1 00:00:52,667 --> 00:00:57,502 SAGAN: We are drifting in a great ocean of space and time. 2 00:00:59,207 --> 00:01:02,108 In that ocean, the events that shape the future... 3 00:01:02,310 --> 00:01:04,710 ...are working themselves out. 4 00:01:06,915 --> 00:01:10,248 Each creature and every world, to the remotest star... 5 00:01:10,451 --> 00:01:12,351 ...owe their existence to... 6 00:01:12,554 --> 00:01:15,387 ...the great, coursing, implacable forces of nature... 7 00:01:15,590 --> 00:01:18,718 ...but also, to minor happenstance. 8 00:01:21,262 --> 00:01:24,527 We are carried with our planet around the sun. 9 00:01:24,732 --> 00:01:28,168 The Earth has made more than 4 billion circuits of our star... 10 00:01:28,369 --> 00:01:30,200 ...since its origin. 11 00:01:32,240 --> 00:01:36,142 The sun itself travels about the core of the Milky Way galaxy. 12 00:01:36,344 --> 00:01:38,972 Our galaxy is moving among the other galaxies. 13 00:01:39,180 --> 00:01:42,547 We have always been space travelers. 14 00:01:44,352 --> 00:01:49,221 These fine sand grains are all, more or less, uniform in size. 15 00:01:50,124 --> 00:01:53,355 They're produced from bigger rocks through ages of... 16 00:01:53,561 --> 00:01:57,395 ...jostling and rubbing, abrasion and erosion. 17 00:01:57,599 --> 00:02:01,467 Driven in part by the distant moon and sun. 18 00:02:01,803 --> 00:02:05,364 So the roots of the present lie buried in the past. 19 00:02:05,573 --> 00:02:09,339 We are also travelers in time. 20 00:02:13,815 --> 00:02:15,043 But trapped on Earth... 21 00:02:15,249 --> 00:02:18,582 ...we've had little to say about where we go in time and space... 22 00:02:18,786 --> 00:02:20,151 ...or how fast. 23 00:02:20,355 --> 00:02:24,086 But now we're thinking about true journeys in time... 24 00:02:24,292 --> 00:02:28,194 ...and real voyages to the distant stars. 25 00:02:29,964 --> 00:02:34,560 A handful of sand contains about 10,000 grains... 26 00:02:34,769 --> 00:02:36,964 ...more than all the stars we can see... 27 00:02:37,171 --> 00:02:39,503 ...with the naked eye on a clear night. 28 00:02:39,707 --> 00:02:41,800 But the number of stars we can see... 29 00:02:42,010 --> 00:02:45,605 ...is only the tiniest fraction of the number of stars that are. 30 00:02:46,414 --> 00:02:49,440 What we see at night is the merest smattering... 31 00:02:49,651 --> 00:02:51,619 ...of the nearest stars... 32 00:02:51,819 --> 00:02:56,085 ...with a few more distant bright stars thrown in for good measure. 33 00:02:56,290 --> 00:03:00,158 Meanwhile, the cosmos is rich beyond measure. 34 00:03:00,361 --> 00:03:02,488 The number of stars in the universe... 35 00:03:02,697 --> 00:03:06,030 ...is larger than all the grains of sand on all the beaches... 36 00:03:06,234 --> 00:03:07,826 ...of the planet Earth. 37 00:03:11,706 --> 00:03:16,507 Long ago, before we had figured out that the stars are distant suns... 38 00:03:16,711 --> 00:03:19,839 ...they seemed to us to make pictures in the sky. 39 00:03:20,048 --> 00:03:22,846 Just follow the dots. 40 00:03:24,152 --> 00:03:27,315 The Big Dipper constellation today in North America... 41 00:03:27,522 --> 00:03:29,683 ...has had many other incarnations. 42 00:03:29,891 --> 00:03:32,155 Every culture, ancient and modern... 43 00:03:32,427 --> 00:03:36,193 ...has placed its totems and concerns among the stars. 44 00:03:36,397 --> 00:03:40,390 From a Chinese bureaucrat to a German wagon. 45 00:03:44,439 --> 00:03:48,239 But very ancient cultures would have seen different constellations... 46 00:03:48,443 --> 00:03:51,742 ...because the stars move with respect to one another. 47 00:03:51,946 --> 00:03:56,713 We can give a computer the present positions and motions of stars... 48 00:03:56,918 --> 00:04:00,615 ...and then run the patterns back into time. 49 00:04:02,623 --> 00:04:06,320 Every constellation is a single frame in a cosmic movie... 50 00:04:06,527 --> 00:04:08,893 ...but because our lives are so short... 51 00:04:09,097 --> 00:04:11,088 ...because star patterns change slowly... 52 00:04:11,299 --> 00:04:14,234 ...we tend not to notice it's a movie. 53 00:04:14,435 --> 00:04:18,235 A million years ago, there was no Big Dipper. 54 00:04:18,639 --> 00:04:21,938 Our ancestors, looking up and wondering about the stars... 55 00:04:22,143 --> 00:04:25,772 ...saw some other pattern in the northern skies. 56 00:04:26,814 --> 00:04:31,183 We can also run a constellation, Leo the Lion, say, forward in time... 57 00:04:31,385 --> 00:04:35,287 ...and see what the patterns in the stars will be in the future. 58 00:04:37,058 --> 00:04:39,458 A million years from now, Leo might be renamed... 59 00:04:39,660 --> 00:04:42,356 ...the constellation of the Radio Telescope. 60 00:04:42,563 --> 00:04:45,862 Although I suspect radio telescopes then will be as obsolete... 61 00:04:46,067 --> 00:04:47,967 ...as stone spears are now. 62 00:04:48,169 --> 00:04:52,196 Or, here's the constellation of Cetus the Whale. 63 00:05:01,215 --> 00:05:05,948 A million years ago, it may have been called something else. 64 00:05:06,154 --> 00:05:07,644 Perhaps the Spear. 65 00:05:12,760 --> 00:05:17,288 Now, let's run fast-forward through a billion nights. 66 00:05:22,436 --> 00:05:24,063 Millions of years from now... 67 00:05:24,272 --> 00:05:28,470 ...some other very different image will be featured in this cosmic movie. 68 00:05:36,417 --> 00:05:39,443 In Orion the Hunter, things are changing... 69 00:05:39,654 --> 00:05:41,588 ...not only because the stars are moving... 70 00:05:41,789 --> 00:05:44,553 ...but also because the stars are evolving. 71 00:05:44,759 --> 00:05:48,320 Many of Orion's stars are hot, young and short-lived. 72 00:05:48,529 --> 00:05:52,590 They're born, live and die within a span of only a few million years. 73 00:05:52,800 --> 00:05:55,633 If we run Orion forward in time... 74 00:05:55,837 --> 00:05:58,670 ...we see the births and explosive deaths... 75 00:05:58,873 --> 00:06:00,397 ...of dozens of stars... 76 00:06:00,608 --> 00:06:04,738 ...flashing on and winking off like fireflies in the night. 77 00:06:07,181 --> 00:06:10,708 If we wait long enough, we see the constellations change. 78 00:06:10,918 --> 00:06:14,854 But if we go far enough, we also see the star patterns alter. 79 00:06:15,056 --> 00:06:16,921 Two-dimensional constellations... 80 00:06:17,124 --> 00:06:21,117 ...are only the appearance of stars strewn through three dimensions. 81 00:06:21,329 --> 00:06:25,629 Some are dim and near, others are bright but farther away. 82 00:06:26,868 --> 00:06:29,200 Could a space traveler actually see... 83 00:06:29,403 --> 00:06:32,099 ...the patterns of the constellations change? 84 00:06:32,306 --> 00:06:37,175 For that, you must travel roughly as far as the constellation is from us. 85 00:06:37,645 --> 00:06:40,671 Here, we're traveling hundreds of light-years... 86 00:06:40,882 --> 00:06:45,182 ...circling all the way around the stars of the Big Dipper. 87 00:06:48,723 --> 00:06:50,850 Inhabitants of planets around other stars... 88 00:06:51,058 --> 00:06:53,492 ...will see different constellations than us... 89 00:06:53,694 --> 00:06:56,663 ...because their vantage points are different. 90 00:07:04,739 --> 00:07:08,402 Here we are in the constellation Andromeda... 91 00:07:08,609 --> 00:07:13,410 ...or at least a model of it next to the constellation Perseus. 92 00:07:13,614 --> 00:07:15,775 Andromeda, in the Greek myth... 93 00:07:15,983 --> 00:07:19,783 ...was the maiden who was saved by Perseus... 94 00:07:19,987 --> 00:07:21,921 ...from a sea monster. 95 00:07:22,123 --> 00:07:27,026 This star just above me is Beta Andromedae... 96 00:07:27,228 --> 00:07:29,856 ...the second brightest star in the constellation... 97 00:07:30,064 --> 00:07:33,227 ...75 light-years from the Earth. 98 00:07:33,434 --> 00:07:36,528 The light by which we see this star... 99 00:07:36,737 --> 00:07:40,867 ...has spent 75 years traversing interstellar space... 100 00:07:41,075 --> 00:07:43,066 ...on its journey to the Earth. 101 00:07:43,277 --> 00:07:46,872 In the unlikely event that Beta Andromedae... 102 00:07:47,081 --> 00:07:49,606 ...blew itself up a week ago Tuesday... 103 00:07:49,817 --> 00:07:52,581 ...we will not know of it for another 75 years... 104 00:07:52,787 --> 00:07:56,780 ...as this interesting information, traveling at the speed of light... 105 00:07:56,991 --> 00:08:00,620 ...crosses the enormous interstellar distances. 106 00:08:00,828 --> 00:08:03,319 When the light we see from this star set out... 107 00:08:03,531 --> 00:08:06,261 ...on its long interstellar voyage... 108 00:08:06,467 --> 00:08:09,027 ...the young Albert Einstein... 109 00:08:09,236 --> 00:08:12,103 ...working as a Swiss patent clerk... 110 00:08:12,306 --> 00:08:16,208 ...had just published his epochal special theory of relativity... 111 00:08:16,410 --> 00:08:17,741 ...here on Earth. 112 00:08:18,612 --> 00:08:19,840 We see... 113 00:08:20,047 --> 00:08:24,177 ...that space and time are intertwined. 114 00:08:24,385 --> 00:08:27,252 We cannot look out into space... 115 00:08:27,455 --> 00:08:30,754 ...without looking back into time. 116 00:08:31,025 --> 00:08:34,119 The speed of light is very fast... 117 00:08:34,328 --> 00:08:37,525 ...but space is very empty... 118 00:08:37,732 --> 00:08:41,031 ...and the stars are very far apart. 119 00:08:41,235 --> 00:08:44,363 The distances that we've been talking about up to now... 120 00:08:44,572 --> 00:08:48,372 ...are very small by the usual astronomical standards. 121 00:08:48,576 --> 00:08:51,204 In fact, the distance from the Earth... 122 00:08:51,412 --> 00:08:53,403 ...to the center of the Milky Way galaxy... 123 00:08:53,614 --> 00:08:56,481 ...is 30,000 light-years. 124 00:08:57,451 --> 00:09:02,388 From our galaxy to the nearest spiral galaxy like our own... 125 00:09:02,723 --> 00:09:04,520 ...called M31 ... 126 00:09:04,725 --> 00:09:07,626 ...and which is also within, that means behind... 127 00:09:07,828 --> 00:09:09,796 ...the constellation Andromeda... 128 00:09:10,531 --> 00:09:14,262 ...is 2 million light-years. 129 00:09:15,503 --> 00:09:18,700 When the light we see today from M31 ... 130 00:09:18,906 --> 00:09:21,431 ...left on its journey for Earth... 131 00:09:21,642 --> 00:09:23,576 ...there were no human beings... 132 00:09:23,778 --> 00:09:27,077 ...although our ancestors were nicely evolving... 133 00:09:27,281 --> 00:09:30,341 ...and very rapidly, to our present form. 134 00:09:31,018 --> 00:09:33,509 There are much greater distances in astronomy. 135 00:09:33,721 --> 00:09:37,316 The distance from the Earth to the most distant quasars... 136 00:09:37,525 --> 00:09:40,961 ...is 8 or 10 billion light-years. 137 00:09:41,162 --> 00:09:45,861 We see them as they were before the Earth itself accumulated... 138 00:09:46,067 --> 00:09:49,628 ...before the Milky Way galaxy was formed. 139 00:09:49,837 --> 00:09:53,466 The fastest space vehicles ever launched by the human species... 140 00:09:53,674 --> 00:09:55,608 ...are the Voyager spacecraft. 141 00:09:55,810 --> 00:09:57,505 They are traveling so fast... 142 00:09:57,711 --> 00:10:01,306 ...that it's only 10,000 times slower... 143 00:10:01,816 --> 00:10:03,113 ...than the speed of light. 144 00:10:03,317 --> 00:10:05,945 The Voyager spacecraft will take 40,000 years... 145 00:10:06,153 --> 00:10:08,212 ...to go the distance to the nearest stars... 146 00:10:08,422 --> 00:10:11,619 ...and they're not even headed towards the nearest stars. 147 00:10:11,826 --> 00:10:14,420 But is there a method by which we could travel... 148 00:10:14,628 --> 00:10:18,120 ...in a conveniently short time to the stars? 149 00:10:18,332 --> 00:10:21,165 Can we travel close to the speed of light? 150 00:10:21,368 --> 00:10:24,098 And what's magic about the speed of light? 151 00:10:24,305 --> 00:10:26,899 Can't we travel faster than that? 152 00:10:28,909 --> 00:10:32,743 It turns out that there is something very strange... 153 00:10:32,947 --> 00:10:34,414 ...about the speed of light. 154 00:10:34,615 --> 00:10:36,674 Something that provides the key... 155 00:10:36,884 --> 00:10:40,285 ...to our understanding of time and space. 156 00:10:42,022 --> 00:10:43,717 The story of its discovery... 157 00:10:43,924 --> 00:10:47,519 ...takes us to Tuscany in northern Italy. 158 00:10:49,830 --> 00:10:52,264 There's something timeless about this place. 159 00:10:52,466 --> 00:10:56,129 A century ago, it probably looked very much the same. 160 00:11:08,249 --> 00:11:12,447 If you had traveled these roads in the summer of 1895... 161 00:11:12,653 --> 00:11:16,783 ...you might have come upon a 16-year-old German high-school dropout. 162 00:11:16,991 --> 00:11:20,154 His teacher told him that he'd never amount to anything... 163 00:11:20,361 --> 00:11:23,819 ...that his attitude destroyed classroom discipline... 164 00:11:24,031 --> 00:11:25,760 ...that he should drop out. 165 00:11:25,966 --> 00:11:27,957 So he left and came here... 166 00:11:28,169 --> 00:11:30,603 ...where he enjoyed wandering these roads... 167 00:11:30,804 --> 00:11:33,272 ...and giving his mind free rein to explore. 168 00:11:35,276 --> 00:11:37,744 One day, he began to think about light... 169 00:11:37,945 --> 00:11:39,936 ...about how fast it travels. 170 00:11:40,147 --> 00:11:43,207 We always measure the speed of a moving object... 171 00:11:43,417 --> 00:11:45,385 ...relative to something else. 172 00:11:45,586 --> 00:11:49,613 I'm moving at about 10 kilometers an hour relative to the ground. 173 00:11:49,823 --> 00:11:51,450 But the ground isn't at rest. 174 00:11:51,659 --> 00:11:55,425 The Earth is turning at more than 1600 kilometers an hour. 175 00:11:55,629 --> 00:11:58,097 The Earth itself is in orbit around the sun. 176 00:11:58,299 --> 00:12:02,360 The sun is moving among the drifting stars, and so on. 177 00:12:02,570 --> 00:12:05,971 It was hard for the young man to imagine some absolute standard... 178 00:12:06,173 --> 00:12:09,040 ...to measure all these relative motions against. 179 00:12:18,852 --> 00:12:22,720 He knew that sound waves are a vibration of the air... 180 00:12:22,923 --> 00:12:26,086 ...and their speed is measured relative to the air itself. 181 00:12:26,293 --> 00:12:29,490 But sunlight travels across the vacuum of empty space. 182 00:12:29,697 --> 00:12:31,961 "Do light waves move relative to something else? 183 00:12:32,166 --> 00:12:35,761 And if so," he wondered, "relative to what?" 184 00:12:39,306 --> 00:12:43,436 That teenage dropout's name... 185 00:12:44,111 --> 00:12:45,772 ...was Albert Einstein. 186 00:12:45,980 --> 00:12:48,915 And his ruminations changed the world. 187 00:12:54,255 --> 00:12:58,385 He had been fascinated by Bernstein's 1869... 188 00:12:59,260 --> 00:13:02,855 ...People's Book of Natural Science. 189 00:13:03,063 --> 00:13:06,226 Here, on its very first page... 190 00:13:07,101 --> 00:13:11,231 ...it describes the astonishing speed of electricity through wires... 191 00:13:11,572 --> 00:13:13,506 ...and light through space. 192 00:13:13,841 --> 00:13:17,777 Einstein wondered, perhaps for the first time, in northern Italy... 193 00:13:18,545 --> 00:13:22,743 ...what the world would look like if you could travel on a wave of light. 194 00:13:23,317 --> 00:13:25,512 To travel at the speed of light. 195 00:13:25,719 --> 00:13:30,520 What an engaging and magical thought for a teenage boy on the road... 196 00:13:30,724 --> 00:13:34,592 ...where the countryside is dappled and rippling in sunlight. 197 00:13:45,606 --> 00:13:50,009 You couldn't tell you were on a light wave if you were traveling with it. 198 00:13:50,210 --> 00:13:52,838 If you started on a wave crest... 199 00:13:53,047 --> 00:13:57,848 ...you would stay on the crest and lose all notion of it being a wave. 200 00:13:58,052 --> 00:14:02,887 Something funny happens at the speed of light. 201 00:14:33,487 --> 00:14:37,253 The more Einstein thought about it, the more troubling it became. 202 00:14:37,458 --> 00:14:40,018 Paradoxes seemed to pop up all over... 203 00:14:40,227 --> 00:14:42,195 ...if you could travel at the speed of light. 204 00:14:42,396 --> 00:14:45,797 Certain ideas had been accepted as true... 205 00:14:45,999 --> 00:14:48,467 ...without sufficiently careful thought. 206 00:14:50,604 --> 00:14:54,631 One of those ideas had to do with the light from a moving object. 207 00:14:56,110 --> 00:14:59,341 The images by which we see the world are made of light... 208 00:14:59,546 --> 00:15:01,673 ...and are carried at the speed of light... 209 00:15:01,882 --> 00:15:05,147 ...300,000 kilometers a second. 210 00:15:05,753 --> 00:15:09,553 You might think that the image of me should be moving out ahead of me... 211 00:15:09,757 --> 00:15:12,988 ...at the speed of light plus the speed of the bicycle. 212 00:15:13,193 --> 00:15:16,720 If I'm moving towards you faster than a horse-and-cart... 213 00:15:16,930 --> 00:15:19,899 ...then my image should be approaching you that much faster. 214 00:15:20,100 --> 00:15:22,694 My image ought to arrive earlier. 215 00:15:24,838 --> 00:15:27,363 But in reality you don't see any time delay. 216 00:15:27,808 --> 00:15:31,676 In a near collision, for example, you see everything happen at once. 217 00:15:31,879 --> 00:15:35,542 Horse, cart, swerve, bicycle. All simultaneous. 218 00:15:36,283 --> 00:15:40,242 But how would it look if it were proper to add the velocities? 219 00:15:40,454 --> 00:15:43,890 Since I'm heading toward you, you'd add my speed to the speed of light. 220 00:15:44,091 --> 00:15:48,858 So my image ought to arrive before the image of the horse-and-cart. 221 00:15:49,763 --> 00:15:52,254 I'd be cycling towards you quite normally. 222 00:15:52,466 --> 00:15:55,958 To me, a collision would seem imminent. 223 00:15:56,170 --> 00:15:59,071 But you'd see me swerve for no apparent reason... 224 00:15:59,273 --> 00:16:01,605 ...and have a collision with nothing. 225 00:16:02,643 --> 00:16:05,441 Now, the horse-and-cart aren't headed towards you. 226 00:16:05,646 --> 00:16:09,377 Their image would arrive only at the speed of light. 227 00:16:10,117 --> 00:16:12,711 Could it seem to me that I just missed colliding... 228 00:16:13,253 --> 00:16:15,813 ...while to you it wasn't even close? 229 00:16:16,023 --> 00:16:18,184 In precise laboratory experiments... 230 00:16:18,392 --> 00:16:21,520 ...scientists have never observed any such thing. 231 00:16:22,129 --> 00:16:24,495 If the world is to be understood... 232 00:16:24,832 --> 00:16:29,735 ...if we are to avoid logical paradoxes when traveling at high speeds... 233 00:16:29,937 --> 00:16:32,462 ...then there are rules which must be obeyed. 234 00:16:32,673 --> 00:16:37,406 Einstein called these rules the special theory of relativity. 235 00:16:37,611 --> 00:16:40,136 Light from a moving object travels at the same speed... 236 00:16:40,347 --> 00:16:43,976 ...no matter whether the object is at rest or in motion. 237 00:16:44,184 --> 00:16:48,848 "Thou shalt not add my speed to the speed of light." 238 00:16:49,056 --> 00:16:53,891 Also, no material object can travel at or beyond the speed of light. 239 00:16:54,094 --> 00:16:58,155 Nothing in physics prevents you from traveling close to the speed of light. 240 00:16:58,365 --> 00:17:02,165 99.9 percent the speed of light is just fine. 241 00:17:02,369 --> 00:17:04,735 But no matter how hard you try... 242 00:17:04,938 --> 00:17:07,702 ...you can never gain that last decimal point. 243 00:17:07,908 --> 00:17:10,172 For the world to be logically consistent... 244 00:17:10,377 --> 00:17:13,505 ...there must be a cosmic speed limit. 245 00:17:14,281 --> 00:17:16,977 The crack of a whip is, due to its tip... 246 00:17:17,184 --> 00:17:19,175 ...moving faster than the speed of sound. 247 00:17:21,822 --> 00:17:23,050 It makes a shock wave... 248 00:17:23,257 --> 00:17:27,091 ...a small sonic boom in the Italian countryside. 249 00:17:27,294 --> 00:17:29,626 A thunderclap has a similar origin. 250 00:17:29,830 --> 00:17:33,163 So does the sound of a supersonic airplane. 251 00:17:35,202 --> 00:17:39,764 So why is the speed of light a barrier any more than the speed of sound? 252 00:17:39,973 --> 00:17:41,838 The answer is not just that... 253 00:17:42,042 --> 00:17:44,636 ...light travels a million times faster than sound. 254 00:17:44,845 --> 00:17:48,508 It's not merely an engineering problem like the supersonic airplane. 255 00:17:49,082 --> 00:17:53,143 Instead, the light barrier is a fundamental law of nature... 256 00:17:53,353 --> 00:17:55,253 ...as basic as gravity. 257 00:17:55,455 --> 00:17:58,822 Einstein found his absolute framework for the world: 258 00:17:59,026 --> 00:18:03,395 This sturdy pillar among all the relative motions of the cosmos. 259 00:18:03,597 --> 00:18:07,658 Light travels just as fast, no matter how its source is moving. 260 00:18:07,868 --> 00:18:11,895 The speed of light is constant, relative to everything else. 261 00:18:12,105 --> 00:18:15,404 Nothing can ever catch up with light. 262 00:18:18,378 --> 00:18:21,905 Einstein's prohibition against traveling faster than light... 263 00:18:22,115 --> 00:18:24,845 ...seems to clash with our common sense notions. 264 00:18:25,052 --> 00:18:27,850 But why should we expect our common sense notions... 265 00:18:28,055 --> 00:18:30,956 ...to have any reliability in a matter of this sort? 266 00:18:31,158 --> 00:18:35,026 Why should our experience at 10 kilometers an hour... 267 00:18:35,228 --> 00:18:37,423 ...constrain the laws of nature... 268 00:18:37,631 --> 00:18:40,930 ...at 300,000 kilometers a second? 269 00:18:43,170 --> 00:18:45,764 Relativity sets limits... 270 00:18:45,973 --> 00:18:49,033 ...on what humans ultimately can do. 271 00:18:49,576 --> 00:18:52,204 The universe is not required... 272 00:18:52,412 --> 00:18:56,781 ...to be in perfect harmony with human ambition. 273 00:19:00,253 --> 00:19:03,154 Imagine a place where the speed of light... 274 00:19:03,357 --> 00:19:06,724 ...isn't its true value of 300,000 kilometers a second... 275 00:19:06,927 --> 00:19:09,487 ...but something a lot less. 276 00:19:09,696 --> 00:19:13,097 Let's say, 40 kilometers an hour... 277 00:19:13,300 --> 00:19:15,359 ...and strictly enforced. 278 00:19:16,269 --> 00:19:20,171 Just as in the real world we can never reach the speed of light... 279 00:19:20,374 --> 00:19:22,239 ...the commandment here is still... 280 00:19:22,442 --> 00:19:25,775 ..."Thou shalt not travel faster than light." 281 00:19:25,979 --> 00:19:30,473 We can do thought experiments on what happens near the speed of light... 282 00:19:30,684 --> 00:19:35,018 ...here 40 kilometers per hour, the speed of a motor scooter. 283 00:19:38,358 --> 00:19:42,351 You can't break the laws of nature. There are no penalties for doing so. 284 00:19:42,729 --> 00:19:44,594 The real world and this one... 285 00:19:44,931 --> 00:19:49,061 ...are merely so arranged that transgressions can't happen. 286 00:19:49,269 --> 00:19:53,205 The job of physics is to find out what those laws are. 287 00:19:55,542 --> 00:19:58,511 Before Einstein, physicists thought that... 288 00:19:58,712 --> 00:20:01,112 ...there were privileged frames of reference... 289 00:20:01,314 --> 00:20:03,805 ...some special places and times... 290 00:20:04,017 --> 00:20:06,577 ...against which everything else had to be measured. 291 00:20:06,787 --> 00:20:10,154 Einstein encountered a similar notion in human affairs. 292 00:20:10,357 --> 00:20:13,053 The idea that the customs of a particular nation... 293 00:20:13,260 --> 00:20:16,821 ...his native Germany or Italy or anywhere... 294 00:20:17,030 --> 00:20:20,966 ...are the standard which all other societies must be measured. 295 00:20:21,835 --> 00:20:25,271 But Einstein rejected the strident nationalism of his time. 296 00:20:25,472 --> 00:20:28,566 He believed every culture had its own validity. 297 00:20:28,775 --> 00:20:30,709 Also in physics, he understood that... 298 00:20:30,911 --> 00:20:33,175 ...there are no privileged frames of reference. 299 00:20:33,380 --> 00:20:36,508 Every observer, in any place, time or motion... 300 00:20:36,717 --> 00:20:39,345 ...must deduce the same laws of nature. 301 00:20:39,553 --> 00:20:41,453 (SPEAKING IN ITALIAN) 302 00:20:42,989 --> 00:20:46,823 A speed is simply how much space you cover in a given time... 303 00:20:47,027 --> 00:20:49,689 ...as any kid on a motor scooter knows. 304 00:20:53,133 --> 00:20:54,998 Since near the velocity of light... 305 00:20:55,202 --> 00:20:57,693 ...we cannot simply add speeds... 306 00:20:57,904 --> 00:21:01,533 ...the familiar notions of absolute space and absolute time... 307 00:21:01,742 --> 00:21:04,734 ...independent of your relative motion, must give way. 308 00:21:04,945 --> 00:21:07,505 That's why, as Einstein showed... 309 00:21:07,714 --> 00:21:11,673 ...funny things have to happen close to the speed of light. 310 00:21:12,419 --> 00:21:16,253 There, our conventional perspectives of space and time... 311 00:21:16,456 --> 00:21:18,515 ...strangely change. 312 00:21:20,961 --> 00:21:24,897 Your nose is just a little closer to me than your ears. 313 00:21:25,098 --> 00:21:27,328 Light reflected off your nose reaches me... 314 00:21:27,534 --> 00:21:29,764 ...an instant in time before your ears. 315 00:21:29,970 --> 00:21:33,201 But suppose I had a magic camera... 316 00:21:33,406 --> 00:21:36,398 ...so that I could see your nose and your ears... 317 00:21:36,610 --> 00:21:38,578 ...at precisely the same instant? 318 00:21:38,779 --> 00:21:40,246 (SCOOTER STARTS UP) 319 00:21:40,447 --> 00:21:41,914 (SCOOTER HONKS) 320 00:21:42,115 --> 00:21:46,552 With such a camera you could take some pretty interesting pictures. 321 00:21:47,921 --> 00:21:51,322 Paolo says goodbye to his little brother, Vincenzo... 322 00:21:51,691 --> 00:21:53,784 -Ciao, Vincenzo. -Ciao, Paolo. 323 00:21:54,594 --> 00:21:56,221 ...and rides off. 324 00:21:56,429 --> 00:21:58,954 He's now going more than half the speed of light. 325 00:21:59,166 --> 00:22:01,726 He is almost catching up with his own light waves. 326 00:22:01,935 --> 00:22:04,495 This compresses the light waves in front of him... 327 00:22:04,704 --> 00:22:06,433 ...and his image becomes blue. 328 00:22:06,740 --> 00:22:10,437 The shorter wavelength is what makes blue light waves blue. 329 00:22:11,011 --> 00:22:14,742 Also Paolo becomes skinny in the direction of motion. 330 00:22:14,948 --> 00:22:17,007 This isn't just some optical illusion. 331 00:22:17,217 --> 00:22:20,448 It really happens when you travel near the speed of light. 332 00:22:21,221 --> 00:22:25,555 As he roars away, he leaves his own light waves stretched out behind him. 333 00:22:25,759 --> 00:22:27,124 Long light waves are red. 334 00:22:27,327 --> 00:22:30,888 We say that his receding image is red-shifted. 335 00:22:32,465 --> 00:22:37,129 Now Paolo leaves for a short tour of the countryside. 336 00:22:37,571 --> 00:22:41,098 He experiences something even stranger. 337 00:22:43,710 --> 00:22:45,940 Everything he can see is squeezed... 338 00:22:46,146 --> 00:22:48,512 ...into a moving window just ahead of him... 339 00:22:48,715 --> 00:22:52,116 ...blue-shifted at the center, red-shifted at the edges. 340 00:22:52,319 --> 00:22:55,618 To a passerby, Paolo appears blue-shifted when approaching... 341 00:22:55,822 --> 00:22:57,551 ...red-shifted when receding. 342 00:22:57,757 --> 00:23:00,817 But to him, the entire world is both coming and going... 343 00:23:01,027 --> 00:23:02,756 ...at nearly the speed of light. 344 00:23:02,963 --> 00:23:06,592 Roadside houses and trees that has already gone past... 345 00:23:06,800 --> 00:23:10,236 ...still appear to him at the edge of his forward field of view... 346 00:23:10,437 --> 00:23:13,133 ...but distorted and red-shifted. 347 00:23:14,307 --> 00:23:18,038 When he slows down, everything again looks normal. 348 00:23:19,579 --> 00:23:21,945 Only very close to the speed of light... 349 00:23:22,148 --> 00:23:25,743 ...does the visible world get squeezed into a kind of tunnel. 350 00:23:26,653 --> 00:23:30,111 You'd see these distortions if you traveled near the speed of light. 351 00:23:30,323 --> 00:23:33,019 Someday, perhaps, interstellar navigators... 352 00:23:33,226 --> 00:23:35,751 ...will take their bearings on stars behind them... 353 00:23:35,962 --> 00:23:40,456 ...whose images have all crowded together on the forward view screen. 354 00:23:42,736 --> 00:23:45,864 The most bizarre aspect of traveling near the speed of light... 355 00:23:46,072 --> 00:23:48,666 ...is that time slows down. 356 00:23:49,509 --> 00:23:51,807 All clocks, mechanical and biological... 357 00:23:52,012 --> 00:23:54,810 ...tick more slowly near the speed of light. 358 00:23:55,015 --> 00:23:58,348 But stationary clocks tick at their usual rate. 359 00:23:58,551 --> 00:24:00,712 If we travel close to light speed... 360 00:24:00,921 --> 00:24:04,015 ...we age more slowly than those we left behind. 361 00:24:09,462 --> 00:24:12,295 Paolo's watch and his internal sense of time show... 362 00:24:12,832 --> 00:24:16,359 ...that he has been gone from his friends for only a few minutes. 363 00:24:16,870 --> 00:24:21,136 But from their point of view, he has been away for many decades. 364 00:24:21,341 --> 00:24:24,936 His friends have grown up, moved on and died. 365 00:24:25,912 --> 00:24:27,539 And his younger brother has been... 366 00:24:27,747 --> 00:24:31,080 ...patiently waiting for him all this time. 367 00:24:33,219 --> 00:24:38,156 The two brothers experience the paradox of time dilation. 368 00:24:38,358 --> 00:24:42,055 They've encountered Einstein's special relativity. 369 00:24:43,096 --> 00:24:44,063 Vincenzo. 370 00:24:57,944 --> 00:25:00,378 This was just a thought experiment. 371 00:25:00,580 --> 00:25:03,743 But atomic particles traveling near the speed of light... 372 00:25:03,950 --> 00:25:07,147 ...do decay more slowly than stationary particles. 373 00:25:07,354 --> 00:25:10,551 As strange and counterintuitive as it seems... 374 00:25:10,757 --> 00:25:14,022 ...time dilation is a law of nature. 375 00:25:15,895 --> 00:25:18,693 Traveling close to the speed of light... 376 00:25:18,999 --> 00:25:21,900 ...is a kind of elixir of life. 377 00:25:22,869 --> 00:25:25,963 Because time slows down close to the speed of light... 378 00:25:26,172 --> 00:25:29,039 ...special relativity provides us... 379 00:25:29,242 --> 00:25:31,938 ...with a means of going to the stars. 380 00:25:32,979 --> 00:25:36,176 This region of northern Italy is not only the caldron... 381 00:25:36,383 --> 00:25:39,682 ...of some of the thinking of the young Albert Einstein... 382 00:25:40,020 --> 00:25:43,353 ...it is also the home of another great genius... 383 00:25:43,556 --> 00:25:45,854 ...who lived 400 years earlier. 384 00:25:46,059 --> 00:25:48,357 Leonardo da Vinci. 385 00:25:49,629 --> 00:25:53,998 Leonardo delighted in climbing these hills... 386 00:25:54,200 --> 00:25:57,567 ...and viewing the ground from a great height... 387 00:25:57,771 --> 00:26:00,069 ...as if he were soaring like a bird. 388 00:26:00,273 --> 00:26:02,867 He drew the first aerial views... 389 00:26:03,076 --> 00:26:06,773 ...of landscapes, villages, fortifications. 390 00:26:07,247 --> 00:26:11,183 I've been talking about Einstein in and around this town of Vinci... 391 00:26:11,384 --> 00:26:13,545 ...in which Leonardo grew up. 392 00:26:13,753 --> 00:26:16,813 Einstein greatly respected Leonardo... 393 00:26:17,023 --> 00:26:19,685 ...and their spirits, in some sense... 394 00:26:19,893 --> 00:26:23,192 ...inhabit this countryside still. 395 00:26:48,154 --> 00:26:51,214 Among Leonardo's many accomplishments... 396 00:26:51,424 --> 00:26:54,791 ...in painting, sculpture, architecture, natural history... 397 00:26:54,994 --> 00:26:59,431 ...anatomy, geology, civil and military engineering... 398 00:26:59,933 --> 00:27:01,798 ...he had a great passion. 399 00:27:02,035 --> 00:27:05,527 He wished to construct a machine... 400 00:27:05,738 --> 00:27:07,399 ...which would fly. 401 00:27:07,941 --> 00:27:11,741 He made sketches of such machines, built miniature models... 402 00:27:11,945 --> 00:27:16,075 ...constructed great, full-scale prototypes. 403 00:27:17,884 --> 00:27:21,547 And not a one of them ever worked. 404 00:27:22,755 --> 00:27:27,124 There were no machines of adequate capacity available in his time. 405 00:27:27,327 --> 00:27:30,694 The technology was just not ready. 406 00:27:31,598 --> 00:27:34,829 The designs, however, were brilliant. 407 00:27:35,034 --> 00:27:38,197 For example, this bird-like machine... 408 00:27:38,404 --> 00:27:42,397 ...here in the Leonardo Museum in the town of Vinci. 409 00:27:43,576 --> 00:27:48,513 Leonardo's great designs encouraged engineers in later epochs... 410 00:27:48,715 --> 00:27:53,084 ...although Leonardo himself was very depressed at these failures. 411 00:27:53,286 --> 00:27:55,117 But it's not his fault... 412 00:27:55,321 --> 00:27:58,347 ...he was trapped in the 15th century. 413 00:27:59,192 --> 00:28:02,889 A somewhat similar case occurred in 1939... 414 00:28:03,096 --> 00:28:07,430 ...when a group of engineers called the British Interplanetary Society... 415 00:28:07,634 --> 00:28:09,898 ...decided to design a ship... 416 00:28:10,103 --> 00:28:13,072 ...which would carry people to the moon. 417 00:28:13,273 --> 00:28:15,571 Now, it was by no means the same design... 418 00:28:15,775 --> 00:28:20,439 ...as the Apollo ship which actually took people to the moon years later. 419 00:28:20,647 --> 00:28:23,207 But that design suggested that... 420 00:28:23,416 --> 00:28:25,213 ...a mission to the moon might one day... 421 00:28:25,418 --> 00:28:28,012 ...be a practical engineering possibility. 422 00:28:28,488 --> 00:28:29,682 Today... 423 00:28:30,690 --> 00:28:34,319 ...we have preliminary designs of ships... 424 00:28:34,527 --> 00:28:37,724 ...which will take people to the stars. 425 00:28:37,931 --> 00:28:42,630 They are constructed in Earth orbit and from there... 426 00:28:42,835 --> 00:28:47,772 ...they venture on their great interstellar journeys. 427 00:28:48,141 --> 00:28:49,540 One of them... 428 00:28:50,009 --> 00:28:53,001 ...is called Project Orion. 429 00:28:54,147 --> 00:28:56,081 It utilizes nuclear weapons... 430 00:28:56,282 --> 00:29:00,719 ...hydrogen bombs against an inertial plate. 431 00:29:00,920 --> 00:29:04,651 Each explosion providing a kind of "putt-putt"... 432 00:29:04,857 --> 00:29:08,793 ...a vast nuclear motorboat in space. 433 00:29:09,195 --> 00:29:12,631 Orion seems entirely practical... 434 00:29:12,832 --> 00:29:15,266 ...and was under development in the U.S... 435 00:29:15,468 --> 00:29:18,835 ...until the signing of the international treaty... 436 00:29:19,038 --> 00:29:22,235 ...forbidding nuclear weapons explosions in space. 437 00:29:22,842 --> 00:29:27,779 I think, the Orion starship is the best use of nuclear weapons... 438 00:29:27,981 --> 00:29:31,815 ...provided the ships don't depart from very near the Earth. 439 00:29:41,394 --> 00:29:44,124 Project Daedalus is a recent design... 440 00:29:44,330 --> 00:29:46,730 ...of the British Interplanetary Society. 441 00:29:46,933 --> 00:29:50,425 It assumes the existence of a nuclear fusion reactor... 442 00:29:50,637 --> 00:29:52,935 ...something much safer and more efficient... 443 00:29:53,139 --> 00:29:56,939 ...than the existing nuclear fission power plants. 444 00:30:01,180 --> 00:30:03,444 We do not yet have fusion reactors. 445 00:30:03,650 --> 00:30:06,380 One day, quite soon, we may. 446 00:30:12,091 --> 00:30:15,492 Orion and Daedalus might go... 447 00:30:15,695 --> 00:30:18,323 ...10 percent the speed of light. 448 00:30:19,465 --> 00:30:22,332 So a trip to Alpha Centauri, 4 1/2 light-years away... 449 00:30:22,535 --> 00:30:26,369 ...would take 45 years, less than a human lifetime. 450 00:30:26,939 --> 00:30:30,773 Such ships could not travel close enough to the speed of light... 451 00:30:30,977 --> 00:30:33,912 ...for the time-slowing effects of special relativity... 452 00:30:34,113 --> 00:30:35,774 ...to become important. 453 00:30:36,349 --> 00:30:39,045 It does not seem likely that such ships... 454 00:30:39,252 --> 00:30:41,846 ...would be built before the middle of the 21 st century... 455 00:30:42,055 --> 00:30:45,855 ...although we could build an Orion starship now. 456 00:30:46,292 --> 00:30:50,353 For voyages beyond the nearest stars, something must be added. 457 00:30:50,563 --> 00:30:53,657 Perhaps they could be used as multigeneration ships... 458 00:30:54,033 --> 00:30:57,025 ...so those arriving would be the remote descendants... 459 00:30:57,236 --> 00:31:00,967 ...of those who had originally set out centuries before. 460 00:31:01,607 --> 00:31:05,634 Or perhaps some safe means of human hibernation might be found... 461 00:31:05,845 --> 00:31:09,713 ...so that space travelers might be frozen and then thawed out... 462 00:31:09,916 --> 00:31:13,818 ...when they arrive at the destination centuries later. 463 00:31:14,520 --> 00:31:18,820 But fast interstellar space flight approaching the speed of light... 464 00:31:19,025 --> 00:31:20,890 ...is much more difficult. 465 00:31:21,094 --> 00:31:23,824 That's an objective not for a hundred years... 466 00:31:24,030 --> 00:31:27,158 ...but for a thousand or for 10 thousand... 467 00:31:27,367 --> 00:31:29,835 ...but it also is possible. 468 00:31:32,205 --> 00:31:35,174 A kind of interstellar ramjet has been proposed... 469 00:31:35,375 --> 00:31:37,809 ...which scoops up the hydrogen atoms... 470 00:31:38,010 --> 00:31:39,944 ...which float between the stars... 471 00:31:40,146 --> 00:31:44,207 ...accelerates them into an engine and spits them out the back. 472 00:31:45,051 --> 00:31:47,451 But in deep space, there is one atom... 473 00:31:47,653 --> 00:31:51,714 ...for every 10 cubic centimeters of space. 474 00:31:51,924 --> 00:31:53,391 For the ramjet to work... 475 00:31:53,893 --> 00:31:56,453 ...it has to have a frontal scoop... 476 00:31:56,662 --> 00:31:59,529 ...hundreds of kilometers across. 477 00:31:59,799 --> 00:32:03,997 Reaching relativistic velocities, the hydrogen atoms will be moving... 478 00:32:04,203 --> 00:32:06,763 ...with respect to the interstellar spaceship... 479 00:32:06,973 --> 00:32:08,998 ...at close to the speed of light. 480 00:32:09,208 --> 00:32:10,937 If precautions aren't taken... 481 00:32:11,144 --> 00:32:15,706 ...the passengers will be fried by these induced cosmic rays. 482 00:32:15,915 --> 00:32:17,780 There's a proposed solution: 483 00:32:17,984 --> 00:32:21,283 A laser is used to strip electrons off the atoms... 484 00:32:21,487 --> 00:32:24,718 ...and electrically charge them while they're some distance away. 485 00:32:25,291 --> 00:32:27,919 And an extremely strong magnetic field... 486 00:32:28,161 --> 00:32:31,426 ...is used to deflect the charged atoms into the scoop... 487 00:32:31,631 --> 00:32:33,258 ...and away from the spacecraft. 488 00:32:33,466 --> 00:32:34,763 This is engineering... 489 00:32:34,967 --> 00:32:38,767 ...on a scale so far unprecedented on the Earth. 490 00:32:38,971 --> 00:32:43,567 We are talking of engines the size of small worlds. 491 00:32:52,985 --> 00:32:57,888 Suppose that the spacecraft is designed to accelerate at 1 g... 492 00:32:58,090 --> 00:33:00,388 ...so we'd be comfortable aboard it. 493 00:33:00,593 --> 00:33:03,027 We'd go closer and closer to the speed of light... 494 00:33:03,229 --> 00:33:05,493 ...until the midpoint of the journey. 495 00:33:05,698 --> 00:33:08,258 Then the spacecraft is turned around... 496 00:33:08,468 --> 00:33:12,029 ...and we decelerate at 1 g to the destination. 497 00:33:12,605 --> 00:33:16,735 For most of the trip, the velocity would be close to the speed of light... 498 00:33:16,943 --> 00:33:20,106 ...and time would slow down enormously. 499 00:33:20,446 --> 00:33:21,970 By how much? 500 00:33:22,949 --> 00:33:26,385 Barnard's Star could be reached by such a ship... 501 00:33:26,586 --> 00:33:29,214 ...in eight years, ship time. 502 00:33:29,822 --> 00:33:33,781 The center of the Milky Way galaxy in 21 years. 503 00:33:33,993 --> 00:33:37,861 The Andromeda galaxy in 28 years. 504 00:33:38,397 --> 00:33:40,331 Of course, the people left behind on the Earth... 505 00:33:40,533 --> 00:33:42,831 ...would see things somewhat differently. 506 00:33:43,102 --> 00:33:45,070 Instead of 21 years to the galaxy... 507 00:33:45,271 --> 00:33:48,832 ...they would measure it as 30,000 years. 508 00:33:49,041 --> 00:33:50,406 When we got back... 509 00:33:50,610 --> 00:33:54,068 ...very few of our friends would be around to greet us. 510 00:33:54,914 --> 00:33:56,677 In principle, such a journey... 511 00:33:56,883 --> 00:34:01,286 ...mounting the decimal points closer and closer to the speed of light... 512 00:34:01,487 --> 00:34:05,253 ...would even permit us to circumnavigate the known universe... 513 00:34:05,458 --> 00:34:08,518 ...in 56 years, ship time. 514 00:34:09,462 --> 00:34:13,558 We would return tens of billions of years... 515 00:34:13,766 --> 00:34:15,757 ...in the far future... 516 00:34:15,968 --> 00:34:19,028 ...with the Earth a charred cinder... 517 00:34:19,238 --> 00:34:21,706 ...and the sun dead. 518 00:34:22,341 --> 00:34:25,742 Relativistic space flight makes the universe accessible... 519 00:34:25,945 --> 00:34:28,311 ...to advanced civilizations... 520 00:34:28,514 --> 00:34:30,709 ...but only to those who go on the journey... 521 00:34:30,917 --> 00:34:33,283 ...not to those who stay home. 522 00:34:34,120 --> 00:34:38,454 These designs are probably further... 523 00:34:38,658 --> 00:34:42,389 ...from the actual interstellar spacecraft of the future... 524 00:34:43,596 --> 00:34:46,394 ...than Leonardo's models are... 525 00:34:46,599 --> 00:34:49,932 ...from the supersonic transports of the present. 526 00:34:50,503 --> 00:34:52,368 But if we do not destroy ourselves... 527 00:34:52,572 --> 00:34:57,475 ...I believe that we will, one day, venture to the stars. 528 00:34:58,144 --> 00:35:00,704 When our solar system is all explored... 529 00:35:00,913 --> 00:35:04,041 ...the planets of other stars will beckon. 530 00:35:37,850 --> 00:35:42,048 Space travel and time travel are connected. 531 00:35:43,189 --> 00:35:45,123 To travel fast into space... 532 00:35:45,324 --> 00:35:48,521 ...is to travel fast into the future. 533 00:35:51,397 --> 00:35:55,561 We travel into the future, although slowly, all the time. 534 00:35:55,768 --> 00:36:00,068 But what about the past? Could we journey into yesterday? 535 00:36:00,272 --> 00:36:03,400 Many physicists think this is fundamentally impossible... 536 00:36:03,609 --> 00:36:05,941 ...that we could not build a device... 537 00:36:06,145 --> 00:36:08,875 ...which would carry us backwards into time. 538 00:36:09,081 --> 00:36:12,482 Some say that even if we were to build such a device... 539 00:36:12,685 --> 00:36:13,947 ...it wouldn't do much good. 540 00:36:14,153 --> 00:36:16,678 We couldn't significantly affect the past. 541 00:36:16,889 --> 00:36:20,552 For example, suppose you traveled into the past... 542 00:36:20,760 --> 00:36:22,853 ...and somehow or other prevented... 543 00:36:23,062 --> 00:36:25,860 ...your own parents from meeting. 544 00:36:26,065 --> 00:36:29,626 Why, then you would probably never have been born... 545 00:36:29,835 --> 00:36:31,860 ...which is something of a contradiction, isn't it... 546 00:36:32,071 --> 00:36:34,198 ...since you are clearly there. 547 00:36:34,807 --> 00:36:36,138 Other people think that... 548 00:36:36,342 --> 00:36:39,470 ...the two alternative histories have equal validity... 549 00:36:39,679 --> 00:36:43,080 ...that they're parallel threads, skeins of time... 550 00:36:43,282 --> 00:36:45,842 ...that they could exist side by side. 551 00:36:49,722 --> 00:36:52,247 The history in which you were never born... 552 00:36:52,458 --> 00:36:55,427 ...and the history that you know all about. 553 00:36:55,661 --> 00:36:58,858 Perhaps time itself has many potential dimensions... 554 00:36:59,065 --> 00:37:02,330 ...despite the fact that we are condemned to experience... 555 00:37:02,535 --> 00:37:04,730 ...only one of those dimensions. 556 00:37:05,371 --> 00:37:08,636 Now, suppose you could go back into the past... 557 00:37:08,841 --> 00:37:12,299 ...and really change it by, let's say something like... 558 00:37:12,511 --> 00:37:17,005 ...persuading Queen Isabella not to bankroll Christopher Columbus. 559 00:37:17,216 --> 00:37:19,480 Then you would have set into motion... 560 00:37:19,685 --> 00:37:22,711 ...a different sequence of historical events... 561 00:37:22,922 --> 00:37:25,982 ...which those people you left behind you in our time... 562 00:37:26,192 --> 00:37:28,353 ...would never get to know about. 563 00:37:28,561 --> 00:37:31,291 If that kind of time travel were possible... 564 00:37:31,497 --> 00:37:34,694 ...then every imaginable sequence... 565 00:37:34,900 --> 00:37:36,868 ...of alternative history... 566 00:37:37,069 --> 00:37:39,264 ...might in some sense really exist. 567 00:37:40,239 --> 00:37:42,298 Would it be possible for a time traveler... 568 00:37:42,508 --> 00:37:45,875 ...to change the course of history in a major way? 569 00:37:46,078 --> 00:37:48,308 Well, let's think about that. 570 00:37:51,884 --> 00:37:54,148 History consists for the most part... 571 00:37:54,353 --> 00:37:58,414 ...of a complex multitude of deeply interwoven threads... 572 00:37:58,624 --> 00:38:01,286 ...biological, economic and social forces... 573 00:38:01,494 --> 00:38:03,985 ...that are not so easily unraveled. 574 00:38:05,264 --> 00:38:09,894 The ancient Greeks imagined the course of human events to be a tapestry... 575 00:38:10,102 --> 00:38:13,868 ...created by three goddesses: the Fates. 576 00:38:15,541 --> 00:38:19,773 Random minor events generally have no long-range consequences. 577 00:38:19,979 --> 00:38:23,176 But some which occur at critical junctures... 578 00:38:23,382 --> 00:38:25,907 ...may alter the weave of history. 579 00:38:26,118 --> 00:38:29,110 There may be cases where profound changes can be made... 580 00:38:29,321 --> 00:38:31,812 ...by relatively trivial adjustments. 581 00:38:32,024 --> 00:38:36,791 The further in the past such an event is, the more powerful its influence. 582 00:38:37,296 --> 00:38:40,754 What if our time traveler had persuaded Queen Isabella that... 583 00:38:40,966 --> 00:38:42,866 ...Columbus' geography was wrong? 584 00:38:43,068 --> 00:38:47,266 Almost certainly, some other European would have sailed to the New World. 585 00:38:47,473 --> 00:38:49,134 There were many inducements: 586 00:38:49,341 --> 00:38:52,401 The lure of the spice trade, improvements in navigation... 587 00:38:52,611 --> 00:38:55,045 ...competition among rival European powers. 588 00:38:55,247 --> 00:38:59,149 The discovery of America around 1500 was inevitable. 589 00:38:59,351 --> 00:39:02,752 Of course, there wouldn't be any postage stamps showing Columbus... 590 00:39:02,955 --> 00:39:05,890 ...and the Republic of Colombia would have another name. 591 00:39:06,091 --> 00:39:10,084 But the big picture would have turned out more or less the same. 592 00:39:14,700 --> 00:39:17,897 In order to affect the future profoundly... 593 00:39:18,103 --> 00:39:20,663 ...a time traveler has to pick and choose. 594 00:39:20,873 --> 00:39:24,331 He'd probably have to intervene in a number of events... 595 00:39:24,543 --> 00:39:27,239 ...which are very carefully selected... 596 00:39:27,446 --> 00:39:32,281 ...so he could change the weave of history. 597 00:39:32,685 --> 00:39:35,210 It's a lovely fantasy... 598 00:39:35,421 --> 00:39:39,357 ...to explore those other worlds that never were. 599 00:39:41,861 --> 00:39:45,456 If you had H.G. Wells' time machine... 600 00:39:45,664 --> 00:39:48,690 ...maybe you could understand how history really works. 601 00:39:48,901 --> 00:39:51,768 If an apparently pivotal person had never lived... 602 00:39:51,971 --> 00:39:56,135 ...Paul the Apostle or Peter the Great or Pythagoras... 603 00:39:56,342 --> 00:39:59,038 ...how different would the world really be? 604 00:39:59,845 --> 00:40:01,938 What if the scientific tradition... 605 00:40:02,147 --> 00:40:04,911 ...of the ancient Ionian Greeks... 606 00:40:05,117 --> 00:40:07,779 ...had prospered and flourished? 607 00:40:07,987 --> 00:40:10,854 It would have required many social factors at the time... 608 00:40:11,056 --> 00:40:12,785 ...to have been different... 609 00:40:12,992 --> 00:40:15,426 ...including the common feeling... 610 00:40:15,628 --> 00:40:18,324 ...that slavery was right and natural. 611 00:40:18,530 --> 00:40:22,091 But what if that light that had dawned... 612 00:40:22,301 --> 00:40:25,668 ...on the eastern Mediterranean some 2500 years ago... 613 00:40:25,871 --> 00:40:27,839 ...had not flickered out? 614 00:40:28,040 --> 00:40:31,339 What if scientific method and experiment... 615 00:40:31,543 --> 00:40:33,534 ...had been vigorously pursued... 616 00:40:33,746 --> 00:40:36,010 ...2000 years before the industrial revolution... 617 00:40:36,215 --> 00:40:38,183 ...our industrial revolution? 618 00:40:38,384 --> 00:40:42,445 What if the power of this new mode of thought, the scientific method... 619 00:40:42,655 --> 00:40:44,953 ...had been generally appreciated? 620 00:40:45,324 --> 00:40:49,055 I think we might have saved 10 or 20 centuries. 621 00:40:49,261 --> 00:40:51,957 Perhaps the contributions that Leonardo made... 622 00:40:52,164 --> 00:40:54,997 ...would have been made 1000 years earlier... 623 00:40:55,200 --> 00:40:59,000 ...and the contributions of Einstein 500 years ago. 624 00:40:59,204 --> 00:41:01,331 Not that it would have been those people... 625 00:41:01,540 --> 00:41:03,974 ...who would've made those contributions... 626 00:41:04,176 --> 00:41:07,168 ...because they lived only in our timeline. 627 00:41:07,746 --> 00:41:10,681 If the Ionians had won... 628 00:41:10,883 --> 00:41:14,876 ...we might by now, I think, be going to the stars. 629 00:41:15,087 --> 00:41:19,820 We might at this moment have the first survey ships... 630 00:41:20,025 --> 00:41:24,689 ...returning with astonishing results from Alpha Centauri... 631 00:41:24,897 --> 00:41:29,493 ...and Barnard's Star, Sirius and Tau Ceti. 632 00:41:29,702 --> 00:41:33,001 There would now be great fleets... 633 00:41:33,205 --> 00:41:35,173 ...of interstellar transports... 634 00:41:35,374 --> 00:41:37,865 ...being constructed in Earth orbit... 635 00:41:38,077 --> 00:41:41,069 ...small, unmanned survey ships... 636 00:41:41,280 --> 00:41:44,738 ...liners for immigrants, perhaps... 637 00:41:44,950 --> 00:41:46,349 ...great trading ships... 638 00:41:46,552 --> 00:41:50,010 ...to ply the spaces between the stars. 639 00:41:50,489 --> 00:41:53,686 On all these ships there would be symbols... 640 00:41:53,892 --> 00:41:56,622 ...and inscriptions on the sides. 641 00:41:56,829 --> 00:41:59,093 The inscriptions, if we looked closely... 642 00:41:59,298 --> 00:42:01,926 ...would be written in Greek. 643 00:42:02,434 --> 00:42:03,765 The symbol... 644 00:42:03,969 --> 00:42:07,427 ...perhaps, would be the dodecahedron. 645 00:42:07,639 --> 00:42:12,269 And the inscription on the sides of the ships to the stars... 646 00:42:12,478 --> 00:42:13,945 ...something like: 647 00:42:14,146 --> 00:42:18,879 "Starship Theodorus of the Planet Earth." 648 00:42:21,887 --> 00:42:24,685 If you were a really ambitious time traveler... 649 00:42:27,893 --> 00:42:30,657 ...you might not dally with human history... 650 00:42:30,863 --> 00:42:33,491 ...or even pause to examine the evolution on Earth. 651 00:42:33,699 --> 00:42:35,929 Instead, you would journey back... 652 00:42:36,135 --> 00:42:37,534 ...to witness the origin of our solar system... 653 00:42:38,003 --> 00:42:42,531 ...from the gas and dust between the stars. 654 00:42:43,709 --> 00:42:45,142 Five billion years ago... 655 00:42:45,344 --> 00:42:48,939 ...an interstellar cloud was collapsing to form our solar system. 656 00:42:49,148 --> 00:42:52,379 Most clumps of matter gravitated towards the center... 657 00:42:52,584 --> 00:42:55,144 ...and were destined to form the sun. 658 00:42:55,354 --> 00:42:59,586 Smaller peripheral clumps would become the planets. 659 00:42:59,792 --> 00:43:04,195 Long ago, there was a kind of natural selection among the worlds. 660 00:43:04,396 --> 00:43:08,594 Those on highly elliptical orbits tended to collide and be destroyed... 661 00:43:08,801 --> 00:43:12,396 ...but planets in circular orbits tended to survive. 662 00:43:12,604 --> 00:43:14,868 But if events had been a little different... 663 00:43:15,074 --> 00:43:16,735 ...the Earth would never have formed... 664 00:43:16,942 --> 00:43:20,901 ...and another planet at another distance from the sun would be around. 665 00:43:21,113 --> 00:43:23,377 We owe the existence of our world... 666 00:43:23,582 --> 00:43:27,416 ...to random collisions in a long-vanished cloud. 667 00:43:30,255 --> 00:43:33,315 Soon, the central mass became very hot. 668 00:43:33,525 --> 00:43:37,359 Thermonuclear reactions were initiated and the sun turned on... 669 00:43:37,563 --> 00:43:40,430 ...flooding the solar system with light. 670 00:43:42,801 --> 00:43:44,769 But the growing smaller lumps... 671 00:43:44,970 --> 00:43:47,200 ...would never achieve such high temperatures... 672 00:43:47,406 --> 00:43:50,239 ...and would never generate thermonuclear reactions. 673 00:43:50,442 --> 00:43:54,173 They would become the Earth and the other planets... 674 00:43:54,379 --> 00:43:58,611 ...heated not from within, but mainly by the distant sun. 675 00:44:03,288 --> 00:44:05,085 The accretion continued until... 676 00:44:05,290 --> 00:44:08,555 ...almost all the gas and dust and small worldlets... 677 00:44:08,760 --> 00:44:12,161 ...were swept up by the surviving planets. 678 00:44:14,533 --> 00:44:16,831 Our time traveler would witness... 679 00:44:17,035 --> 00:44:19,970 ...the collisions that made the worlds. 680 00:44:26,145 --> 00:44:28,170 Except for the comets and asteroids... 681 00:44:28,380 --> 00:44:31,008 ...the chaos of the early solar system was reduced... 682 00:44:31,216 --> 00:44:33,548 ...to a remarkable simplicity: 683 00:44:33,752 --> 00:44:37,586 Nine or so principal planets in almost circular orbits... 684 00:44:37,789 --> 00:44:39,848 ...and a few dozen moons. 685 00:44:44,329 --> 00:44:47,264 Now, let's take a different look. 686 00:44:48,901 --> 00:44:51,369 If we view the solar system edge on... 687 00:44:51,570 --> 00:44:53,834 ...and move the sun off-screen to the left... 688 00:44:54,039 --> 00:44:56,701 ...we see that the small terrestrial planets... 689 00:44:56,909 --> 00:45:00,538 ...the ones about as massive as Earth, tend to be close to the sun. 690 00:45:00,746 --> 00:45:04,580 The big Jupiter-like planets tend to be much further from the sun. 691 00:45:04,783 --> 00:45:07,650 But is that the way it has to be? 692 00:45:08,854 --> 00:45:10,515 Computer studies suggest... 693 00:45:10,722 --> 00:45:13,486 ...that there may be many similar systems about stars... 694 00:45:13,692 --> 00:45:18,061 ...with the terrestrials in close and the Jovian planets further away. 695 00:45:21,934 --> 00:45:25,665 But some systems might have Jovians and terrestrials mixed together. 696 00:45:25,871 --> 00:45:30,399 There may be great worlds like Jupiter looming in other skies. 697 00:45:31,710 --> 00:45:35,669 Rarely, the Jovian planets may form close to the star... 698 00:45:35,881 --> 00:45:40,215 ...the terrestrials trailing away towards interstellar space. 699 00:45:41,553 --> 00:45:43,544 Our familiar arrangement of planets... 700 00:45:43,755 --> 00:45:46,519 ...is only one, perhaps typical, case... 701 00:45:46,725 --> 00:45:50,786 ...in the vast expanse of systems. 702 00:45:50,996 --> 00:45:55,626 Often, one fledgling planet accumulates so much gas and dust... 703 00:45:55,834 --> 00:45:58,029 ...that thermonuclear reactions do occur. 704 00:45:58,237 --> 00:46:00,330 It becomes a second sun. 705 00:46:00,539 --> 00:46:03,372 A binary star system has formed. 706 00:46:07,879 --> 00:46:11,781 From most of these worlds, the vistas will be dazzling. 707 00:46:11,984 --> 00:46:14,350 Not one of them will be identical to the Earth. 708 00:46:14,553 --> 00:46:18,922 A few will be hospitable. Many will appear hostile. 709 00:46:20,325 --> 00:46:22,350 Where there are two suns in the sky... 710 00:46:22,561 --> 00:46:26,258 ...every object will cast two shadows. 711 00:46:30,435 --> 00:46:33,063 What wonders are waiting for us... 712 00:46:33,272 --> 00:46:35,638 ...on the planets of the nearby stars? 713 00:46:35,841 --> 00:46:38,708 Are there radically different kinds of worlds... 714 00:46:38,910 --> 00:46:41,970 ...unimaginably exotic forms of life? 715 00:46:45,250 --> 00:46:48,151 Perhaps in another century or two... 716 00:46:48,353 --> 00:46:50,344 ...when our solar system is all explored... 717 00:46:50,555 --> 00:46:53,786 ...we will also have put our own planet in order. 718 00:46:53,992 --> 00:46:57,450 Then we will set sail for the stars... 719 00:46:57,663 --> 00:47:00,325 ...and the beckoning worlds around them. 720 00:47:03,769 --> 00:47:06,897 In that day, our machines and our descendants... 721 00:47:07,105 --> 00:47:10,802 ...approaching the speed of light, will skim the light-years... 722 00:47:11,009 --> 00:47:15,639 ...leaping ahead through time, seeking new worlds. 723 00:47:15,847 --> 00:47:19,374 Einstein has shown us that it's possible. 724 00:47:20,619 --> 00:47:22,712 We will journey simultaneously... 725 00:47:22,921 --> 00:47:26,322 ...to distant planets and to the far future. 726 00:47:27,125 --> 00:47:28,820 Some worlds, like this one... 727 00:47:29,027 --> 00:47:32,292 ...will look out onto a vast gaseous nebula... 728 00:47:32,497 --> 00:47:34,226 ...the remains of a star... 729 00:47:34,433 --> 00:47:37,561 ...that once was and is no longer. 730 00:47:40,072 --> 00:47:42,597 In all those skies, rich and distant... 731 00:47:42,808 --> 00:47:45,333 ...and exotic constellations... 732 00:47:45,544 --> 00:47:49,503 ...there may be a faint yellow star... 733 00:47:49,715 --> 00:47:52,616 ...perhaps barely visible to the naked eye... 734 00:47:52,818 --> 00:47:55,651 ...perhaps seen only through the telescope. 735 00:47:55,854 --> 00:47:59,722 The home star of a fleet of interstellar transports... 736 00:47:59,925 --> 00:48:02,086 ...exploring this tiny region... 737 00:48:02,294 --> 00:48:05,388 ...of the great Milky Way galaxy. 738 00:48:05,831 --> 00:48:10,097 The themes of space and time are intertwined. 739 00:48:10,302 --> 00:48:13,294 Worlds and stars, like people... 740 00:48:13,505 --> 00:48:17,305 ...are born, live and die. 741 00:48:17,509 --> 00:48:20,239 The lifetime of a human being is measured in decades. 742 00:48:20,445 --> 00:48:22,675 But the lifetime of the sun... 743 00:48:22,881 --> 00:48:25,714 ...is a hundred million times longer. 744 00:48:27,886 --> 00:48:30,446 Matter is much older than life. 745 00:48:30,655 --> 00:48:33,886 Billions of years before the sun and Earth even formed... 746 00:48:34,092 --> 00:48:37,459 ...atoms were being synthesized in the insides of hot stars... 747 00:48:37,662 --> 00:48:42,031 ...and then returned to space when the stars blew themselves up. 748 00:48:42,234 --> 00:48:45,567 Newly formed planets were made of this stellar debris. 749 00:48:45,771 --> 00:48:50,071 The Earth and every living thing are made of star stuff. 750 00:48:54,913 --> 00:48:58,747 But how slowly, in our human perspective, life evolved... 751 00:48:58,950 --> 00:49:03,387 ...from the molecules of the early oceans to the first bacteria. 752 00:49:07,159 --> 00:49:10,094 Evolution is not immediately obvious to everybody... 753 00:49:10,295 --> 00:49:13,594 ...because it moves so slowly and takes so long. 754 00:49:13,799 --> 00:49:16,927 How can creatures who live for only 70 years... 755 00:49:17,135 --> 00:49:20,866 ...detect events that take 70 million years to unfold? 756 00:49:21,072 --> 00:49:22,869 Or 4 billion? 757 00:49:27,913 --> 00:49:30,279 By the time one-celled animals had evolved... 758 00:49:30,482 --> 00:49:33,713 ...the history of life on Earth was half over. 759 00:49:38,156 --> 00:49:40,920 Not very far along to us, you might think... 760 00:49:41,126 --> 00:49:43,788 ...but by now almost all the basic chemistry of life... 761 00:49:43,995 --> 00:49:46,020 ...had been established. 762 00:49:47,265 --> 00:49:49,233 Forget our human time perspective. 763 00:49:49,434 --> 00:49:51,425 From the point of view of a star... 764 00:49:51,636 --> 00:49:54,901 ...evolution was weaving intricate new patterns... 765 00:49:55,106 --> 00:49:59,065 ...from the star stuff on the planet Earth, and very rapidly. 766 00:50:01,780 --> 00:50:04,476 Most evolutionary lines became extinct. 767 00:50:04,683 --> 00:50:06,776 Many lines became stagnant. 768 00:50:06,985 --> 00:50:08,976 If things had gone a bit differently... 769 00:50:09,187 --> 00:50:11,280 ...a small change of climate, say, or... 770 00:50:11,490 --> 00:50:12,548 ...a new mutation... 771 00:50:12,757 --> 00:50:16,158 ...or the accidental death of a different humble organism... 772 00:50:16,361 --> 00:50:20,559 ...the entire future history of life might have been very different. 773 00:50:23,235 --> 00:50:26,136 Maybe the line to an intelligent technological species... 774 00:50:26,338 --> 00:50:29,000 ...would have passed through worms. 775 00:50:31,843 --> 00:50:34,004 Maybe the present masters of the planet... 776 00:50:34,212 --> 00:50:37,875 ...would have had ancestors who were tunicates. 777 00:50:40,352 --> 00:50:41,876 We might not have evolved. 778 00:50:42,087 --> 00:50:45,113 Someone else, someone very different... 779 00:50:45,323 --> 00:50:50,158 ...would be here now in our stead, maybe pondering their origins. 780 00:50:52,264 --> 00:50:54,357 But that's not what happened. 781 00:50:54,566 --> 00:50:57,626 There's a particular sequence of environmental accidents... 782 00:50:57,836 --> 00:51:00,964 ...and random mutations in the hereditary material. 783 00:51:01,172 --> 00:51:04,869 One particular timeline for life on Earth... 784 00:51:05,076 --> 00:51:06,907 ...in this universe. 785 00:51:10,882 --> 00:51:14,716 As a result, the dominant organisms on the planet today... 786 00:51:14,920 --> 00:51:16,854 ...come from fish. 787 00:51:18,690 --> 00:51:22,387 Along the way, many more species became extinct than now exist. 788 00:51:22,594 --> 00:51:25,825 If history had a slightly different weave... 789 00:51:26,031 --> 00:51:30,491 ...some of those extinct organisms might have survived and prospered. 790 00:51:31,303 --> 00:51:34,466 But occasionally, a creature thought to have become extinct... 791 00:51:34,673 --> 00:51:36,573 ...hundreds of millions of years ago... 792 00:51:36,775 --> 00:51:39,642 ...turns out to be alive and well. 793 00:51:39,844 --> 00:51:42,506 The coelacanth, for example. 794 00:51:44,616 --> 00:51:49,451 For 3 1/2 billion years, life had lived exclusively in the water. 795 00:51:49,654 --> 00:51:52,088 But now, in a great breathtaking adventure... 796 00:51:52,290 --> 00:51:53,518 ...it took to the land. 797 00:51:53,725 --> 00:51:55,852 But if things had gone a little differently... 798 00:51:56,061 --> 00:51:58,791 ...the dominant species might still be in the ocean... 799 00:51:58,997 --> 00:52:03,366 ...or developed spaceships to carry them off the planet altogether. 800 00:52:09,007 --> 00:52:11,066 From our ancestors, the reptiles... 801 00:52:11,276 --> 00:52:13,642 ...there developed many successful lines... 802 00:52:13,845 --> 00:52:16,405 ...including the dinosaurs. 803 00:52:16,715 --> 00:52:19,684 Some were fast, dexterous and intelligent. 804 00:52:19,884 --> 00:52:21,852 A visitor from another world or time... 805 00:52:22,053 --> 00:52:24,817 ...might have thought them the wave of the future. 806 00:52:25,023 --> 00:52:29,585 But after nearly 200 million years, they were suddenly all wiped out. 807 00:52:29,794 --> 00:52:32,558 Perhaps it was a great meteorite colliding with the Earth... 808 00:52:32,764 --> 00:52:35,494 ...spewing debris into the air, blotting out the sun... 809 00:52:35,700 --> 00:52:38,225 ...and killing the plants that the dinosaurs ate. 810 00:52:38,436 --> 00:52:42,930 I wonder when they first sensed that something was wrong. 811 00:52:45,043 --> 00:52:48,809 The successors of the dinosaurs came from the same reptilian stock... 812 00:52:49,014 --> 00:52:53,451 ...but they survived the catastrophe that destroyed their cousins. 813 00:52:56,054 --> 00:52:58,750 Again, there were many branches which became extinct. 814 00:52:58,957 --> 00:53:01,255 And had events been a little different... 815 00:53:01,459 --> 00:53:04,917 ...those branches might have led to the dominant form today. 816 00:53:07,799 --> 00:53:10,859 For 40 million years, a visitor would not have been impressed... 817 00:53:11,069 --> 00:53:13,196 ...by these timid little creatures... 818 00:53:13,405 --> 00:53:17,034 ...but they led to all the familiar mammals of today. 819 00:53:19,544 --> 00:53:22,513 And that includes the primates. 820 00:53:23,415 --> 00:53:26,782 About 20 million years ago, a space time traveler... 821 00:53:26,985 --> 00:53:29,613 ...might have recognized these guys as promising... 822 00:53:29,821 --> 00:53:33,416 ...bright, quick, agile, sociable, curious. 823 00:53:33,658 --> 00:53:36,718 Their ancestors were once atoms made in stars... 824 00:53:36,928 --> 00:53:39,761 ...then simple molecules, single cells... 825 00:53:39,964 --> 00:53:42,228 ...polyps stuck to the ocean floor... 826 00:53:42,434 --> 00:53:45,835 ...fish, amphibians, reptiles, shrews. 827 00:53:46,237 --> 00:53:50,640 But then they came down from the trees and stood upright. 828 00:53:50,842 --> 00:53:53,208 They grew an enormous brain... 829 00:53:53,411 --> 00:53:56,539 ...they developed culture, invented tools... 830 00:53:56,748 --> 00:53:58,807 ...domesticated fire. 831 00:54:02,353 --> 00:54:05,015 They discovered language and writing. 832 00:54:05,223 --> 00:54:07,214 They developed agriculture. 833 00:54:07,425 --> 00:54:11,020 They built cities and forged metal. 834 00:54:12,797 --> 00:54:16,631 And ultimately, they set out for the stars... 835 00:54:16,835 --> 00:54:21,169 ...from which they had come 5 billion years earlier. 836 00:54:23,575 --> 00:54:25,099 We are star stuff... 837 00:54:25,310 --> 00:54:28,711 ...which has taken its destiny into its own hands. 838 00:54:31,516 --> 00:54:33,643 The loom of time and space... 839 00:54:33,852 --> 00:54:37,481 ...works the most astonishing transformations of matter. 840 00:54:38,656 --> 00:54:41,648 Our own planet is only a tiny part... 841 00:54:41,860 --> 00:54:43,987 ...of the vast cosmic tapestry... 842 00:54:44,195 --> 00:54:48,928 ...a starry fabric of worlds yet untold. 843 00:54:55,807 --> 00:54:59,834 Those worlds in space are as countless... 844 00:55:00,044 --> 00:55:03,741 ...as all the grains of sand on all the beaches of the Earth. 845 00:55:04,415 --> 00:55:07,350 Each of those worlds is as real as ours. 846 00:55:07,552 --> 00:55:10,043 In every one of them, there's a succession of... 847 00:55:10,255 --> 00:55:14,885 ...incidents, events, occurrences which influence its future. 848 00:55:15,093 --> 00:55:18,688 Countless worlds, numberless moments... 849 00:55:18,897 --> 00:55:22,355 ...an immensity of space and time. 850 00:55:22,567 --> 00:55:25,365 And our small planet, at this moment... 851 00:55:25,570 --> 00:55:30,007 ...here, we face a critical branchpoint in history. 852 00:55:30,208 --> 00:55:33,109 What we do with our world right now... 853 00:55:33,311 --> 00:55:35,609 ...will propagate down through the centuries... 854 00:55:35,814 --> 00:55:39,215 ...and powerfully affect the destiny of our descendants. 855 00:55:39,417 --> 00:55:43,217 It is well within our power to destroy our civilization... 856 00:55:43,421 --> 00:55:46,254 ...and perhaps our species as well. 857 00:55:46,457 --> 00:55:48,857 If we capitulate to superstition... 858 00:55:49,060 --> 00:55:51,426 ...or greed or stupidity... 859 00:55:51,629 --> 00:55:55,565 ...we can plunge our world into a darkness deeper than the time... 860 00:55:55,767 --> 00:56:00,101 ...between the collapse of classical civilization and Italian Renaissance. 861 00:56:00,305 --> 00:56:02,569 But we are also capable... 862 00:56:02,774 --> 00:56:05,265 ...of using our compassion and our intelligence... 863 00:56:05,476 --> 00:56:08,070 ...our technology and our wealth... 864 00:56:08,279 --> 00:56:10,747 ...to make an abundant and meaningful life... 865 00:56:10,949 --> 00:56:13,247 ...for every inhabitant of this planet... 866 00:56:13,451 --> 00:56:17,945 ...to enhance enormously our understanding of the universe... 867 00:56:18,356 --> 00:56:21,416 ...and to carry us to the stars. 868 00:56:37,275 --> 00:56:38,902 In our motorbike sequence... 869 00:56:39,110 --> 00:56:41,601 ...we showed how the landscape might look... 870 00:56:41,813 --> 00:56:44,543 ...if we barreled through it at close to light speed. 871 00:56:44,749 --> 00:56:47,684 Since then, inspired by this sequence... 872 00:56:47,886 --> 00:56:51,481 ...Ping-Kang Hsiung at Carnegie Mellon University... 873 00:56:51,689 --> 00:56:53,623 ...produced an exact computer animation. 874 00:56:54,125 --> 00:56:57,356 This is what you'd see if you traveled at ordinary speeds... 875 00:56:57,562 --> 00:56:59,792 ...through this red and white lattice. 876 00:56:59,998 --> 00:57:01,829 But this is how it would appear... 877 00:57:02,033 --> 00:57:05,969 ...if you were traveling at close to the speed of light. 878 00:57:06,838 --> 00:57:10,865 We're probably many centuries away from traveling close to light speed... 879 00:57:11,075 --> 00:57:13,942 ...and experiencing time dilation. 880 00:57:14,145 --> 00:57:17,012 But even then, it might not be fast enough... 881 00:57:17,215 --> 00:57:20,616 ...if we wanted to travel to some distant place in the galaxy... 882 00:57:20,818 --> 00:57:23,548 ...and then come back to Earth in our own epoch. 883 00:57:24,055 --> 00:57:27,149 Some years after completing Cosmos... 884 00:57:27,358 --> 00:57:31,886 ...I took time out from my scientific work to write a novel. 885 00:57:32,397 --> 00:57:33,989 A novel about travel... 886 00:57:34,198 --> 00:57:37,395 ...to the center of the Milky Way galaxy. 887 00:57:37,669 --> 00:57:41,002 I was willing to imagine beings and civilizations... 888 00:57:41,205 --> 00:57:43,230 ...far more advanced than we... 889 00:57:43,441 --> 00:57:46,740 ...but I wasn't willing to ignore the laws of physics. 890 00:57:46,978 --> 00:57:51,540 Was there, even in principle, a way to get very quickly... 891 00:57:51,749 --> 00:57:54,718 ...to 30,000 light-years from Earth? 892 00:57:54,919 --> 00:57:56,784 So I asked my friend... 893 00:57:56,988 --> 00:58:00,219 ...Kip Thorne of the California Institute of Technology. 894 00:58:00,425 --> 00:58:03,451 He's a leading expert on the nature of space and time. 895 00:58:03,661 --> 00:58:06,129 Kip thought about it for a while... 896 00:58:06,331 --> 00:58:09,425 ...and then answered with about 50 lines of equations... 897 00:58:09,634 --> 00:58:12,364 ...which showed that a really advanced civilization... 898 00:58:12,570 --> 00:58:16,506 ...might establish and hold open wormholes... 899 00:58:18,710 --> 00:58:22,407 ...which we might think of as tubes through the fourth dimension... 900 00:58:22,613 --> 00:58:25,446 ...which connect the Earth with another place... 901 00:58:25,650 --> 00:58:29,108 ...without having to traverse the intervening distance. 902 00:58:29,354 --> 00:58:33,222 Something like crawling through a wormhole in an apple. 903 00:58:33,658 --> 00:58:35,523 I was happy with this result... 904 00:58:35,727 --> 00:58:39,219 ...and used it as a key plot device in Contact. 905 00:58:39,697 --> 00:58:41,722 But such wormholes through space... 906 00:58:41,933 --> 00:58:44,868 ...would also be time machines, it seemed to me. 907 00:58:45,069 --> 00:58:47,936 And I used that notion in my novel Contact as well. 908 00:58:48,406 --> 00:58:52,502 Kip Thorne and his colleagues later proved, or so it seemed... 909 00:58:52,710 --> 00:58:55,178 ...that time travel of this sort was possible. 910 00:58:55,380 --> 00:58:57,712 Here, look at this. 911 00:58:58,916 --> 00:59:01,544 The key question being explored now... 912 00:59:01,753 --> 00:59:05,211 ...is whether such time travel can be done consistently... 913 00:59:05,423 --> 00:59:09,723 ...with causes preceding effects, say, rather than following them. 914 00:59:09,927 --> 00:59:11,394 Does nature contrive it... 915 00:59:11,596 --> 00:59:14,827 ...so that even with a time machine, you can't intervene... 916 00:59:15,033 --> 00:59:18,127 ...to prevent your own conception, for example? 917 00:59:18,336 --> 00:59:21,362 Even if time travel of this sort is really possible... 918 00:59:21,572 --> 00:59:24,439 ...it's far in our technological future. 919 00:59:24,642 --> 00:59:28,601 But maybe other beings much more advanced than we... 920 00:59:28,813 --> 00:59:31,748 ...are voyaging to the far future and the remote past... 921 00:59:31,949 --> 00:59:34,941 ...not a measly 40 years ago on Earth... 922 00:59:35,153 --> 00:59:37,644 ...but to witness the death of the sun, say... 923 00:59:37,855 --> 00:59:40,050 ...or the origin of the cosmos.