:33:43,947 --> 00:33:46,582 had taken a picture of the whole Earth-- 581 00:33:46,616 --> 00:33:49,785 the planet as a world without borders. 582 00:33:49,820 --> 00:33:53,189 It became an icon of a new consciousness. 583 00:33:53,223 --> 00:33:57,360 Carl realized the next step in this process. 584 00:33:57,394 --> 00:34:00,429 He convinced NASA to turn the Voyager 1 camera 585 00:34:00,464 --> 00:34:04,233 back towards Earth when the spacecraft went beyond Neptune 586 00:34:04,267 --> 00:34:07,903 for one last look homeward at what he called... 587 00:34:07,938 --> 00:34:10,940 the pale blue dot. 588 00:34:16,747 --> 00:34:18,314 That's here. 589 00:34:18,348 --> 00:34:19,882 That's home. 590 00:34:19,916 --> 00:34:21,484 That's us. 591 00:34:21,518 --> 00:34:24,220 On it, everyone you love, 592 00:34:24,254 --> 00:34:26,255 everyone you know, 593 00:34:26,289 --> 00:34:28,257 everyone you ever heard of, 594 00:34:28,291 --> 00:34:30,926 every human being who ever was, 595 00:34:30,961 --> 00:34:33,329 lived out their lives. 596 00:34:33,363 --> 00:34:37,099 The aggregate of our joy and suffering, 597 00:34:37,134 --> 00:34:39,935 thousands of confident religions, ideologies, 598 00:34:39,970 --> 00:34:41,937 and economic doctrines, 599 00:34:41,972 --> 00:34:44,106 every hunter and forager, 600 00:34:44,141 --> 00:34:46,108 every hero and coward, 601 00:34:46,143 --> 00:34:49,578 every creator and destroyer of civilization, 602 00:34:49,613 --> 00:34:51,580 every king and peasant, 603 00:34:51,615 --> 00:34:53,616 every young couple in love, 604 00:34:53,650 --> 00:34:56,919 every mother and father, hopeful child, 605 00:34:56,953 --> 00:34:59,188 inventor and explorer, 606 00:34:59,222 --> 00:35:02,858 every teacher of morals, every corrupt politician, 607 00:35:02,893 --> 00:35:06,962 every superstar, every supreme leader, 608 00:35:06,997 --> 00:35:10,833 every saint and sinner in the history of our species, 609 00:35:10,867 --> 00:35:12,835 lived there... 610 00:35:12,869 --> 00:35:14,670 on a mote of dust 611 00:35:14,705 --> 00:35:17,940 suspended... in a sunbeam. 612 00:35:17,974 --> 00:35:22,178 The Earth is a very small stage 613 00:35:22,212 --> 00:35:26,015 in a vast, cosmic arena. 614 00:35:26,049 --> 00:35:29,285 Think of the rivers of blood 615 00:35:29,319 --> 00:35:33,022 spilled by all those generals and emperors 616 00:35:33,056 --> 00:35:35,491 so that in glory and triumph 617 00:35:35,525 --> 00:35:38,461 they could become the momentary masters 618 00:35:38,495 --> 00:35:41,731 of a fraction... of a dot. 619 00:35:41,765 --> 00:35:44,867 Think of the endless cruelties visited 620 00:35:44,901 --> 00:35:48,404 by the inhabitants of one corner of this pixel 621 00:35:48,406 --> 00:35:51,307 on the scarcely distinguishable inhabitants 622 00:35:51,341 --> 00:35:53,409 of some other corner. 623 00:35:53,443 --> 00:35:56,479 How frequent their misunderstandings, 624 00:35:56,513 --> 00:35:59,482 how eager they are to kill one another, 625 00:35:59,516 --> 00:36:02,818 how fervent their hatreds. 626 00:36:02,853 --> 00:36:05,354 Our posturings, 627 00:36:05,389 --> 00:36:08,424 our imagined self-importance, 628 00:36:08,458 --> 00:36:11,027 the delusion that we have some privileged position 629 00:36:11,061 --> 00:36:12,662 in the universe, 630 00:36:12,696 --> 00:36:17,366 are challenged by this point of pale light. 631 00:36:17,401 --> 00:36:19,402 Our planet... 632 00:36:19,436 --> 00:36:22,438 is a lonely speck in the great, 633 00:36:22,472 --> 00:36:25,541 enveloping cosmic dark. 634 00:36:25,575 --> 00:36:28,377 In our obscurity, 635 00:36:28,412 --> 00:36:31,180 in all this vastness, 636 00:36:31,214 --> 00:36:34,917 there is no hint that help will come from elsewhere 637 00:36:34,951 --> 00:36:37,887 to save us from ourselves. 638 00:36:37,921 --> 00:36:42,124 The Earth is the only world known so far to harbor life. 639 00:36:42,159 --> 00:36:44,126 There is nowhere else, 640 00:36:44,161 --> 00:36:46,128 at least in the near future, 641 00:36:46,163 --> 00:36:48,964 to which our species could migrate. 642 00:36:48,999 --> 00:36:51,734 Visit, yes. 643 00:36:51,768 --> 00:36:53,970 Settle, not yet. 644 00:36:54,004 --> 00:36:56,038 Like it or not, 645 00:36:56,073 --> 00:37:00,609 for the moment, the Earth is where we make our stand. 646 00:37:00,644 --> 00:37:03,946 It has been said that astronomy is a humbling 647 00:37:03,981 --> 00:37:08,084 and character-building experience. 648 00:37:08,118 --> 00:37:10,586 There is perhaps no better demonstration 649 00:37:10,621 --> 00:37:13,456 of the folly of human conceits 650 00:37:13,490 --> 00:37:16,993 than this distant image. 651 00:37:17,027 --> 00:37:20,596 To me, it underscores our responsibility 652 00:37:20,631 --> 00:37:23,299 to deal more kindly with one another 653 00:37:23,333 --> 00:37:25,801 and to preserve and cherish 654 00:37:25,836 --> 00:37:27,970 the pale blue dot, 655 00:37:28,005 --> 00:37:31,140 the only home we've ever known. 656 00:37:38,048 --> 00:37:40,016 How did we, 657 00:37:40,050 --> 00:37:42,418 tiny creatures living on that speck of dust, 658 00:37:42,452 --> 00:37:45,755 ever manage to figure out how to send spacecraft 659 00:37:45,789 --> 00:37:48,591 out among the stars of the Milky Way? 660 00:37:48,625 --> 00:37:52,995 Only a few centuries ago, a mere second of cosmic time, 661 00:37:53,030 --> 00:37:55,631 we knew nothing of where or when we were. 662 00:37:55,666 --> 00:37:58,434 Oblivious to the rest of the cosmos, 663 00:37:58,468 --> 00:38:01,037 we inhabited a kind of prison-- 664 00:38:01,071 --> 00:38:03,039 a tiny universe 665 00:38:03,073 --> 00:38:06,575 bounded by a nutshell. 666 00:38:08,679 --> 00:38:12,949 How did we escape from the prison? 667 00:38:12,983 --> 00:38:16,886 It was the work of generations of searchers 668 00:38:16,920 --> 00:38:20,756 who took five simple rules to heart. 669 00:38:24,661 --> 00:38:26,629 Question authority. 670 00:38:26,663 --> 00:38:29,966 No idea is true just because someone says so, 671 00:38:30,000 --> 00:38:31,667 including me. 672 00:38:32,669 --> 00:38:34,503 Think for yourself. 673 00:38:37,941 --> 00:38:39,909 Question yourself. 674 00:38:42,279 --> 00:38:44,914 Don't believe anything just because you want to. 675 00:38:44,948 --> 00:38:49,719 Believing something doesn't make it so. 676 00:38:49,753 --> 00:38:51,721 Test ideas by the evidence gained 677 00:38:51,755 --> 00:38:55,424 from observation and experiment. 678 00:38:55,459 --> 00:38:58,828 If a favorite idea fails a well-designed test, 679 00:38:58,862 --> 00:39:01,030 it's wrong! 680 00:39:01,064 --> 00:39:02,665 Get over it. 681 00:39:02,699 --> 00:39:07,536 Follow the evidence, wherever it leads. 682 00:39:09,139 --> 00:39:12,541 If you have no evidence, reserve judgment. 683 00:39:12,576 --> 00:39:15,611 And perhaps the most important rule of all... 684 00:39:15,646 --> 00:39:17,380 Remember, you could be wrong. 685 00:39:17,414 --> 00:39:20,116 Even the best scientists 686 00:39:20,150 --> 00:39:22,051 have been wrong about some things. 687 00:39:22,085 --> 00:39:24,787 Newton, Einstein, 688 00:39:24,821 --> 00:39:27,556 and every other great scientist in history, 689 00:39:27,591 --> 00:39:30,059 they all made mistakes. 690 00:39:30,093 --> 00:39:33,262 Of course they did-- they were human. 691 00:39:33,297 --> 00:39:37,600 Science is a way to keep from fooling ourselves... 692 00:39:37,634 --> 00:39:40,403 and each other. 693 00:39:40,437 --> 00:39:42,772 Have scientists known sin? 694 00:39:45,943 --> 00:39:47,410 Of course. 695 00:39:47,444 --> 00:39:49,779 We have misused science, just as we have 696 00:39:49,813 --> 00:39:53,049 every other tool at our disposal, 697 00:39:53,083 --> 00:39:55,251 and that's why we can't afford 698 00:39:55,285 --> 00:39:59,222 to leave it in the hands of a powerful few. 699 00:39:59,256 --> 00:40:02,158 The more science belongs to all of us, 700 00:40:02,192 --> 00:40:05,261 the less likely it is to be misused. 701 00:40:07,497 --> 00:40:10,132 These values undermine the appeals 702 00:40:10,167 --> 00:40:12,435 of fanaticism and ignorance 703 00:40:12,469 --> 00:40:14,337 and, after all, 704 00:40:14,371 --> 00:40:16,806 the universe is mostly dark, 705 00:40:16,840 --> 00:40:20,076 dotted by islands of light. 706 00:40:20,110 --> 00:40:23,746 Learning the age of the Earth or the distance to the stars 707 00:40:23,780 --> 00:40:25,748 or how life evolves-- 708 00:40:25,782 --> 00:40:28,017 what difference does that make? 709 00:40:28,051 --> 00:40:30,753 Well, part of it depends on how big a universe 710 00:40:30,787 --> 00:40:32,655 you're willing to live in. 711 00:40:32,689 --> 00:40:34,657 Some of us like it small. 712 00:40:34,691 --> 00:40:36,292 That's fine. 713 00:40:36,326 --> 00:40:37,693 Understandable. 714 00:40:37,728 --> 00:40:39,428 But I like it big. 715 00:40:39,463 --> 00:40:42,598 And when I take all of this into my heart and my mind, 716 00:40:42,633 --> 00:40:44,534 I'm uplifted by it. 717 00:40:44,568 --> 00:40:48,337 And when I have that feeling, I want to know that it's real, 718 00:40:48,372 --> 00:40:51,841 that it's not just something happening inside my own head, 719 00:40:51,875 --> 00:40:54,010 because it matters what's true, 720 00:40:54,044 --> 00:40:56,479 and our imagination is nothing 721 00:40:56,513 --> 00:40:59,882 compared with Nature's awesome reality. 722 00:41:04,021 --> 00:41:08,024 I want to know what's in those dark places, 723 00:41:08,058 --> 00:41:11,761 and what happened before the Big Bang. 724 00:41:13,830 --> 00:41:16,666 I want to know what lies beyond the cosmic horizon, 725 00:41:16,700 --> 00:41:19,569 and how life began. 726 00:41:19,603 --> 00:41:22,071 Are there other places in the cosmos 727 00:41:22,105 --> 00:41:26,075 where matter and energy have become alive... 728 00:41:26,109 --> 00:41:28,544 and aware? 729 00:41:35,586 --> 00:41:37,754 I want to know my ancestors-- 730 00:41:37,788 --> 00:41:39,255 all of them. 731 00:41:39,290 --> 00:41:41,524 I want to be a good, strong link 732 00:41:41,559 --> 00:41:43,426 in the chain of generations. 733 00:41:43,460 --> 00:41:45,261 I want to protect my children 734 00:41:45,296 --> 00:41:48,431 and the children of ages to come. 735 00:41:48,465 --> 00:41:51,634 We, who embody the local eyes and ears 736 00:41:51,669 --> 00:41:54,370 and thoughts and feelings of the cosmos, 737 00:41:54,405 --> 00:41:57,640 we've begun to learn the story of our origins-- 738 00:41:57,675 --> 00:42:01,544 star stuff contemplating the evolution of matter, 739 00:42:01,579 --> 00:42:05,248 tracing that long path by which it arrived at consciousness. 740 00:42:05,282 --> 00:42:08,251 We and the other living things on this planet 741 00:42:08,285 --> 00:42:10,453 carry a legacy of cosmic evolution 742 00:42:10,487 --> 00:42:12,889 spanning billions of years. 743 00:42:12,923 --> 00:42:15,124 If we take that knowledge to heart, 744 00:42:15,159 --> 00:42:18,661 if we come to know and love nature as it really is, 745 00:42:18,696 --> 00:42:21,731 then we will surely be remembered by our descendants 746 00:42:21,765 --> 00:42:24,834 as good, strong links in the chain of life. 747 00:42:24,868 --> 00:42:28,171 And our children will continue this sacred searching, 748 00:42:28,205 --> 00:42:31,941 seeing for us as we have seen for those who came before, 749 00:42:31,976 --> 00:42:35,245 discovering wonders yet undreamt of... 750 00:42:35,279 --> 00:42:37,480 in the cosmos.