1 00:00:52,667 --> 00:00:57,503 SAGAN: We are drifting in a great ocean of space and time. 2 00:00:59,208 --> 00:01:02,110 In that ocean, the events that shape the future... 3 00:01:02,312 --> 00:01:04,712 ...are working themselves out. 4 00:01:06,917 --> 00:01:10,251 Each creature and every world, to the remotest star... 5 00:01:10,454 --> 00:01:12,354 ...owe their existence to... 6 00:01:12,557 --> 00:01:15,391 ...the great, coursing, implacable forces of nature... 7 00:01:15,594 --> 00:01:18,722 ...but also, to minor happenstance. 8 00:01:21,267 --> 00:01:24,532 We are carried with our planet around the sun. 9 00:01:24,737 --> 00:01:28,174 The Earth has made more than 4 billion circuits of our star... 10 00:01:28,375 --> 00:01:30,206 ...since its origin. 11 00:01:32,247 --> 00:01:36,149 The sun itself travels about the core of the Milky Way galaxy. 12 00:01:36,351 --> 00:01:38,980 Our galaxy is moving among the other galaxies. 13 00:01:39,188 --> 00:01:42,555 We have always been space travelers. 14 00:01:44,361 --> 00:01:49,231 These fine sand grains are all, more or less, uniform in size. 15 00:01:50,134 --> 00:01:53,365 They're produced from bigger rocks through ages of... 16 00:01:53,571 --> 00:01:57,406 ...jostling and rubbing, abrasion and erosion. 17 00:01:57,610 --> 00:02:01,479 Driven in part by the distant moon and sun. 18 00:02:01,815 --> 00:02:05,376 So the roots of the present lie buried in the past. 19 00:02:05,585 --> 00:02:09,352 We are also travelers in time. 20 00:02:13,829 --> 00:02:15,057 But trapped on Earth... 21 00:02:15,263 --> 00:02:18,597 ...we've had little to say about where we go in time and space... 22 00:02:18,801 --> 00:02:20,166 ...or how fast. 23 00:02:20,370 --> 00:02:24,102 But now we're thinking about true journeys in time... 24 00:02:24,308 --> 00:02:28,210 ...and real voyages to the distant stars. 25 00:02:29,981 --> 00:02:34,577 A handful of sand contains about 10,000 grains... 26 00:02:34,786 --> 00:02:36,982 ...more than all the stars we can see... 27 00:02:37,189 --> 00:02:39,521 ...with the naked eye on a clear night. 28 00:02:39,725 --> 00:02:41,819 But the number of stars we can see... 29 00:02:42,029 --> 00:02:45,624 ...is only the tiniest fraction of the number of stars that are. 30 00:02:46,433 --> 00:02:49,460 What we see at night is the merest smattering... 31 00:02:49,671 --> 00:02:51,639 ...of the nearest stars... 32 00:02:51,839 --> 00:02:56,106 ...with a few more distant bright stars thrown in for good measure. 33 00:02:56,311 --> 00:03:00,180 Meanwhile, the cosmos is rich beyond measure. 34 00:03:00,383 --> 00:03:02,510 The number of stars in the universe... 35 00:03:02,719 --> 00:03:06,053 ...is larger than all the grains of sand on all the beaches... 36 00:03:06,257 --> 00:03:07,849 ...of the planet Earth. 37 00:03:11,730 --> 00:03:16,531 Long ago, before we had figured out that the stars are distant suns... 38 00:03:16,736 --> 00:03:19,864 ...they seemed to us to make pictures in the sky. 39 00:03:20,073 --> 00:03:22,872 Just follow the dots. 40 00:03:24,178 --> 00:03:27,341 The Big Dipper constellation today in North America... 41 00:03:27,548 --> 00:03:29,710 ...has had many other incarnations. 42 00:03:29,918 --> 00:03:32,182 Every culture, ancient and modern... 43 00:03:32,454 --> 00:03:36,221 ...has placed its totems and concerns among the stars. 44 00:03:36,425 --> 00:03:40,419 From a Chinese bureaucrat to a German wagon. 45 00:03:44,468 --> 00:03:48,269 But very ancient cultures would have seen different constellations... 46 00:03:48,473 --> 00:03:51,772 ...because the stars move with respect to one another. 47 00:03:51,977 --> 00:03:56,744 We can give a computer the present positions and motions of stars... 48 00:03:56,949 --> 00:04:00,647 ...and then run the patterns back into time. 49 00:04:02,655 --> 00:04:06,353 Every constellation is a single frame in a cosmic movie... 50 00:04:06,560 --> 00:04:08,926 ...but because our lives are so short... 51 00:04:09,130 --> 00:04:11,122 ...because star patterns change slowly... 52 00:04:11,333 --> 00:04:14,268 ...we tend not to notice it's a movie. 53 00:04:14,469 --> 00:04:18,270 A million years ago, there was no Big Dipper. 54 00:04:18,674 --> 00:04:21,974 Our ancestors, looking up and wondering about the stars... 55 00:04:22,179 --> 00:04:25,808 ...saw some other pattern in the northern skies. 56 00:04:26,850 --> 00:04:31,220 We can also run a constellation, Leo the Lion, say, forward in time... 57 00:04:31,422 --> 00:04:35,325 ...and see what the patterns in the stars will be in the future. 58 00:04:37,096 --> 00:04:39,497 A million years from now, Leo might be renamed... 59 00:04:39,699 --> 00:04:42,395 ...the constellation of the Radio Telescope. 60 00:04:42,602 --> 00:04:45,902 Although I suspect radio telescopes then will be as obsolete... 61 00:04:46,107 --> 00:04:48,007 ...as stone spears are now. 62 00:04:48,209 --> 00:04:52,237 Or, here's the constellation of Cetus the Whale. 63 00:05:01,257 --> 00:05:05,991 A million years ago, it may have been called something else. 64 00:05:06,197 --> 00:05:07,687 Perhaps the Spear. 65 00:05:12,804 --> 00:05:17,333 Now, let's run fast-forward through a billion nights. 66 00:05:22,482 --> 00:05:24,109 Millions of years from now... 67 00:05:24,318 --> 00:05:28,517 ...some other very different image will be featured in this cosmic movie. 68 00:05:36,465 --> 00:05:39,492 In Orion the Hunter, things are changing... 69 00:05:39,703 --> 00:05:41,637 ...not only because the stars are moving... 70 00:05:41,838 --> 00:05:44,603 ...but also because the stars are evolving. 71 00:05:44,809 --> 00:05:48,370 Many of Orion's stars are hot, young and short-lived. 72 00:05:48,579 --> 00:05:52,641 They're born, live and die within a span of only a few million years. 73 00:05:52,851 --> 00:05:55,685 If we run Orion forward in time... 74 00:05:55,889 --> 00:05:58,722 ...we see the births and explosive deaths... 75 00:05:58,925 --> 00:06:00,449 ...of dozens of stars... 76 00:06:00,660 --> 00:06:04,791 ...flashing on and winking off like fireflies in the night. 77 00:06:07,235 --> 00:06:10,762 If we wait long enough, we see the constellations change. 78 00:06:10,972 --> 00:06:14,909 But if we go far enough, we also see the star patterns alter. 79 00:06:15,111 --> 00:06:16,976 Two-dimensional constellations... 80 00:06:17,179 --> 00:06:21,173 ...are only the appearance of stars strewn through three dimensions. 81 00:06:21,385 --> 00:06:25,686 Some are dim and near, others are bright but farther away. 82 00:06:26,925 --> 00:06:29,257 Could a space traveler actually see... 83 00:06:29,460 --> 00:06:32,157 ...the patterns of the constellations change? 84 00:06:32,364 --> 00:06:37,234 For that, you must travel roughly as far as the constellation is from us. 85 00:06:37,704 --> 00:06:40,730 Here, we're traveling hundreds of light-years... 86 00:06:40,941 --> 00:06:45,242 ...circling all the way around the stars of the Big Dipper. 87 00:06:48,784 --> 00:06:50,911 Inhabitants of planets around other stars... 88 00:06:51,119 --> 00:06:53,553 ...will see different constellations than us... 89 00:06:53,755 --> 00:06:56,725 ...because their vantage points are different. 90 00:07:04,802 --> 00:07:08,466 Here we are in the constellation Andromeda... 91 00:07:08,673 --> 00:07:13,475 ...or at least a model of it next to the constellation Perseus. 92 00:07:13,679 --> 00:07:15,840 Andromeda, in the Greek myth... 93 00:07:16,048 --> 00:07:19,849 ...was the maiden who was saved by Perseus... 94 00:07:20,053 --> 00:07:21,987 ...from a sea monster. 95 00:07:22,189 --> 00:07:27,093 This star just above me is Beta Andromedae... 96 00:07:27,295 --> 00:07:29,924 ...the second brightest star in the constellation... 97 00:07:30,132 --> 00:07:33,295 ...75 light-years from the Earth. 98 00:07:33,502 --> 00:07:36,597 The light by which we see this star... 99 00:07:36,806 --> 00:07:40,936 ...has spent 75 years traversing interstellar space... 100 00:07:41,145 --> 00:07:43,136 ...on its journey to the Earth. 101 00:07:43,347 --> 00:07:46,942 In the unlikely event that Beta Andromedae... 102 00:07:47,152 --> 00:07:49,677 ...blew itself up a week ago Tuesday... 103 00:07:49,888 --> 00:07:52,652 ...we will not know of it for another 75 years... 104 00:07:52,859 --> 00:07:56,852 ...as this interesting information, traveling at the speed of light... 105 00:07:57,063 --> 00:08:00,693 ...crosses the enormous interstellar distances. 106 00:08:00,901 --> 00:08:03,392 When the light we see from this star set out... 107 00:08:03,604 --> 00:08:06,335 ...on its long interstellar voyage... 108 00:08:06,541 --> 00:08:09,101 ...the young Albert Einstein... 109 00:08:09,310 --> 00:08:12,178 ...working as a Swiss patent clerk... 110 00:08:12,381 --> 00:08:16,283 ...had just published his epochal special theory of relativity... 111 00:08:16,486 --> 00:08:17,817 ...here on Earth. 112 00:08:18,688 --> 00:08:19,916 We see... 113 00:08:20,123 --> 00:08:24,254 ...that space and time are intertwined. 114 00:08:24,462 --> 00:08:27,329 We cannot look out into space... 115 00:08:27,532 --> 00:08:30,832 ...without looking back into time. 116 00:08:31,103 --> 00:08:34,198 The speed of light is very fast... 117 00:08:34,407 --> 00:08:37,604 ...but space is very empty... 118 00:08:37,811 --> 00:08:41,111 ...and the stars are very far apart. 119 00:08:41,315 --> 00:08:44,443 The distances that we've been talking about up to now... 120 00:08:44,652 --> 00:08:48,453 ...are very small by the usual astronomical standards. 121 00:08:48,657 --> 00:08:51,285 In fact, the distance from the Earth... 122 00:08:51,493 --> 00:08:53,485 ...to the center of the Milky Way galaxy... 123 00:08:53,696 --> 00:08:56,563 ...is 30,000 light-years. 124 00:08:57,534 --> 00:09:02,471 From our galaxy to the nearest spiral galaxy like our own... 125 00:09:02,806 --> 00:09:04,604 ...called M31... 126 00:09:04,809 --> 00:09:07,710 ...and which is also within, that means behind... 127 00:09:07,912 --> 00:09:09,881 ...the constellation Andromeda... 128 00:09:10,616 --> 00:09:14,347 ...is 2 million light-years. 129 00:09:15,589 --> 00:09:18,786 When the light we see today from M31... 130 00:09:18,992 --> 00:09:21,518 ...left on its journey for Earth... 131 00:09:21,729 --> 00:09:23,663 ...there were no human beings... 132 00:09:23,865 --> 00:09:27,165 ...although our ancestors were nicely evolving... 133 00:09:27,369 --> 00:09:30,429 ...and very rapidly, to our present form. 134 00:09:31,106 --> 00:09:33,598 There are much greater distances in astronomy. 135 00:09:33,810 --> 00:09:37,405 The distance from the Earth to the most distant quasars... 136 00:09:37,614 --> 00:09:41,051 ...is 8 or 10 billion light-years. 137 00:09:41,252 --> 00:09:45,952 We see them as they were before the Earth itself accumulated... 138 00:09:46,158 --> 00:09:49,719 ...before the Milky Way galaxy was formed. 139 00:09:49,928 --> 00:09:53,558 The fastest space vehicles ever launched by the human species... 140 00:09:53,766 --> 00:09:55,700 ...are the Voyager spacecraft. 141 00:09:55,902 --> 00:09:57,598 They are traveling so fast... 142 00:09:57,804 --> 00:10:01,399 ...that it's only 10,000 times slower... 143 00:10:01,909 --> 00:10:03,207 ...than the speed of light. 144 00:10:03,411 --> 00:10:06,039 The Voyager spacecraft will take 40,000 years... 145 00:10:06,247 --> 00:10:08,307 ...to go the distance to the nearest stars... 146 00:10:08,517 --> 00:10:11,714 ...and they're not even headed towards the nearest stars. 147 00:10:11,921 --> 00:10:14,516 But is there a method by which we could travel... 148 00:10:14,724 --> 00:10:18,216 ...in a conveniently short time to the stars? 149 00:10:18,428 --> 00:10:21,262 Can we travel close to the speed of light? 150 00:10:21,465 --> 00:10:24,195 And what's magic about the speed of light? 151 00:10:24,402 --> 00:10:26,997 Can't we travel faster than that? 152 00:10:29,007 --> 00:10:32,842 It turns out that there is something very strange... 153 00:10:33,046 --> 00:10:34,513 ...about the speed of light. 154 00:10:34,714 --> 00:10:36,773 Something that provides the key... 155 00:10:36,983 --> 00:10:40,385 ...to our understanding of time and space. 156 00:10:42,122 --> 00:10:43,818 The story of its discovery... 157 00:10:44,025 --> 00:10:47,620 ...takes us to Tuscany in northern Italy. 158 00:10:49,932 --> 00:10:52,366 There's something timeless about this place. 159 00:10:52,568 --> 00:10:56,232 A century ago, it probably looked very much the same. 160 00:11:08,354 --> 00:11:12,552 If you had traveled these roads in the summer of 1895... 161 00:11:12,759 --> 00:11:16,889 ...you might have come upon a 16-year-old German high-school dropout. 162 00:11:17,097 --> 00:11:20,261 His teacher told him that he'd never amount to anything... 163 00:11:20,468 --> 00:11:23,926 ...that his attitude destroyed classroom discipline... 164 00:11:24,138 --> 00:11:25,868 ...that he should drop out. 165 00:11:26,074 --> 00:11:28,065 So he left and came here... 166 00:11:28,277 --> 00:11:30,712 ...where he enjoyed wandering these roads... 167 00:11:30,913 --> 00:11:33,381 ...and giving his mind free rein to explore. 168 00:11:35,385 --> 00:11:37,854 One day, he began to think about light... 169 00:11:38,055 --> 00:11:40,046 ...about how fast it travels. 170 00:11:40,257 --> 00:11:43,318 We always measure the speed of a moving object... 171 00:11:43,528 --> 00:11:45,496 ...relative to something else. 172 00:11:45,697 --> 00:11:49,725 I'm moving at about 10 kilometers an hour relative to the ground. 173 00:11:49,935 --> 00:11:51,562 But the ground isn't at rest. 174 00:11:51,771 --> 00:11:55,538 The Earth is turning at more than 1600 kilometers an hour. 175 00:11:55,742 --> 00:11:58,210 The Earth itself is in orbit around the sun. 176 00:11:58,412 --> 00:12:02,474 The sun is moving among the drifting stars, and so on. 177 00:12:02,684 --> 00:12:06,086 It was hard for the young man to imagine some absolute standard... 178 00:12:06,288 --> 00:12:09,155 ...to measure all these relative motions against. 179 00:12:18,969 --> 00:12:22,837 He knew that sound waves are a vibration of the air... 180 00:12:23,040 --> 00:12:26,204 ...and their speed is measured relative to the air itself. 181 00:12:26,411 --> 00:12:29,609 But sunlight travels across the vacuum of empty space. 182 00:12:29,816 --> 00:12:32,080 "Do light waves move relative to something else? 183 00:12:32,285 --> 00:12:35,881 And if so," he wondered, "relative to what?" 184 00:12:39,426 --> 00:12:43,557 That teenage dropout's name... 185 00:12:44,232 --> 00:12:45,893 ...was Albert Einstein. 186 00:12:46,101 --> 00:12:49,037 And his ruminations changed the world. 187 00:12:54,378 --> 00:12:58,509 He had been fascinated by Bernstein's 1869... 188 00:12:59,384 --> 00:13:02,979 ...People's Book of Natural Science. 189 00:13:03,187 --> 00:13:06,351 Here, on its very first page... 190 00:13:07,226 --> 00:13:11,357 ...it describes the astonishing speed of electricity through wires... 191 00:13:11,698 --> 00:13:13,632 ...and light through space. 192 00:13:13,967 --> 00:13:17,904 Einstein wondered, perhaps for the first time, in northern Italy... 193 00:13:18,672 --> 00:13:22,871 ...what the world would look like if you could travel on a wave of light. 194 00:13:23,445 --> 00:13:25,640 To travel at the speed of light. 195 00:13:25,847 --> 00:13:30,649 What an engaging and magical thought for a teenage boy on the road... 196 00:13:30,853 --> 00:13:34,722 ...where the countryside is dappled and rippling in sunlight. 197 00:13:45,738 --> 00:13:50,141 You couldn't tell you were on a light wave if you were traveling with it. 198 00:13:50,342 --> 00:13:52,971 If you started on a wave crest... 199 00:13:53,180 --> 00:13:57,982 ...you would stay on the crest and lose all notion of it being a wave. 200 00:13:58,186 --> 00:14:03,021 Something funny happens at the speed of light. 201 00:14:33,627 --> 00:14:37,393 The more Einstein thought about it, the more troubling it became. 202 00:14:37,598 --> 00:14:40,159 Paradoxes seemed to pop up all over... 203 00:14:40,368 --> 00:14:42,336 ...if you could travel at the speed of light. 204 00:14:42,537 --> 00:14:45,939 Certain ideas had been accepted as true... 205 00:14:46,141 --> 00:14:48,609 ...without sufficiently careful thought. 206 00:14:50,747 --> 00:14:54,774 One of those ideas had to do with the light from a moving object. 207 00:14:56,254 --> 00:14:59,485 The images by which we see the world are made of light... 208 00:14:59,690 --> 00:15:01,818 ...and are carried at the speed of light... 209 00:15:02,027 --> 00:15:05,292 ...300,000 kilometers a second. 210 00:15:05,898 --> 00:15:09,699 You might think that the image of me should be moving out ahead of me... 211 00:15:09,903 --> 00:15:13,134 ...at the speed of light plus the speed of the bicycle. 212 00:15:13,339 --> 00:15:16,867 If I'm moving towards you faster than a horse-and-cart... 213 00:15:17,077 --> 00:15:20,047 ...then my image should be approaching you that much faster. 214 00:15:20,248 --> 00:15:22,842 My image ought to arrive earlier. 215 00:15:24,986 --> 00:15:27,512 But in reality you don't see any time delay. 216 00:15:27,957 --> 00:15:31,826 In a near collision, for example, you see everything happen at once. 217 00:15:32,029 --> 00:15:35,692 Horse, cart, swerve, bicycle. All simultaneous. 218 00:15:36,433 --> 00:15:40,393 But how would it look if it were proper to add the velocities? 219 00:15:40,605 --> 00:15:44,042 Since I'm heading toward you, you'd add my speed to the speed of light. 220 00:15:44,243 --> 00:15:49,011 So my image ought to arrive before the image of the horse-and-cart. 221 00:15:49,916 --> 00:15:52,407 I'd be cycling towards you quite normally. 222 00:15:52,619 --> 00:15:56,112 To me, a collision would seem imminent. 223 00:15:56,324 --> 00:15:59,225 But you'd see me swerve for no apparent reason... 224 00:15:59,427 --> 00:16:01,760 ...and have a collision with nothing. 225 00:16:02,798 --> 00:16:05,596 Now, the horse-and-cart aren't headed towards you. 226 00:16:05,801 --> 00:16:09,533 Their image would arrive only at the speed of light. 227 00:16:10,273 --> 00:16:12,868 Could it seem to me that I just missed colliding... 228 00:16:13,410 --> 00:16:15,970 ...while to you it wasn't even close? 229 00:16:16,180 --> 00:16:18,342 In precise laboratory experiments... 230 00:16:18,550 --> 00:16:21,678 ...scientists have never observed any such thing. 231 00:16:22,287 --> 00:16:24,654 If the world is to be understood... 232 00:16:24,991 --> 00:16:29,894 ...if we are to avoid logical paradoxes when traveling at high speeds... 233 00:16:30,097 --> 00:16:32,622 ...then there are rules which must be obeyed. 234 00:16:32,833 --> 00:16:37,567 Einstein called these rules the special theory of relativity. 235 00:16:37,772 --> 00:16:40,297 Light from a moving object travels at the same speed... 236 00:16:40,508 --> 00:16:44,138 ...no matter whether the object is at rest or in motion. 237 00:16:44,346 --> 00:16:49,011 "Thou shalt not add my speed to the speed of light." 238 00:16:49,219 --> 00:16:54,055 Also, no material object can travel at or beyond the speed of light. 239 00:16:54,258 --> 00:16:58,319 Nothing in physics prevents you from traveling close to the speed of light. 240 00:16:58,529 --> 00:17:02,330 99.9 percent the speed of light is just fine. 241 00:17:02,534 --> 00:17:04,900 But no matter how hard you try... 242 00:17:05,103 --> 00:17:07,868 ...you can never gain that last decimal point. 243 00:17:08,074 --> 00:17:10,338 For the world to be logically consistent... 244 00:17:10,543 --> 00:17:13,672 ...there must be a cosmic speed limit. 245 00:17:14,448 --> 00:17:17,145 The crack of a whip is, due to its tip... 246 00:17:17,352 --> 00:17:19,343 ...moving faster than the speed of sound. 247 00:17:21,990 --> 00:17:23,219 It makes a shock wave... 248 00:17:23,426 --> 00:17:27,260 ...a small sonic boom in the Italian countryside. 249 00:17:27,463 --> 00:17:29,796 A thunderclap has a similar origin. 250 00:17:30,000 --> 00:17:33,333 So does the sound of a supersonic airplane. 251 00:17:35,373 --> 00:17:39,935 So why is the speed of light a barrier any more than the speed of sound? 252 00:17:40,144 --> 00:17:42,010 The answer is not just that... 253 00:17:42,214 --> 00:17:44,808 ...light travels a million times faster than sound. 254 00:17:45,017 --> 00:17:48,681 It's not merely an engineering problem like the supersonic airplane. 255 00:17:49,255 --> 00:17:53,317 Instead, the light barrier is a fundamental law of nature... 256 00:17:53,527 --> 00:17:55,427 ...as basic as gravity. 257 00:17:55,629 --> 00:17:58,997 Einstein found his absolute framework for the world: 258 00:17:59,201 --> 00:18:03,570 This sturdy pillar among all the relative motions of the cosmos. 259 00:18:03,772 --> 00:18:07,834 Light travels just as fast, no matter how its source is moving. 260 00:18:08,044 --> 00:18:12,072 The speed of light is constant, relative to everything else. 261 00:18:12,282 --> 00:18:15,581 Nothing can ever catch up with light. 262 00:18:18,556 --> 00:18:22,084 Einstein's prohibition against traveling faster than light... 263 00:18:22,294 --> 00:18:25,024 ...seems to clash with our common sense notions. 264 00:18:25,231 --> 00:18:28,030 But why should we expect our common sense notions... 265 00:18:28,235 --> 00:18:31,136 ...to have any reliability in a matter of this sort? 266 00:18:31,338 --> 00:18:35,207 Why should our experience at 10 kilometers an hour... 267 00:18:35,409 --> 00:18:37,604 ...constrain the laws of nature... 268 00:18:37,812 --> 00:18:41,112 ...at 300,000 kilometers a second? 269 00:18:43,352 --> 00:18:45,947 Relativity sets limits... 270 00:18:46,156 --> 00:18:49,216 ...on what humans ultimately can do. 271 00:18:49,759 --> 00:18:52,388 The universe is not required... 272 00:18:52,596 --> 00:18:56,966 ...to be in perfect harmony with human ambition. 273 00:19:00,438 --> 00:19:03,340 Imagine a place where the speed of light... 274 00:19:03,543 --> 00:19:06,910 ...isn't its true value of 300,000 kilometers a second... 275 00:19:07,113 --> 00:19:09,674 ...but something a lot less. 276 00:19:09,883 --> 00:19:13,284 Let's say, 40 kilometers an hour... 277 00:19:13,487 --> 00:19:15,547 ...and strictly enforced. 278 00:19:16,457 --> 00:19:20,360 Just as in the real world we can never reach the speed of light... 279 00:19:20,563 --> 00:19:22,428 ...the commandment here is still... 280 00:19:22,631 --> 00:19:25,964 ..."Thou shalt not travel faster than light." 281 00:19:26,168 --> 00:19:30,663 We can do thought experiments on what happens near the speed of light... 282 00:19:30,874 --> 00:19:35,209 ...here 40 kilometers per hour, the speed of a motor scooter. 283 00:19:38,550 --> 00:19:42,543 You can't break the laws of nature. There are no penalties for doing so. 284 00:19:42,921 --> 00:19:44,787 The real world and this one... 285 00:19:45,124 --> 00:19:49,254 ...are merely so arranged that transgressions can't happen. 286 00:19:49,462 --> 00:19:53,399 The job of physics is to find out what those laws are. 287 00:19:55,737 --> 00:19:58,706 Before Einstein, physicists thought that... 288 00:19:58,907 --> 00:20:01,307 ...there were privileged frames of reference... 289 00:20:01,510 --> 00:20:04,001 ...some special places and times... 290 00:20:04,213 --> 00:20:06,773 ...against which everything else had to be measured. 291 00:20:06,983 --> 00:20:10,351 Einstein encountered a similar notion in human affairs. 292 00:20:10,554 --> 00:20:13,251 The idea that the customs of a particular nation... 293 00:20:13,458 --> 00:20:17,019 ...his native Germany or Italy or anywhere... 294 00:20:17,228 --> 00:20:21,165 ...are the standard which all other societies must be measured. 295 00:20:22,034 --> 00:20:25,471 But Einstein rejected the strident nationalism of his time. 296 00:20:25,672 --> 00:20:28,766 He believed every culture had its own validity. 297 00:20:28,975 --> 00:20:30,910 Also in physics, he understood that... 298 00:20:31,112 --> 00:20:33,376 ...there are no privileged frames of reference. 299 00:20:33,581 --> 00:20:36,709 Every observer, in any place, time or motion... 300 00:20:36,919 --> 00:20:39,547 ...must deduce the same laws of nature. 301 00:20:39,755 --> 00:20:41,655 (SPEAKING IN ITALIAN) 302 00:20:43,192 --> 00:20:47,026 A speed is simply how much space you cover in a given time... 303 00:20:47,230 --> 00:20:49,893 ...as any kid on a motor scooter knows. 304 00:20:53,337 --> 00:20:55,203 Since near the velocity of light... 305 00:20:55,407 --> 00:20:57,898 ...we cannot simply add speeds... 306 00:20:58,109 --> 00:21:01,739 ...the familiar notions of absolute space and absolute time... 307 00:21:01,948 --> 00:21:04,940 ...independent of your relative motion, must give way. 308 00:21:05,151 --> 00:21:07,712 That's why, as Einstein showed... 309 00:21:07,921 --> 00:21:11,880 ...funny things have to happen close to the speed of light. 310 00:21:12,627 --> 00:21:16,461 There, our conventional perspectives of space and time... 311 00:21:16,664 --> 00:21:18,724 ...strangely change. 312 00:21:21,170 --> 00:21:25,107 Your nose is just a little closer to me than your ears. 313 00:21:25,308 --> 00:21:27,538 Light reflected off your nose reaches me... 314 00:21:27,744 --> 00:21:29,975 ...an instant in time before your ears. 315 00:21:30,181 --> 00:21:33,412 But suppose I had a magic camera... 316 00:21:33,617 --> 00:21:36,610 ...so that I could see your nose and your ears... 317 00:21:36,822 --> 00:21:38,790 ...at precisely the same instant? 318 00:21:38,991 --> 00:21:40,458 (SCOOTER STARTS UP) 319 00:21:40,659 --> 00:21:42,127 (SCOOTER HONKS) 320 00:21:42,328 --> 00:21:46,765 With such a camera you could take some pretty interesting pictures. 321 00:21:48,135 --> 00:21:51,536 Paolo says goodbye to his little brother, Vincenzo... 322 00:21:51,905 --> 00:21:53,999 - Ciao, Vincenzo. - Ciao, Paolo. 323 00:21:54,809 --> 00:21:56,436 ...and rides off. 324 00:21:56,644 --> 00:21:59,170 He's now going more than half the speed of light. 325 00:21:59,382 --> 00:22:01,942 He is almost catching up with his own light waves. 326 00:22:02,151 --> 00:22:04,711 This compresses the light waves in front of him... 327 00:22:04,921 --> 00:22:06,650 ...and his image becomes blue. 328 00:22:06,957 --> 00:22:10,654 The shorter wavelength is what makes blue light waves blue. 329 00:22:11,229 --> 00:22:14,960 Also Paolo becomes skinny in the direction of motion. 330 00:22:15,166 --> 00:22:17,226 This isn't just some optical illusion. 331 00:22:17,436 --> 00:22:20,667 It really happens when you travel near the speed of light. 332 00:22:21,440 --> 00:22:25,775 As he roars away, he leaves his own light waves stretched out behind him. 333 00:22:25,979 --> 00:22:27,344 Long light waves are red. 334 00:22:27,547 --> 00:22:31,109 We say that his receding image is red-shifted. 335 00:22:32,686 --> 00:22:37,351 Now Paolo leaves for a short tour of the countryside. 336 00:22:37,793 --> 00:22:41,321 He experiences something even stranger. 337 00:22:43,933 --> 00:22:46,164 Everything he can see is squeezed... 338 00:22:46,370 --> 00:22:48,736 ...into a moving window just ahead of him... 339 00:22:48,939 --> 00:22:52,341 ...blue-shifted at the center, red-shifted at the edges. 340 00:22:52,544 --> 00:22:55,843 To a passerby, Paolo appears blue-shifted when approaching... 341 00:22:56,047 --> 00:22:57,777 ...red-shifted when receding. 342 00:22:57,983 --> 00:23:01,043 But to him, the entire world is both coming and going... 343 00:23:01,253 --> 00:23:02,982 ...at nearly the speed of light. 344 00:23:03,189 --> 00:23:06,819 Roadside houses and trees that has already gone past... 345 00:23:07,027 --> 00:23:10,464 ...still appear to him at the edge of his forward field of view... 346 00:23:10,665 --> 00:23:13,361 ...but distorted and red-shifted. 347 00:23:14,535 --> 00:23:18,267 When he slows down, everything again looks normal. 348 00:23:19,808 --> 00:23:22,175 Only very close to the speed of light... 349 00:23:22,378 --> 00:23:25,973 ...does the visible world get squeezed into a kind of tunnel. 350 00:23:26,883 --> 00:23:30,342 You'd see these distortions if you traveled near the speed of light. 351 00:23:30,554 --> 00:23:33,251 Someday, perhaps, interstellar navigators... 352 00:23:33,458 --> 00:23:35,983 ...will take their bearings on stars behind them... 353 00:23:36,194 --> 00:23:40,689 ...whose images have all crowded together on the forward view screen. 354 00:23:42,969 --> 00:23:46,098 The most bizarre aspect of traveling near the speed of light... 355 00:23:46,306 --> 00:23:48,900 ...is that time slows down. 356 00:23:49,743 --> 00:23:52,042 All clocks, mechanical and biological... 357 00:23:52,247 --> 00:23:55,045 ...tick more slowly near the speed of light. 358 00:23:55,250 --> 00:23:58,584 But stationary clocks tick at their usual rate. 359 00:23:58,787 --> 00:24:00,948 If we travel close to light speed... 360 00:24:01,157 --> 00:24:04,252 ...we age more slowly than those we left behind. 361 00:24:09,700 --> 00:24:12,533 Paolo's watch and his internal sense of time show... 362 00:24:13,070 --> 00:24:16,598 ...that he has been gone from his friends for only a few minutes. 363 00:24:17,109 --> 00:24:21,376 But from their point of view, he has been away for many decades. 364 00:24:21,581 --> 00:24:25,176 His friends have grown up, moved on and died. 365 00:24:26,153 --> 00:24:27,780 And his younger brother has been... 366 00:24:27,988 --> 00:24:31,321 ...patiently waiting for him all this time. 367 00:24:33,461 --> 00:24:38,399 The two brothers experience the paradox of time dilation. 368 00:24:38,601 --> 00:24:42,298 They've encountered Einstein's special relativity. 369 00:24:43,339 --> 00:24:44,307 Vincenzo. 370 00:24:58,190 --> 00:25:00,624 This was just a thought experiment. 371 00:25:00,826 --> 00:25:03,990 But atomic particles traveling near the speed of light... 372 00:25:04,197 --> 00:25:07,395 ...do decay more slowly than stationary particles. 373 00:25:07,602 --> 00:25:10,799 As strange and counterintuitive as it seems... 374 00:25:11,005 --> 00:25:14,271 ...time dilation is a law of nature. 375 00:25:16,144 --> 00:25:18,943 Traveling close to the speed of light... 376 00:25:19,249 --> 00:25:22,150 ...is a kind of elixir of life. 377 00:25:23,119 --> 00:25:26,214 Because time slows down close to the speed of light... 378 00:25:26,423 --> 00:25:29,290 ...special relativity provides us... 379 00:25:29,493 --> 00:25:32,190 ...with a means of going to the stars. 380 00:25:33,231 --> 00:25:36,428 This region of northern Italy is not only the caldron... 381 00:25:36,636 --> 00:25:39,935 ...of some of the thinking of the young Albert Einstein... 382 00:25:40,273 --> 00:25:43,607 ...it is also the home of another great genius... 383 00:25:43,810 --> 00:25:46,108 ...who lived 400 years earlier. 384 00:25:46,313 --> 00:25:48,612 Leonardo da Vinci. 385 00:25:49,884 --> 00:25:54,254 Leonardo delighted in climbing these hills... 386 00:25:54,456 --> 00:25:57,823 ...and viewing the ground from a great height... 387 00:25:58,027 --> 00:26:00,326 ...as if he were soaring like a bird. 388 00:26:00,530 --> 00:26:03,124 He drew the first aerial views... 389 00:26:03,333 --> 00:26:07,031 ...of landscapes, villages, fortifications. 390 00:26:07,505 --> 00:26:11,441 I've been talking about Einstein in and around this town of Vinci... 391 00:26:11,642 --> 00:26:13,804 ...in which Leonardo grew up. 392 00:26:14,012 --> 00:26:17,072 Einstein greatly respected Leonardo... 393 00:26:17,282 --> 00:26:19,945 ...and their spirits, in some sense... 394 00:26:20,153 --> 00:26:23,453 ...inhabit this countryside still. 395 00:26:48,419 --> 00:26:51,479 Among Leonardo's many accomplishments... 396 00:26:51,689 --> 00:26:55,057 ...in painting, sculpture, architecture, natural history... 397 00:26:55,260 --> 00:26:59,698 ...anatomy, geology, civil and military engineering... 398 00:27:00,200 --> 00:27:02,065 ...he had a great passion. 399 00:27:02,302 --> 00:27:05,795 He wished to construct a machine... 400 00:27:06,006 --> 00:27:07,667 ...which would fly. 401 00:27:08,209 --> 00:27:12,010 He made sketches of such machines, built miniature models... 402 00:27:12,214 --> 00:27:16,345 ...constructed great, full-scale prototypes. 403 00:27:18,154 --> 00:27:21,817 And not a one of them ever worked. 404 00:27:23,026 --> 00:27:27,395 There were no machines of adequate capacity available in his time. 405 00:27:27,598 --> 00:27:30,966 The technology was just not ready. 406 00:27:31,870 --> 00:27:35,102 The designs, however, were brilliant. 407 00:27:35,307 --> 00:27:38,470 For example, this bird-like machine... 408 00:27:38,677 --> 00:27:42,671 ...here in the Leonardo Museum in the town of Vinci. 409 00:27:43,850 --> 00:27:48,788 Leonardo's great designs encouraged engineers in later epochs... 410 00:27:48,990 --> 00:27:53,360 ...although Leonardo himself was very depressed at these failures. 411 00:27:53,562 --> 00:27:55,393 But it's not his fault... 412 00:27:55,597 --> 00:27:58,624 ...he was trapped in the 15th century. 413 00:27:59,469 --> 00:28:03,166 A somewhat similar case occurred in 1939... 414 00:28:03,374 --> 00:28:07,708 ...when a group of engineers called the British Interplanetary Society... 415 00:28:07,912 --> 00:28:10,177 ...decided to design a ship... 416 00:28:10,382 --> 00:28:13,351 ...which would carry people to the moon. 417 00:28:13,552 --> 00:28:15,851 Now, it was by no means the same design... 418 00:28:16,055 --> 00:28:20,719 ...as the Apollo ship which actually took people to the moon years later. 419 00:28:20,927 --> 00:28:23,488 But that design suggested that... 420 00:28:23,697 --> 00:28:25,494 ...a mission to the moon might one day... 421 00:28:25,699 --> 00:28:28,294 ...be a practical engineering possibility. 422 00:28:28,770 --> 00:28:29,964 Today... 423 00:28:30,972 --> 00:28:34,602 ...we have preliminary designs of ships... 424 00:28:34,810 --> 00:28:38,007 ...which will take people to the stars. 425 00:28:38,214 --> 00:28:42,914 They are constructed in Earth orbit and from there... 426 00:28:43,119 --> 00:28:48,057 ...they venture on their great interstellar journeys. 427 00:28:48,426 --> 00:28:49,825 One of them... 428 00:28:50,294 --> 00:28:53,287 ...is called Project Orion. 429 00:28:54,433 --> 00:28:56,368 It utilizes nuclear weapons... 430 00:28:56,569 --> 00:29:01,006 ...hydrogen bombs against an inertial plate. 431 00:29:01,207 --> 00:29:04,939 Each explosion providing a kind of "putt-putt"... 432 00:29:05,145 --> 00:29:09,082 ...a vast nuclear motorboat in space. 433 00:29:09,484 --> 00:29:12,920 Orion seems entirely practical... 434 00:29:13,121 --> 00:29:15,556 ...and was under development in the U.S... 435 00:29:15,758 --> 00:29:19,125 ...until the signing of the international treaty... 436 00:29:19,328 --> 00:29:22,526 ...forbidding nuclear weapons explosions in space. 437 00:29:23,133 --> 00:29:28,071 I think, the Orion starship is the best use of nuclear weapons... 438 00:29:28,273 --> 00:29:32,108 ...provided the ships don't depart from very near the Earth. 439 00:29:41,688 --> 00:29:44,419 Project Daedalus is a recent design... 440 00:29:44,625 --> 00:29:47,025 ...of the British Interplanetary Society. 441 00:29:47,228 --> 00:29:50,721 It assumes the existence of a nuclear fusion reactor... 442 00:29:50,933 --> 00:29:53,231 ...something much safer and more efficient... 443 00:29:53,435 --> 00:29:57,236 ...than the existing nuclear fission power plants. 444 00:30:01,478 --> 00:30:03,742 We do not yet have fusion reactors. 445 00:30:03,948 --> 00:30:06,678 One day, quite soon, we may. 446 00:30:12,390 --> 00:30:15,792 Orion and Daedalus might go... 447 00:30:15,995 --> 00:30:18,624 ...10 percent the speed of light. 448 00:30:19,766 --> 00:30:22,633 So a trip to Alpha Centauri, 4 1/2 light-years away... 449 00:30:22,836 --> 00:30:26,671 ...would take 45 years, less than a human lifetime. 450 00:30:27,241 --> 00:30:31,076 Such ships could not travel close enough to the speed of light... 451 00:30:31,280 --> 00:30:34,215 ...for the time-slowing effects of special relativity... 452 00:30:34,416 --> 00:30:36,077 ...to become important. 453 00:30:36,653 --> 00:30:39,349 It does not seem likely that such ships... 454 00:30:39,556 --> 00:30:42,151 ...would be built before the middle of the 21 st century... 455 00:30:42,360 --> 00:30:46,160 ...although we could build an Orion starship now. 456 00:30:46,597 --> 00:30:50,659 For voyages beyond the nearest stars, something must be added. 457 00:30:50,869 --> 00:30:53,964 Perhaps they could be used as multigeneration ships... 458 00:30:54,340 --> 00:30:57,332 ...so those arriving would be the remote descendants... 459 00:30:57,543 --> 00:31:01,275 ...of those who had originally set out centuries before. 460 00:31:01,915 --> 00:31:05,943 Or perhaps some safe means of human hibernation might be found... 461 00:31:06,154 --> 00:31:10,022 ...so that space travelers might be frozen and then thawed out... 462 00:31:10,225 --> 00:31:14,128 ...when they arrive at the destination centuries later. 463 00:31:14,830 --> 00:31:19,131 But fast interstellar space flight approaching the speed of light... 464 00:31:19,336 --> 00:31:21,201 ...is much more difficult. 465 00:31:21,405 --> 00:31:24,136 That's an objective not for a hundred years... 466 00:31:24,342 --> 00:31:27,470 ...but for a thousand or for 10 thousand... 467 00:31:27,679 --> 00:31:30,148 ...but it also is possible. 468 00:31:32,518 --> 00:31:35,488 A kind of interstellar ramjet has been proposed... 469 00:31:35,689 --> 00:31:38,123 ...which scoops up the hydrogen atoms... 470 00:31:38,324 --> 00:31:40,258 ...which float between the stars... 471 00:31:40,460 --> 00:31:44,522 ...accelerates them into an engine and spits them out the back. 472 00:31:45,366 --> 00:31:47,767 But in deep space, there is one atom... 473 00:31:47,969 --> 00:31:52,030 ...for every 10 cubic centimeters of space. 474 00:31:52,240 --> 00:31:53,708 For the ramjet to work... 475 00:31:54,210 --> 00:31:56,770 ...it has to have a frontal scoop... 476 00:31:56,979 --> 00:31:59,847 ...hundreds of kilometers across. 477 00:32:00,117 --> 00:32:04,316 Reaching relativistic velocities, the hydrogen atoms will be moving... 478 00:32:04,522 --> 00:32:07,082 ...with respect to the interstellar spaceship... 479 00:32:07,292 --> 00:32:09,317 ...at close to the speed of light. 480 00:32:09,527 --> 00:32:11,257 If precautions aren't taken... 481 00:32:11,464 --> 00:32:16,027 ...the passengers will be fried by these induced cosmic rays. 482 00:32:16,236 --> 00:32:18,101 There's a proposed solution: 483 00:32:18,305 --> 00:32:21,604 A laser is used to strip electrons off the atoms... 484 00:32:21,808 --> 00:32:25,040 ...and electrically charge them while they're some distance away. 485 00:32:25,613 --> 00:32:28,242 And an extremely strong magnetic field... 486 00:32:28,484 --> 00:32:31,749 ...is used to deflect the charged atoms into the scoop... 487 00:32:31,954 --> 00:32:33,581 ...and away from the spacecraft. 488 00:32:33,790 --> 00:32:35,087 This is engineering... 489 00:32:35,291 --> 00:32:39,091 ...on a scale so far unprecedented on the Earth. 490 00:32:39,295 --> 00:32:43,892 We are talking of engines the size of small worlds. 491 00:32:53,312 --> 00:32:58,216 Suppose that the spacecraft is designed to accelerate at 1 g... 492 00:32:58,418 --> 00:33:00,716 ...so we'd be comfortable aboard it. 493 00:33:00,921 --> 00:33:03,356 We'd go closer and closer to the speed of light... 494 00:33:03,558 --> 00:33:05,822 ...until the midpoint of the journey. 495 00:33:06,027 --> 00:33:08,587 Then the spacecraft is turned around... 496 00:33:08,797 --> 00:33:12,359 ...and we decelerate at 1 g to the destination. 497 00:33:12,935 --> 00:33:17,066 For most of the trip, the velocity would be close to the speed of light... 498 00:33:17,274 --> 00:33:20,437 ...and time would slow down enormously. 499 00:33:20,778 --> 00:33:22,302 By how much? 500 00:33:23,281 --> 00:33:26,718 Barnard's Star could be reached by such a ship... 501 00:33:26,919 --> 00:33:29,547 ...in eight years, ship time. 502 00:33:30,155 --> 00:33:34,115 The center of the Milky Way galaxy in 21 years. 503 00:33:34,327 --> 00:33:38,195 The Andromeda galaxy in 28 years. 504 00:33:38,732 --> 00:33:40,666 Of course, the people left behind on the Earth... 505 00:33:40,868 --> 00:33:43,166 ...would see things somewhat differently. 506 00:33:43,437 --> 00:33:45,406 Instead of 21 years to the galaxy... 507 00:33:45,607 --> 00:33:49,168 ...they would measure it as 30,000 years. 508 00:33:49,377 --> 00:33:50,743 When we got back... 509 00:33:50,947 --> 00:33:54,405 ...very few of our friends would be around to greet us. 510 00:33:55,251 --> 00:33:57,015 In principle, such a journey... 511 00:33:57,221 --> 00:34:01,624 ...mounting the decimal points closer and closer to the speed of light... 512 00:34:01,826 --> 00:34:05,592 ...would even permit us to circumnavigate the known universe... 513 00:34:05,797 --> 00:34:08,858 ...in 56 years, ship time. 514 00:34:09,802 --> 00:34:13,899 We would return tens of billions of years... 515 00:34:14,107 --> 00:34:16,098 ...in the far future... 516 00:34:16,309 --> 00:34:19,369 ...with the Earth a charred cinder... 517 00:34:19,580 --> 00:34:22,048 ...and the sun dead. 518 00:34:22,683 --> 00:34:26,085 Relativistic space flight makes the universe accessible... 519 00:34:26,288 --> 00:34:28,654 ...to advanced civilizations... 520 00:34:28,857 --> 00:34:31,052 ...but only to those who go on the journey... 521 00:34:31,261 --> 00:34:33,627 ...not to those who stay home. 522 00:34:34,464 --> 00:34:38,799 These designs are probably further... 523 00:34:39,003 --> 00:34:42,734 ...from the actual interstellar spacecraft of the future... 524 00:34:43,942 --> 00:34:46,740 ...than Leonardo's models are... 525 00:34:46,945 --> 00:34:50,279 ...from the supersonic transports of the present. 526 00:34:50,850 --> 00:34:52,715 But if we do not destroy ourselves... 527 00:34:52,919 --> 00:34:57,823 ...I believe that we will, one day, venture to the stars. 528 00:34:58,492 --> 00:35:01,053 When our solar system is all explored... 529 00:35:01,262 --> 00:35:04,390 ...the planets of other stars will beckon. 530 00:35:38,205 --> 00:35:42,404 Space travel and time travel are connected. 531 00:35:43,545 --> 00:35:45,479 To travel fast into space... 532 00:35:45,680 --> 00:35:48,878 ...is to travel fast into the future. 533 00:35:51,754 --> 00:35:55,919 We travel into the future, although slowly, all the time. 534 00:35:56,126 --> 00:36:00,427 But what about the past? Could we journey into yesterday? 535 00:36:00,631 --> 00:36:03,759 Many physicists think this is fundamentally impossible... 536 00:36:03,968 --> 00:36:06,301 ...that we could not build a device... 537 00:36:06,505 --> 00:36:09,235 ...which would carry us backwards into time. 538 00:36:09,441 --> 00:36:12,843 Some say that even if we were to build such a device... 539 00:36:13,046 --> 00:36:14,308 ...it wouldn't do much good. 540 00:36:14,514 --> 00:36:17,040 We couldn't significantly affect the past. 541 00:36:17,251 --> 00:36:20,914 For example, suppose you traveled into the past... 542 00:36:21,122 --> 00:36:23,216 ...and somehow or other prevented... 543 00:36:23,425 --> 00:36:26,223 ...your own parents from meeting. 544 00:36:26,428 --> 00:36:29,990 Why, then you would probably never have been born... 545 00:36:30,199 --> 00:36:32,224 ...which is something of a contradiction, isn't it... 546 00:36:32,435 --> 00:36:34,562 ...since you are clearly there. 547 00:36:35,172 --> 00:36:36,503 Other people think that... 548 00:36:36,707 --> 00:36:39,835 ...the two alternative histories have equal validity... 549 00:36:40,044 --> 00:36:43,446 ...that they're parallel threads, skeins of time... 550 00:36:43,648 --> 00:36:46,208 ...that they could exist side by side. 551 00:36:50,089 --> 00:36:52,615 The history in which you were never born... 552 00:36:52,826 --> 00:36:55,795 ...and the history that you know all about. 553 00:36:56,029 --> 00:36:59,227 Perhaps time itself has many potential dimensions... 554 00:36:59,434 --> 00:37:02,699 ...despite the fact that we are condemned to experience... 555 00:37:02,904 --> 00:37:05,100 ...only one of those dimensions. 556 00:37:05,741 --> 00:37:09,006 Now, suppose you could go back into the past... 557 00:37:09,211 --> 00:37:12,670 ...and really change it by, let's say something like... 558 00:37:12,882 --> 00:37:17,377 ...persuading Queen Isabella not to bankroll Christopher Columbus. 559 00:37:17,588 --> 00:37:19,852 Then you would have set into motion... 560 00:37:20,057 --> 00:37:23,084 ...a different sequence of historical events... 561 00:37:23,295 --> 00:37:26,355 ...which those people you left behind you in our time... 562 00:37:26,565 --> 00:37:28,727 ...would never get to know about. 563 00:37:28,935 --> 00:37:31,665 If that kind of time travel were possible... 564 00:37:31,871 --> 00:37:35,069 ...then every imaginable sequence... 565 00:37:35,275 --> 00:37:37,243 ...of alternative history... 566 00:37:37,444 --> 00:37:39,640 ...might in some sense really exist. 567 00:37:40,615 --> 00:37:42,674 Would it be possible for a time traveler... 568 00:37:42,884 --> 00:37:46,252 ...to change the course of history in a major way? 569 00:37:46,455 --> 00:37:48,685 Well, let's think about that. 570 00:37:52,262 --> 00:37:54,526 History consists for the most part... 571 00:37:54,731 --> 00:37:58,793 ...of a complex multitude of deeply interwoven threads... 572 00:37:59,003 --> 00:38:01,665 ...biological, economic and social forces... 573 00:38:01,873 --> 00:38:04,365 ...that are not so easily unraveled. 574 00:38:05,644 --> 00:38:10,275 The ancient Greeks imagined the course of human events to be a tapestry... 575 00:38:10,483 --> 00:38:14,249 ...created by three goddesses: the Fates. 576 00:38:15,923 --> 00:38:20,155 Random minor events generally have no long-range consequences. 577 00:38:20,362 --> 00:38:23,559 But some which occur at critical junctures... 578 00:38:23,765 --> 00:38:26,291 ...may alter the weave of history. 579 00:38:26,502 --> 00:38:29,494 There may be cases where profound changes can be made... 580 00:38:29,705 --> 00:38:32,197 ...by relatively trivial adjustments. 581 00:38:32,409 --> 00:38:37,176 The further in the past such an event is, the more powerful its influence. 582 00:38:37,681 --> 00:38:41,140 What if our time traveler had persuaded Queen Isabella that... 583 00:38:41,352 --> 00:38:43,252 ...Columbus' geography was wrong? 584 00:38:43,454 --> 00:38:47,653 Almost certainly, some other European would have sailed to the New World. 585 00:38:47,860 --> 00:38:49,521 There were many inducements: 586 00:38:49,728 --> 00:38:52,789 The lure of the spice trade, improvements in navigation... 587 00:38:52,999 --> 00:38:55,433 ...competition among rival European powers. 588 00:38:55,636 --> 00:38:59,538 The discovery of America around 1500 was inevitable. 589 00:38:59,740 --> 00:39:03,142 Of course, there wouldn't be any postage stamps showing Columbus... 590 00:39:03,345 --> 00:39:06,280 ...and the Republic of Colombia would have another name. 591 00:39:06,481 --> 00:39:10,475 But the big picture would have turned out more or less the same. 592 00:39:15,092 --> 00:39:18,289 In order to affect the future profoundly... 593 00:39:18,495 --> 00:39:21,056 ...a time traveler has to pick and choose. 594 00:39:21,266 --> 00:39:24,724 He'd probably have to intervene in a number of events... 595 00:39:24,936 --> 00:39:27,633 ...which are very carefully selected... 596 00:39:27,840 --> 00:39:32,676 ...so he could change the weave of history. 597 00:39:33,080 --> 00:39:35,605 It's a lovely fantasy... 598 00:39:35,816 --> 00:39:39,753 ...to explore those other worlds that never were. 599 00:39:42,257 --> 00:39:45,853 If you had H.G. Wells' time machine... 600 00:39:46,061 --> 00:39:49,088 ...maybe you could understand how history really works. 601 00:39:49,299 --> 00:39:52,166 If an apparently pivotal person had never lived... 602 00:39:52,369 --> 00:39:56,534 ...Paul the Apostle or Peter the Great or Pythagoras... 603 00:39:56,741 --> 00:39:59,437 ...how different would the world really be? 604 00:40:00,244 --> 00:40:02,338 What if the scientific tradition... 605 00:40:02,547 --> 00:40:05,311 ...of the ancient Ionian Greeks... 606 00:40:05,517 --> 00:40:08,180 ...had prospered and flourished? 607 00:40:08,388 --> 00:40:11,255 It would have required many social factors at the time... 608 00:40:11,457 --> 00:40:13,187 ...to have been different... 609 00:40:13,394 --> 00:40:15,828 ...including the common feeling... 610 00:40:16,030 --> 00:40:18,727 ...that slavery was right and natural. 611 00:40:18,933 --> 00:40:22,494 But what if that light that had dawned... 612 00:40:22,704 --> 00:40:26,072 ...on the eastern Mediterranean some 2500 years ago... 613 00:40:26,275 --> 00:40:28,243 ...had not flickered out? 614 00:40:28,444 --> 00:40:31,744 What if scientific method and experiment... 615 00:40:31,948 --> 00:40:33,939 ...had been vigorously pursued... 616 00:40:34,151 --> 00:40:36,416 ...2000 years before the industrial revolution... 617 00:40:36,621 --> 00:40:38,589 ...our industrial revolution? 618 00:40:38,790 --> 00:40:42,852 What if the power of this new mode of thought, the scientific method... 619 00:40:43,062 --> 00:40:45,360 ...had been generally appreciated? 620 00:40:45,731 --> 00:40:49,463 I think we might have saved 10 or 20 centuries. 621 00:40:49,669 --> 00:40:52,365 Perhaps the contributions that Leonardo made... 622 00:40:52,572 --> 00:40:55,406 ...would have been made 1000 years earlier... 623 00:40:55,609 --> 00:40:59,410 ...and the contributions of Einstein 500 years ago. 624 00:40:59,614 --> 00:41:01,741 Not that it would have been those people... 625 00:41:01,950 --> 00:41:04,384 ...who would've made those contributions... 626 00:41:04,586 --> 00:41:07,579 ...because they lived only in our timeline. 627 00:41:08,157 --> 00:41:11,093 If the Ionians had won... 628 00:41:11,295 --> 00:41:15,288 ...we might by now, I think, be going to the stars. 629 00:41:15,499 --> 00:41:20,233 We might at this moment have the first survey ships... 630 00:41:20,438 --> 00:41:25,103 ...returning with astonishing results from Alpha Centauri... 631 00:41:25,311 --> 00:41:29,908 ...and Barnard's Star, Sirius and Tau Ceti. 632 00:41:30,117 --> 00:41:33,416 There would now be great fleets... 633 00:41:33,620 --> 00:41:35,589 ...of interstellar transports... 634 00:41:35,790 --> 00:41:38,281 ...being constructed in Earth orbit... 635 00:41:38,493 --> 00:41:41,486 ...small, unmanned survey ships... 636 00:41:41,697 --> 00:41:45,155 ...liners for immigrants, perhaps... 637 00:41:45,367 --> 00:41:46,767 ...great trading ships... 638 00:41:46,970 --> 00:41:50,428 ...to ply the spaces between the stars. 639 00:41:50,907 --> 00:41:54,105 On all these ships there would be symbols... 640 00:41:54,311 --> 00:41:57,041 ...and inscriptions on the sides. 641 00:41:57,248 --> 00:41:59,513 The inscriptions, if we looked closely... 642 00:41:59,718 --> 00:42:02,346 ...would be written in Greek. 643 00:42:02,854 --> 00:42:04,186 The symbol... 644 00:42:04,390 --> 00:42:07,848 ...perhaps, would be the dodecahedron. 645 00:42:08,060 --> 00:42:12,691 And the inscription on the sides of the ships to the stars... 646 00:42:12,900 --> 00:42:14,367 ...something like: 647 00:42:14,568 --> 00:42:19,302 "Starship Theodorus of the Planet Earth." 648 00:42:22,311 --> 00:42:25,109 If you were a really ambitious time traveler... 649 00:42:28,318 --> 00:42:31,082 ...you might not dally with human history... 650 00:42:31,288 --> 00:42:33,917 ...or even pause to examine the evolution on Earth. 651 00:42:34,125 --> 00:42:36,355 Instead, you would journey back... 652 00:42:36,561 --> 00:42:37,960 ...to witness the origin of our solar system... 653 00:42:38,429 --> 00:42:42,958 ...from the gas and dust between the stars. 654 00:42:44,136 --> 00:42:45,570 Five billion years ago... 655 00:42:45,772 --> 00:42:49,367 ...an interstellar cloud was collapsing to form our solar system. 656 00:42:49,576 --> 00:42:52,808 Most clumps of matter gravitated towards the center... 657 00:42:53,013 --> 00:42:55,573 ...and were destined to form the sun. 658 00:42:55,783 --> 00:43:00,016 Smaller peripheral clumps would become the planets. 659 00:43:00,222 --> 00:43:04,626 Long ago, there was a kind of natural selection among the worlds. 660 00:43:04,827 --> 00:43:09,026 Those on highly elliptical orbits tended to collide and be destroyed... 661 00:43:09,233 --> 00:43:12,828 ...but planets in circular orbits tended to survive. 662 00:43:13,036 --> 00:43:15,301 But if events had been a little different... 663 00:43:15,507 --> 00:43:17,168 ...the Earth would never have formed... 664 00:43:17,375 --> 00:43:21,335 ...and another planet at another distance from the sun would be around. 665 00:43:21,547 --> 00:43:23,811 We owe the existence of our world... 666 00:43:24,016 --> 00:43:27,851 ...to random collisions in a long-vanished cloud. 667 00:43:30,690 --> 00:43:33,751 Soon, the central mass became very hot. 668 00:43:33,961 --> 00:43:37,796 Thermonuclear reactions were initiated and the sun turned on... 669 00:43:38,000 --> 00:43:40,867 ...flooding the solar system with light. 670 00:43:43,238 --> 00:43:45,207 But the growing smaller lumps... 671 00:43:45,408 --> 00:43:47,638 ...would never achieve such high temperatures... 672 00:43:47,844 --> 00:43:50,678 ...and would never generate thermonuclear reactions. 673 00:43:50,881 --> 00:43:54,612 They would become the Earth and the other planets... 674 00:43:54,818 --> 00:43:59,051 ...heated not from within, but mainly by the distant sun. 675 00:44:03,729 --> 00:44:05,526 The accretion continued until... 676 00:44:05,731 --> 00:44:08,997 ...almost all the gas and dust and small worldlets... 677 00:44:09,202 --> 00:44:12,603 ...were swept up by the surviving planets. 678 00:44:14,976 --> 00:44:17,274 Our time traveler would witness... 679 00:44:17,478 --> 00:44:20,414 ...the collisions that made the worlds. 680 00:44:26,590 --> 00:44:28,615 Except for the comets and asteroids... 681 00:44:28,825 --> 00:44:31,454 ...the chaos of the early solar system was reduced... 682 00:44:31,662 --> 00:44:33,994 ...to a remarkable simplicity: 683 00:44:34,198 --> 00:44:38,033 Nine or so principal planets in almost circular orbits... 684 00:44:38,236 --> 00:44:40,295 ...and a few dozen moons. 685 00:44:44,777 --> 00:44:47,712 Now, let's take a different look. 686 00:44:49,350 --> 00:44:51,818 If we view the solar system edge on... 687 00:44:52,019 --> 00:44:54,284 ...and move the sun off-screen to the left... 688 00:44:54,489 --> 00:44:57,151 ...we see that the small terrestrial planets... 689 00:44:57,359 --> 00:45:00,989 ...the ones about as massive as Earth, tend to be close to the sun. 690 00:45:01,197 --> 00:45:05,031 The big Jupiter-like planets tend to be much further from the sun. 691 00:45:05,234 --> 00:45:08,102 But is that the way it has to be? 692 00:45:09,306 --> 00:45:10,967 Computer studies suggest... 693 00:45:11,174 --> 00:45:13,939 ...that there may be many similar systems about stars... 694 00:45:14,145 --> 00:45:18,515 ...with the terrestrials in close and the Jovian planets further away. 695 00:45:22,388 --> 00:45:26,120 But some systems might have Jovians and terrestrials mixed together. 696 00:45:26,326 --> 00:45:30,855 There may be great worlds like Jupiter looming in other skies. 697 00:45:32,166 --> 00:45:36,126 Rarely, the Jovian planets may form close to the star... 698 00:45:36,338 --> 00:45:40,672 ...the terrestrials trailing away towards interstellar space. 699 00:45:42,011 --> 00:45:44,002 Our familiar arrangement of planets... 700 00:45:44,213 --> 00:45:46,978 ...is only one, perhaps typical, case... 701 00:45:47,184 --> 00:45:51,245 ...in the vast expanse of systems. 702 00:45:51,455 --> 00:45:56,086 Often, one fledgling planet accumulates so much gas and dust... 703 00:45:56,294 --> 00:45:58,489 ...that thermonuclear reactions do occur. 704 00:45:58,697 --> 00:46:00,791 It becomes a second sun. 705 00:46:01,000 --> 00:46:03,833 A binary star system has formed. 706 00:46:08,341 --> 00:46:12,244 From most of these worlds, the vistas will be dazzling. 707 00:46:12,447 --> 00:46:14,813 Not one of them will be identical to the Earth. 708 00:46:15,016 --> 00:46:19,386 A few will be hospitable. Many will appear hostile. 709 00:46:20,789 --> 00:46:22,815 Where there are two suns in the sky... 710 00:46:23,026 --> 00:46:26,723 ...every object will cast two shadows. 711 00:46:30,901 --> 00:46:33,529 What wonders are waiting for us... 712 00:46:33,738 --> 00:46:36,105 ...on the planets of the nearby stars? 713 00:46:36,308 --> 00:46:39,175 Are there radically different kinds of worlds... 714 00:46:39,377 --> 00:46:42,438 ...unimaginably exotic forms of life? 715 00:46:45,718 --> 00:46:48,620 Perhaps in another century or two... 716 00:46:48,822 --> 00:46:50,813 ...when our solar system is all explored... 717 00:46:51,024 --> 00:46:54,256 ...we will also have put our own planet in order. 718 00:46:54,462 --> 00:46:57,921 Then we will set sail for the stars... 719 00:46:58,134 --> 00:47:00,796 ...and the beckoning worlds around them. 720 00:47:04,241 --> 00:47:07,369 In that day, our machines and our descendants... 721 00:47:07,577 --> 00:47:11,275 ...approaching the speed of light, will skim the light-years... 722 00:47:11,482 --> 00:47:16,113 ...leaping ahead through time, seeking new worlds. 723 00:47:16,321 --> 00:47:19,848 Einstein has shown us that it's possible. 724 00:47:21,094 --> 00:47:23,187 We will journey simultaneously... 725 00:47:23,396 --> 00:47:26,797 ...to distant planets and to the far future. 726 00:47:27,601 --> 00:47:29,296 Some worlds, like this one... 727 00:47:29,503 --> 00:47:32,769 ...will look out onto a vast gaseous nebula... 728 00:47:32,974 --> 00:47:34,703 ...the remains of a star... 729 00:47:34,910 --> 00:47:38,038 ...that once was and is no longer. 730 00:47:40,550 --> 00:47:43,075 In all those skies, rich and distant... 731 00:47:43,286 --> 00:47:45,812 ...and exotic constellations... 732 00:47:46,023 --> 00:47:49,982 ...there may be a faint yellow star... 733 00:47:50,194 --> 00:47:53,096 ...perhaps barely visible to the naked eye... 734 00:47:53,298 --> 00:47:56,131 ...perhaps seen only through the telescope. 735 00:47:56,335 --> 00:48:00,203 The home star of a fleet of interstellar transports... 736 00:48:00,406 --> 00:48:02,568 ...exploring this tiny region... 737 00:48:02,776 --> 00:48:05,870 ...of the great Milky Way galaxy. 738 00:48:06,313 --> 00:48:10,580 The themes of space and time are intertwined. 739 00:48:10,785 --> 00:48:13,777 Worlds and stars, like people... 740 00:48:13,989 --> 00:48:17,789 ...are born, live and die. 741 00:48:17,993 --> 00:48:20,724 The lifetime of a human being is measured in decades. 742 00:48:20,930 --> 00:48:23,160 But the lifetime of the sun... 743 00:48:23,366 --> 00:48:26,200 ...is a hundred million times longer. 744 00:48:28,372 --> 00:48:30,932 Matter is much older than life. 745 00:48:31,141 --> 00:48:34,373 Billions of years before the sun and Earth even formed... 746 00:48:34,579 --> 00:48:37,947 ...atoms were being synthesized in the insides of hot stars... 747 00:48:38,150 --> 00:48:42,519 ...and then returned to space when the stars blew themselves up. 748 00:48:42,722 --> 00:48:46,056 Newly formed planets were made of this stellar debris. 749 00:48:46,260 --> 00:48:50,561 The Earth and every living thing are made of star stuff. 750 00:48:55,404 --> 00:48:59,238 But how slowly, in our human perspective, life evolved... 751 00:48:59,441 --> 00:49:03,879 ...from the molecules of the early oceans to the first bacteria. 752 00:49:07,652 --> 00:49:10,587 Evolution is not immediately obvious to everybody... 753 00:49:10,788 --> 00:49:14,088 ...because it moves so slowly and takes so long. 754 00:49:14,293 --> 00:49:17,421 How can creatures who live for only 70 years... 755 00:49:17,629 --> 00:49:21,361 ...detect events that take 70 million years to unfold? 756 00:49:21,567 --> 00:49:23,364 Or 4 billion? 757 00:49:28,409 --> 00:49:30,776 By the time one-celled animals had evolved... 758 00:49:30,979 --> 00:49:34,210 ...the history of life on Earth was half over. 759 00:49:38,654 --> 00:49:41,418 Not very far along to us, you might think... 760 00:49:41,624 --> 00:49:44,287 ...but by now almost all the basic chemistry of life... 761 00:49:44,494 --> 00:49:46,519 ...had been established. 762 00:49:47,764 --> 00:49:49,733 Forget our human time perspective. 763 00:49:49,934 --> 00:49:51,925 From the point of view of a star... 764 00:49:52,136 --> 00:49:55,402 ...evolution was weaving intricate new patterns... 765 00:49:55,607 --> 00:49:59,566 ...from the star stuff on the planet Earth, and very rapidly. 766 00:50:02,282 --> 00:50:04,978 Most evolutionary lines became extinct. 767 00:50:05,185 --> 00:50:07,279 Many lines became stagnant. 768 00:50:07,488 --> 00:50:09,479 If things had gone a bit differently... 769 00:50:09,690 --> 00:50:11,784 ...a small change of climate, say, or... 770 00:50:11,994 --> 00:50:13,052 ...a new mutation... 771 00:50:13,261 --> 00:50:16,662 ...or the accidental death of a different humble organism... 772 00:50:16,865 --> 00:50:21,064 ...the entire future history of life might have been very different. 773 00:50:23,741 --> 00:50:26,642 Maybe the line to an intelligent technological species... 774 00:50:26,844 --> 00:50:29,507 ...would have passed through worms. 775 00:50:32,350 --> 00:50:34,511 Maybe the present masters of the planet... 776 00:50:34,719 --> 00:50:38,383 ...would have had ancestors who were tunicates. 777 00:50:40,861 --> 00:50:42,385 We might not have evolved. 778 00:50:42,596 --> 00:50:45,622 Someone else, someone very different... 779 00:50:45,832 --> 00:50:50,668 ...would be here now in our stead, maybe pondering their origins. 780 00:50:52,775 --> 00:50:54,868 But that's not what happened. 781 00:50:55,077 --> 00:50:58,137 There's a particular sequence of environmental accidents... 782 00:50:58,347 --> 00:51:01,476 ...and random mutations in the hereditary material. 783 00:51:01,684 --> 00:51:05,382 One particular timeline for life on Earth... 784 00:51:05,589 --> 00:51:07,420 ...in this universe. 785 00:51:11,396 --> 00:51:15,230 As a result, the dominant organisms on the planet today... 786 00:51:15,434 --> 00:51:17,369 ...come from fish. 787 00:51:19,205 --> 00:51:22,903 Along the way, many more species became extinct than now exist. 788 00:51:23,110 --> 00:51:26,341 If history had a slightly different weave... 789 00:51:26,547 --> 00:51:31,008 ...some of those extinct organisms might have survived and prospered. 790 00:51:31,820 --> 00:51:34,984 But occasionally, a creature thought to have become extinct... 791 00:51:35,191 --> 00:51:37,091 ...hundreds of millions of years ago... 792 00:51:37,293 --> 00:51:40,161 ...turns out to be alive and well. 793 00:51:40,363 --> 00:51:43,025 The coelacanth, for example. 794 00:51:45,135 --> 00:51:49,971 For 3 1/2 billion years, life had lived exclusively in the water. 795 00:51:50,174 --> 00:51:52,609 But now, in a great breathtaking adventure... 796 00:51:52,811 --> 00:51:54,039 ...it took to the land. 797 00:51:54,246 --> 00:51:56,373 But if things had gone a little differently... 798 00:51:56,582 --> 00:51:59,313 ...the dominant species might still be in the ocean... 799 00:51:59,519 --> 00:52:03,889 ...or developed spaceships to carry them off the planet altogether. 800 00:52:09,531 --> 00:52:11,590 From our ancestors, the reptiles... 801 00:52:11,800 --> 00:52:14,166 ...there developed many successful lines... 802 00:52:14,369 --> 00:52:16,930 ...including the dinosaurs. 803 00:52:17,240 --> 00:52:20,209 Some were fast, dexterous and intelligent. 804 00:52:20,409 --> 00:52:22,378 A visitor from another world or time... 805 00:52:22,579 --> 00:52:25,343 ...might have thought them the wave of the future. 806 00:52:25,549 --> 00:52:30,112 But after nearly 200 million years, they were suddenly all wiped out. 807 00:52:30,321 --> 00:52:33,086 Perhaps it was a great meteorite colliding with the Earth... 808 00:52:33,292 --> 00:52:36,022 ...spewing debris into the air, blotting out the sun... 809 00:52:36,228 --> 00:52:38,754 ...and killing the plants that the dinosaurs ate. 810 00:52:38,965 --> 00:52:43,459 I wonder when they first sensed that something was wrong. 811 00:52:45,573 --> 00:52:49,339 The successors of the dinosaurs came from the same reptilian stock... 812 00:52:49,544 --> 00:52:53,982 ...but they survived the catastrophe that destroyed their cousins. 813 00:52:56,586 --> 00:52:59,282 Again, there were many branches which became extinct. 814 00:52:59,489 --> 00:53:01,787 And had events been a little different... 815 00:53:01,992 --> 00:53:05,450 ...those branches might have led to the dominant form today. 816 00:53:08,333 --> 00:53:11,393 For 40 million years, a visitor would not have been impressed... 817 00:53:11,603 --> 00:53:13,731 ...by these timid little creatures... 818 00:53:13,940 --> 00:53:17,569 ...but they led to all the familiar mammals of today. 819 00:53:20,080 --> 00:53:23,049 And that includes the primates. 820 00:53:23,951 --> 00:53:27,319 About 20 million years ago, a space time traveler... 821 00:53:27,522 --> 00:53:30,150 ...might have recognized these guys as promising... 822 00:53:30,358 --> 00:53:33,954 ...bright, quick, agile, sociable, curious. 823 00:53:34,196 --> 00:53:37,257 Their ancestors were once atoms made in stars... 824 00:53:37,467 --> 00:53:40,300 ...then simple molecules, single cells... 825 00:53:40,503 --> 00:53:42,767 ...polyps stuck to the ocean floor... 826 00:53:42,974 --> 00:53:46,375 ...fish, amphibians, reptiles, shrews. 827 00:53:46,777 --> 00:53:51,181 But then they came down from the trees and stood upright. 828 00:53:51,383 --> 00:53:53,749 They grew an enormous brain... 829 00:53:53,952 --> 00:53:57,081 ...they developed culture, invented tools... 830 00:53:57,290 --> 00:53:59,349 ...domesticated fire. 831 00:54:02,896 --> 00:54:05,558 They discovered language and writing. 832 00:54:05,766 --> 00:54:07,758 They developed agriculture. 833 00:54:07,969 --> 00:54:11,564 They built cities and forged metal. 834 00:54:13,342 --> 00:54:17,176 And ultimately, they set out for the stars... 835 00:54:17,380 --> 00:54:21,715 ...from which they had come 5 billion years earlier. 836 00:54:24,122 --> 00:54:25,646 We are star stuff... 837 00:54:25,857 --> 00:54:29,258 ...which has taken its destiny into its own hands. 838 00:54:32,064 --> 00:54:34,191 The loom of time and space... 839 00:54:34,400 --> 00:54:38,030 ...works the most astonishing transformations of matter. 840 00:54:39,205 --> 00:54:42,198 Our own planet is only a tiny part... 841 00:54:42,410 --> 00:54:44,537 ...of the vast cosmic tapestry... 842 00:54:44,745 --> 00:54:49,479 ...a starry fabric of worlds yet untold. 843 00:54:56,359 --> 00:55:00,387 Those worlds in space are as countless... 844 00:55:00,597 --> 00:55:04,294 ...as all the grains of sand on all the beaches of the Earth. 845 00:55:04,968 --> 00:55:07,904 Each of those worlds is as real as ours. 846 00:55:08,106 --> 00:55:10,597 In every one of them, there's a succession of... 847 00:55:10,809 --> 00:55:15,440 ...incidents, events, occurrences which influence its future. 848 00:55:15,648 --> 00:55:19,244 Countless worlds, numberless moments... 849 00:55:19,453 --> 00:55:22,912 ...an immensity of space and time. 850 00:55:23,124 --> 00:55:25,922 And our small planet, at this moment... 851 00:55:26,127 --> 00:55:30,565 ...here, we face a critical branchpoint in history. 852 00:55:30,766 --> 00:55:33,667 What we do with our world right now... 853 00:55:33,869 --> 00:55:36,168 ...will propagate down through the centuries... 854 00:55:36,373 --> 00:55:39,774 ...and powerfully affect the destiny of our descendants. 855 00:55:39,976 --> 00:55:43,777 It is well within our power to destroy our civilization... 856 00:55:43,981 --> 00:55:46,815 ...and perhaps our species as well. 857 00:55:47,018 --> 00:55:49,418 If we capitulate to superstition... 858 00:55:49,621 --> 00:55:51,987 ...or greed or stupidity... 859 00:55:52,190 --> 00:55:56,127 ...we can plunge our world into a darkness deeper than the time... 860 00:55:56,329 --> 00:56:00,664 ...between the collapse of classical civilization and Italian Renaissance. 861 00:56:00,868 --> 00:56:03,132 But we are also capable... 862 00:56:03,337 --> 00:56:05,829 ...of using our compassion and our intelligence... 863 00:56:06,040 --> 00:56:08,634 ...our technology and our wealth... 864 00:56:08,843 --> 00:56:11,312 ...to make an abundant and meaningful life... 865 00:56:11,514 --> 00:56:13,812 ...for every inhabitant of this planet... 866 00:56:14,016 --> 00:56:18,511 ...to enhance enormously our understanding of the universe... 867 00:56:18,922 --> 00:56:21,983 ...and to carry us to the stars. 868 00:56:37,844 --> 00:56:39,472 In our motorbike sequence... 869 00:56:39,680 --> 00:56:42,171 ...we showed how the landscape might look... 870 00:56:42,383 --> 00:56:45,114 ...if we barreled through it at close to light speed. 871 00:56:45,320 --> 00:56:48,255 Since then, inspired by this sequence... 872 00:56:48,457 --> 00:56:52,053 ...Ping-Kang Hsiung at Carnegie Mellon University... 873 00:56:52,261 --> 00:56:54,195 ...produced an exact computer animation. 874 00:56:54,697 --> 00:56:57,929 This is what you'd see if you traveled at ordinary speeds... 875 00:56:58,135 --> 00:57:00,365 ...through this red and white lattice. 876 00:57:00,571 --> 00:57:02,402 But this is how it would appear... 877 00:57:02,606 --> 00:57:06,543 ...if you were traveling at close to the speed of light. 878 00:57:07,412 --> 00:57:11,440 We're probably many centuries away from traveling close to light speed... 879 00:57:11,650 --> 00:57:14,518 ...and experiencing time dilation. 880 00:57:14,721 --> 00:57:17,588 But even then, it might not be fast enough... 881 00:57:17,791 --> 00:57:21,193 ...if we wanted to travel to some distant place in the galaxy... 882 00:57:21,395 --> 00:57:24,125 ...and then come back to Earth in our own epoch. 883 00:57:24,632 --> 00:57:27,727 Some years after completing Cosmos... 884 00:57:27,936 --> 00:57:32,465 ...I took time out from my scientific work to write a novel. 885 00:57:32,976 --> 00:57:34,568 A novel about travel... 886 00:57:34,777 --> 00:57:37,974 ...to the center of the Milky Way galaxy. 887 00:57:38,249 --> 00:57:41,582 I was willing to imagine beings and civilizations... 888 00:57:41,785 --> 00:57:43,810 ...far more advanced than we... 889 00:57:44,022 --> 00:57:47,321 ...but I wasn't willing to ignore the laws of physics. 890 00:57:47,559 --> 00:57:52,122 Was there, even in principle, a way to get very quickly... 891 00:57:52,331 --> 00:57:55,300 ...to 30,000 light-years from Earth? 892 00:57:55,501 --> 00:57:57,367 So I asked my friend... 893 00:57:57,571 --> 00:58:00,802 ...Kip Thorne of the California Institute of Technology. 894 00:58:01,008 --> 00:58:04,035 He's a leading expert on the nature of space and time. 895 00:58:04,245 --> 00:58:06,713 Kip thought about it for a while... 896 00:58:06,915 --> 00:58:10,010 ...and then answered with about 50 lines of equations... 897 00:58:10,219 --> 00:58:12,949 ...which showed that a really advanced civilization... 898 00:58:13,155 --> 00:58:17,092 ...might establish and hold open wormholes... 899 00:58:19,297 --> 00:58:22,994 ...which we might think of as tubes through the fourth dimension... 900 00:58:23,200 --> 00:58:26,034 ...which connect the Earth with another place... 901 00:58:26,238 --> 00:58:29,696 ...without having to traverse the intervening distance. 902 00:58:29,942 --> 00:58:33,811 Something like crawling through a wormhole in an apple. 903 00:58:34,247 --> 00:58:36,112 I was happy with this result... 904 00:58:36,316 --> 00:58:39,809 ...and used it as a key plot device in Contact. 905 00:58:40,287 --> 00:58:42,312 But such wormholes through space... 906 00:58:42,523 --> 00:58:45,459 ...would also be time machines, it seemed to me. 907 00:58:45,660 --> 00:58:48,528 And I used that notion in my novel Contact as well. 908 00:58:48,998 --> 00:58:53,094 Kip Thorne and his colleagues later proved, or so it seemed... 909 00:58:53,302 --> 00:58:55,771 ...that time travel of this sort was possible. 910 00:58:55,973 --> 00:58:58,305 Here, look at this. 911 00:58:59,509 --> 00:59:02,138 The key question being explored now... 912 00:59:02,347 --> 00:59:05,805 ...is whether such time travel can be done consistently... 913 00:59:06,017 --> 00:59:10,318 ...with causes preceding effects, say, rather than following them. 914 00:59:10,522 --> 00:59:11,989 Does nature contrive it... 915 00:59:12,192 --> 00:59:15,423 ...so that even with a time machine, you can't intervene... 916 00:59:15,629 --> 00:59:18,724 ...to prevent your own conception, for example? 917 00:59:18,933 --> 00:59:21,959 Even if time travel of this sort is really possible... 918 00:59:22,169 --> 00:59:25,037 ...it's far in our technological future. 919 00:59:25,240 --> 00:59:29,199 But maybe other beings much more advanced than we... 920 00:59:29,411 --> 00:59:32,347 ...are voyaging to the far future and the remote past... 921 00:59:32,548 --> 00:59:35,541 ...not a measly 40 years ago on Earth... 922 00:59:35,753 --> 00:59:38,244 ...but to witness the death of the sun, say... 923 00:59:38,455 --> 00:59:40,650 ...or the origin of the cosmos.