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These are the user uploaded subtitles that are being translated: 1 00:00:12,387 --> 00:00:14,056 PROFESSOR BRIAN COX: Why are we here? 2 00:00:14,139 --> 00:00:16,058 Where do we come from? 3 00:00:16,099 --> 00:00:18,894 These are the most enduring of guestions. 4 00:00:19,228 --> 00:00:22,022 And it's an essential part of human nature 5 00:00:22,105 --> 00:00:24,274 to want to find the answers. 6 00:00:30,072 --> 00:00:33,700 Now, we can trace our ancestry back hundreds of thousands of years 7 00:00:33,742 --> 00:00:35,410 to the dawn of humarıkiınd. 8 00:00:35,702 --> 00:00:40,999 But in reality, our story extends far further back in time. 9 00:00:41,083 --> 00:00:44,545 Our story starts with the beginning of the universe. 10 00:00:49,049 --> 00:00:52,511 It began 13.7 billion years ago. 11 00:00:56,473 --> 00:01:00,269 And today it's filled with oöver a hundred billion galaxies, 12 00:01:00,352 --> 00:01:04,106 each containing hundreds of billions of stars. 13 00:01:10,696 --> 00:01:13,615 In this series, I want to tell that story. 14 00:01:13,699 --> 00:01:17,160 Because ultimately, we are part of the universe. 15 00:01:17,244 --> 00:01:20,455 So its story İis our story. 16 00:01:25,502 --> 00:01:29,256 It's a story that you couldn't tell without something so fundamental 17 00:01:29,339 --> 00:01:32,968 that's it's impossible to imagine the universe without it. 18 00:01:33,051 --> 00:01:37,431 It's woven into the very fabric of the cosmos. Time. 19 00:01:41,893 --> 00:01:46,315 The relentless flow of time has driven the evolution of the universe 20 00:01:46,398 --> 00:01:49,943 and created many extraordinary wonders. 21 00:01:52,821 --> 00:01:56,033 These wonders take us from the very first moments 22 00:01:56,116 --> 00:02:00,078 in the life of the universe to its eventual end. 23 00:02:29,858 --> 00:02:33,028 This is Chankillo on the northwestern coast of Peru. 24 00:02:33,111 --> 00:02:37,824 And it's one of South America's lesser known archaeological sites, 25 00:02:37,908 --> 00:02:41,411 but for me, it is surely one of the most fascinating. 26 00:02:50,504 --> 00:02:52,839 Around two and a half thousand years ago, 27 00:02:52,923 --> 00:02:56,093 a civilisation we know almost nothing about 28 00:02:56,176 --> 00:02:58,970 built this fortified temple in the desert. 29 00:03:03,767 --> 00:03:08,188 Its walls were once brilliant white and covered with painted figures. 30 00:03:12,484 --> 00:03:16,947 Today, all but the smallest fragments of the decorations are göne. 31 00:03:17,030 --> 00:03:22,285 The details of this culture and all traces of its language are lost. 32 00:03:26,748 --> 00:03:29,501 And yet, if you stand in the right place, 33 00:03:29,584 --> 00:03:33,296 you can still experience the true purpose of Chankillo 34 00:03:33,380 --> 00:03:36,383 in fust the same way as you could the day it was built. 35 00:03:41,471 --> 00:03:45,350 But to do that, you have to be here before the sun rises. 36 00:04:00,157 --> 00:04:03,034 These towers form an ancient solar calendar. 37 00:04:04,202 --> 00:04:07,330 Now, at different times of year, the sunrise point 38 00:04:07,414 --> 00:04:09,708 is at a different place on the horizon. 39 00:04:09,791 --> 00:04:11,293 Actually, December 21st, 40 00:04:11,376 --> 00:04:16,006 which here in the Southern Hemisphere is the summer solstice, the longest day, 41 00:04:16,089 --> 00:04:20,677 and the sun Trises just to the right of the rightmost tower. 42 00:04:20,761 --> 00:04:25,432 Then as the year passes, the surı moves through the towers, 43 00:04:25,515 --> 00:04:28,769 until, on June 21st, which is the winter solstice, 44 00:04:28,852 --> 00:04:32,481 the shortest day, it rises just to the left 45 00:04:32,564 --> 00:04:33,732 of the leftmost tower. 46 00:04:33,815 --> 00:04:37,235 Actually, just in between that mountain you can see in the distance 47 00:04:37,319 --> 00:04:38,820 and the leftmost tower. 48 00:04:38,904 --> 00:04:43,033 So at any time of year, if you watch the sun rise, 49 00:04:43,116 --> 00:04:45,827 you can measure its position and you can tell, 50 00:04:45,911 --> 00:04:50,624 within an accuracy of two or three days, the date. 51 00:04:50,707 --> 00:04:53,585 Today's date is September the 15th, 52 00:04:53,668 --> 00:04:59,466 and so that means that the sun will rise between the fifth and the sixth towers. 53 00:05:11,061 --> 00:05:13,438 Chankillo still works as a calendar, 54 00:05:13,522 --> 00:05:17,025 because the sun still rises in the same place today 55 00:05:17,108 --> 00:05:20,278 as it did when these stones were first laid down. 56 00:05:25,450 --> 00:05:30,038 Now, that is a magnificent sight, as the sun burns through the towers. 57 00:05:37,879 --> 00:05:40,382 You can almost feel the presence of the past here. 58 00:05:40,465 --> 00:05:42,259 I mean, imagine what it must have been like. 59 00:05:42,300 --> 00:05:46,221 Thousands of citizens stood here to greet the sun, 60 00:05:46,304 --> 00:05:49,599 which was almost certainly a deity, almost certainly their god. 61 00:05:50,517 --> 00:05:51,935 What a magnificent achievement. 62 00:05:52,018 --> 00:05:54,020 I mean, it's probably one of our earliest attempts 63 00:05:54,062 --> 00:05:57,440 to begin to measure the heavens. 64 00:06:11,580 --> 00:06:13,540 Over the millennia, 65 00:06:13,582 --> 00:06:17,460 that desire to measure what's goling on in the sky 66 00:06:17,502 --> 00:06:20,005 has led to modern astronomy 67 00:06:20,088 --> 00:06:23,133 and the foundations of our modern civilisation. 68 00:06:32,684 --> 00:06:34,978 I might build öone in my garden. 69 00:06:37,647 --> 00:06:40,066 (LAUGHS) I want one. 70 00:06:49,409 --> 00:06:53,121 The 13 towers that line this ridge stand testament 71 00:06:53,204 --> 00:06:57,000 to our enduring fascination with the clockwork of the heavens 72 00:06:58,627 --> 00:07:02,881 and to the direct connection between our lives and the cosmos. 73 00:07:05,300 --> 00:07:09,220 The rising and setting of the sun Pprovides an epic heartbeat 74 00:07:09,262 --> 00:07:12,390 that allows us to mark the passage of time. 75 00:07:20,649 --> 00:07:24,152 A day on Farth is the 24 hours it takes our planet 76 00:07:24,235 --> 00:07:26,571 to rotate once on its axis. 77 00:07:34,788 --> 00:07:38,333 Our months are based on the 29 and a half days 78 00:07:38,416 --> 00:07:42,087 it takes the moon to wax and wane im the night sky. 79 00:07:45,799 --> 00:07:49,844 And a year is the 365 and a guarter days 80 00:07:49,928 --> 00:07:52,764 it takes us to orbit öonce around the sun. 81 00:07:56,059 --> 00:07:59,396 These familiar time scales mark the passing of our lives, 82 00:08:00,939 --> 00:08:04,859 but the life of the universe plays out on a much grander scale. 83 00:08:09,614 --> 00:08:14,077 When you look up into the night sky, you don't just see stars, 84 00:08:14,160 --> 00:08:18,498 those tiny points of light are a million different clocks, 85 00:08:18,540 --> 00:08:20,917 whose life spans mark out the passage of time 86 00:08:21,001 --> 00:08:24,295 over billions or even trillions of years. 87 00:08:33,430 --> 00:08:37,350 This film is about the greatest expanses of time. 88 00:08:37,976 --> 00:08:41,146 The deep time that shapes the universe. 89 00:08:43,231 --> 00:08:45,108 From its fiery beginnings 90 00:08:45,191 --> 00:08:49,821 through countless generations of stars, planets and galaxies 91 00:08:49,904 --> 00:08:51,906 to its eventual demise, 92 00:08:51,990 --> 00:08:56,578 the fate of the universe is determined by the passage of time. 93 00:09:05,295 --> 00:09:09,674 Time scales in the cosmos seem so unimaginabliy vast 94 00:09:09,758 --> 00:09:11,926 it's almost impossible to relate to them. 95 00:09:13,678 --> 00:09:17,849 Yet there are places on Earth where we can begin to encounter time 96 00:09:17,932 --> 00:09:19,976 on these universal scales. 97 00:09:27,984 --> 00:09:32,572 This is Ostional on the northern Pacific coast of Costa Rica, 98 00:09:32,655 --> 00:09:36,910 and I've come here to witmess a natural event 99 00:09:36,993 --> 00:09:40,997 that's been happening long before there were arıy humans here to see it. 100 00:09:41,081 --> 00:09:43,792 And I suppose it really is a window 101 00:09:43,875 --> 00:09:47,378 into the distant past of life on our planet. 102 00:09:53,760 --> 00:09:55,178 (INDISTINCT) 103 00:10:10,610 --> 00:10:13,446 Once the sun has dipped below the horizon 104 00:10:13,530 --> 00:10:17,325 and the moon conspired to make the tides just right, 105 00:10:17,408 --> 00:10:20,829 this beach is visited by prehistoric creatures. 106 00:10:31,881 --> 00:10:35,260 Ünder the cover of darkness, they emerge from the ocean. 107 00:10:44,102 --> 00:10:47,355 Playa Ostional is öone of the few beaches in the world 108 00:10:47,438 --> 00:10:50,525 where large numbers of sea turtles make their nests. 109 00:11:00,869 --> 00:11:03,288 But what makes this truly remarkable 110 00:11:03,371 --> 00:11:07,500 is the sheer length of time scenes like this have been playing out. 111 00:11:13,506 --> 00:11:18,219 This is part of one of the oldest life cycles on Earth. 112 00:11:18,303 --> 00:11:23,433 On nights like these for the last hundred million years, 113 00:11:23,516 --> 00:11:27,103 turtles like this have been hauling themselves out of the ocean 114 00:11:27,187 --> 00:11:28,479 to lay their eggs. 115 00:11:30,732 --> 00:11:33,318 It's an almost incomprehensible time span. 116 00:11:33,401 --> 00:11:37,030 I mean, a hundred million years ago there were dinosaurs roaming the Earth 117 00:11:37,113 --> 00:11:40,200 but the Earth itself looked very different. 118 00:11:40,283 --> 00:11:44,954 I mean, South America was not connected to North America. 119 00:11:45,038 --> 00:11:48,082 North America was somewhere over close to Europe. 120 00:11:48,166 --> 00:11:51,044 Australia was connected to Antarctica. 121 00:11:57,884 --> 00:12:01,930 It really is guite wonderful to be so close 122 00:12:02,013 --> 00:12:05,433 to such an ancient cycle of life. 123 00:12:07,268 --> 00:12:09,520 I can hear her breathing, actually. 124 00:12:11,731 --> 00:12:13,483 (BREATHING HEAVILY) 125 00:12:20,990 --> 00:12:23,618 It's a remarkable experience. 126 00:12:23,660 --> 00:12:26,913 I mean, it really is beautiful to see that 127 00:12:27,747 --> 00:12:32,126 on öone night of many hundreds of millions of nights 128 00:12:32,210 --> 00:12:34,420 stretching back into the past. 129 00:12:42,679 --> 00:12:44,222 And she's gone. 130 00:12:55,566 --> 00:12:57,402 To witness a moment like this 131 00:12:57,485 --> 00:13:00,655 is to open up a connection to the deep past, 132 00:13:03,241 --> 00:13:08,246 to experience time spans far longer than the history of our own species. 133 00:13:10,540 --> 00:13:14,502 Yet even the hundred million years story of the turtles 134 00:13:14,585 --> 00:13:19,590 only begins to connect us with the vast sweep of cosmic time. 135 00:13:26,097 --> 00:13:31,686 Our entire solar system is travelling on an unimagğinabiy vast orbit, 136 00:13:31,769 --> 00:13:34,522 spinning around the centre of our galaxy. 137 00:13:41,237 --> 00:13:44,449 Ittakes 250 million years 138 00:13:44,532 --> 00:13:47,243 to make just one circuit of the Milky Way. 139 00:13:54,709 --> 00:13:57,670 In the entire history of the human race, 140 00:13:57,754 --> 00:14:01,799 we've travelled less than a tenth of one percent of that orbit. 141 00:14:07,138 --> 00:14:11,309 These cycles seem eternal and unchanging, 142 00:14:11,392 --> 00:14:13,770 but as the story of time unfolds, 143 00:14:13,853 --> 00:14:16,147 a fundamental truth is revealed. 144 00:14:21,444 --> 00:14:23,529 Nothing lasts forever. 145 00:14:30,745 --> 00:14:33,581 This is the most profound property of time. 146 00:14:38,086 --> 00:14:41,172 And it plays out Just as vividiy here on Farth 147 00:14:41,255 --> 00:14:43,966 aâs it does in the depths of space. 148 00:15:02,819 --> 00:15:07,573 This is the Perito Moreno Glacier in Patagonia in Southern Argentina, 149 00:15:07,657 --> 00:15:10,743 and it's one of the hundreds of glaciers 150 00:15:10,827 --> 00:15:15,998 that sweep down the continent from the southern Patagonian ice fields. 151 00:15:16,082 --> 00:15:19,544 And you know, if you carry on that way, 152 00:15:19,627 --> 00:15:22,755 so south, about... I don't know about 1000 kilometres 153 00:15:22,839 --> 00:15:25,258 you get to the end of South America 154 00:15:25,299 --> 00:15:28,719 and from then on there's nothing till the Antarctic. 155 00:15:29,345 --> 00:15:31,973 (LAUGHS) And it feels like that today. 156 00:15:41,899 --> 00:15:45,361 The glacier is such a massive expanse of ice, 157 00:15:45,445 --> 00:15:48,948 but at first sight just like the cycles of the heavens, 158 00:15:49,031 --> 00:15:51,784 it appears fixed and unchanging. 159 00:15:56,372 --> 00:15:58,124 (CRACKING) 160 00:16:00,877 --> 00:16:04,964 Yet seen close-up, it's continually on the move, 161 00:16:05,047 --> 00:16:08,468 as it has been for tens of thousands of years. 162 00:16:24,734 --> 00:16:29,697 The whole face of the glacier is moving into the lake, 163 00:16:29,780 --> 00:16:32,450 about something like that much every day. 164 00:16:32,533 --> 00:16:37,205 And that means that well over a guarter of a billion tons of ice 165 00:16:37,288 --> 00:16:40,541 drop off the face of the glacier into the lake every year. 166 00:16:40,625 --> 00:16:42,418 It's about a million tons a day. 167 00:16:42,460 --> 00:16:43,794 And you can hear it happening. 168 00:16:43,878 --> 00:16:48,424 Just every now and again, you hear this tremendous cracking sound. 169 00:16:48,508 --> 00:16:51,677 It really is like the place is alive. 170 00:16:54,514 --> 00:16:56,015 (RUMBLING) 171 00:17:04,607 --> 00:17:05,983 You know, it's guite disturbing 172 00:17:06,067 --> 00:17:09,237 when these enormous chunks of ice fall into the lake. 173 00:17:09,320 --> 00:17:14,408 Although this thing seems stable and the movement seems glacially slow, 174 00:17:14,492 --> 00:17:18,412 actually, there can be really violent collapses. 175 00:17:18,496 --> 00:17:22,583 It's an incredibly dynamic place to be. 176 00:17:33,928 --> 00:17:39,392 This movement of the glacier provides an İnsight into the nature of time. 177 00:17:39,475 --> 00:17:44,814 It is simply the öordering of events into seguences, öone step after another. 178 00:17:49,652 --> 00:17:54,031 As time passes, snow falls, ice forms, 179 00:17:54,115 --> 00:17:57,076 the glacier gradually inches down the valley 180 00:17:57,159 --> 00:18:00,705 and huge chunks of ice fall into the lake below. 181 00:18:03,040 --> 00:18:07,295 But even this simple seguvence contains a profound idea. 182 00:18:10,047 --> 00:18:12,967 Events always happen in the same order. 183 00:18:13,676 --> 00:18:16,846 They're never jumbled up and they never go backwards. 184 00:18:33,237 --> 00:18:37,241 Now, that is something that you would never see in reverse. 185 00:18:37,325 --> 00:18:40,995 But interestingly, there's nothing about the laws of physics 186 00:18:41,078 --> 00:18:45,082 that describe how all those water molecules are moving around 187 00:18:45,166 --> 00:18:49,420 that prevent them from all getting together on the surface of the lake, 188 00:18:49,462 --> 00:18:53,341 jumping out of the water, sticking together into a block of ice 189 00:18:53,424 --> 00:18:57,511 and then gluing themselves back onto the surface of the glacier again. 190 00:18:57,595 --> 00:19:02,058 But interestingly, we do understand 191 00:19:02,099 --> 00:19:05,102 why the world doesn't run in reverse. 192 00:19:05,186 --> 00:19:08,564 There is a reason, we have a scientific explanation, 193 00:19:08,648 --> 00:19:11,359 and it's called the arrow of time. 194 00:19:22,078 --> 00:19:24,955 We never see waves travelling across lakes, 195 00:19:25,039 --> 00:19:29,377 coming together and bouncing chunks of ice back onto glaciers. 196 00:19:32,505 --> 00:19:35,716 We are compelled to travel into the future. 197 00:19:36,926 --> 00:19:39,845 And that's because the arrow of time dictates 198 00:19:39,929 --> 00:19:43,474 that as each moment passes, things change. 199 00:19:43,557 --> 00:19:47,353 And önce these changes have happened, they are never undone. 200 00:19:52,108 --> 00:19:57,988 Permanent change is a fundamental part of what it means to be human. 201 00:19:58,072 --> 00:20:01,325 You know, we all age as the years pass by. 202 00:20:01,409 --> 00:20:04,829 People are born, they live and they die. 203 00:20:05,496 --> 00:20:09,792 I suppose it's part of the joy and tragedy of our lives. 204 00:20:09,875 --> 00:20:14,505 But out there in the universe, those grand and epic cycles 205 00:20:14,588 --> 00:20:19,260 appear eternal and unchanging, but that's an illusion. 206 00:20:19,343 --> 00:20:22,763 See, in the life of the universe, jJust as in our lives, 207 00:20:22,847 --> 00:20:26,517 everything is irreversibly changing. 208 00:20:35,985 --> 00:20:38,696 By building change upon change, 209 00:20:38,779 --> 00:20:43,451 the arrow of time drives the evolutron of the entire universe. 210 00:20:45,161 --> 00:20:47,913 And as we look out deep in to the cosmos, 211 00:20:47,997 --> 00:20:50,332 we can see that story unfold. 212 00:20:54,128 --> 00:20:58,507 This is an image of a tiny piece of night sky 213 00:20:58,591 --> 00:21:00,426 in the constellation of Leo. 214 00:21:00,509 --> 00:21:03,512 It's actually where the mouth of the lion would be. 215 00:21:04,013 --> 00:21:05,973 And despite appearances, 216 00:21:06,056 --> 00:21:11,312 it is oöne of the most interesting images taken in recent astronomical history. 217 00:21:12,354 --> 00:21:15,274 The interesting thing is this little red blob here, 218 00:21:15,357 --> 00:21:18,694 which looks very unremarkable. 219 00:21:18,778 --> 00:21:20,571 But what that red blob is 220 00:21:20,654 --> 00:21:25,659 is the afterglow of an enormous cosmic explosion. 221 00:21:25,868 --> 00:21:29,163 It's the death of a star that was about, what, 222 00:21:29,246 --> 00:21:34,043 40 or even 50 times the mass of our sun. 223 00:21:39,215 --> 00:21:43,677 Poetically named GRB 090423, 224 00:21:43,761 --> 00:21:46,764 it was öonce a Wolf-Rayet star. 225 00:21:51,685 --> 00:21:54,855 Shrouded by rapidiy swirling clouds of gas, 226 00:21:54,939 --> 00:21:59,026 it burned 10,000 times more brightly than our sun. 227 00:22:01,737 --> 00:22:05,950 But because it burned so brightiy, it was extremely short-lived. 228 00:22:08,619 --> 00:22:12,414 As it died, the giant star collapsed in on itself. 229 00:22:13,082 --> 00:22:16,502 That caused massive jets of light and stellar materlal 230 00:22:16,585 --> 00:22:18,587 to be ejected from its poles, 231 00:22:18,671 --> 00:22:23,551 in an explosion that shone with the light of ten million billion suns. 232 00:22:30,891 --> 00:22:34,728 And it's the afterglow of this catastrophic explosion 233 00:22:34,812 --> 00:22:39,525 that is just visible from our planet as a faint red dot. 234 00:22:45,489 --> 00:22:50,327 But that's not what's so interesting about GRB 090423. 235 00:22:50,411 --> 00:22:55,040 You see, when we look up into the sky at distant stars and galaxies, 236 00:22:55,124 --> 00:22:56,959 then we're looking back in time, 237 00:22:57,042 --> 00:23:01,463 because the light takes time to journey from them to us. 238 00:23:01,547 --> 00:23:06,010 And the light from that red dot has been travelling to us 239 00:23:06,093 --> 00:23:10,055 for almost the entire history of the universe. 240 00:23:10,556 --> 00:23:12,391 You see, what we're looking at here 241 00:23:12,474 --> 00:23:17,062 is an event that happened 13 billion years ago. 242 00:23:17,146 --> 00:23:19,607 I mean, that's only about 600 million years 243 00:23:19,690 --> 00:23:22,651 after the Big Bang, after the universe began. 244 00:23:22,735 --> 00:23:27,781 So this is something incredibly early in the universe's history. 245 00:23:27,865 --> 00:23:32,036 In fact, this is the oldest single object 246 00:23:32,119 --> 00:23:34,330 that we've ever seen. 247 00:23:34,830 --> 00:23:37,917 What we're looking at here is the explosive death 248 00:23:38,000 --> 00:23:41,503 of one of the first stars in the universe. 249 00:23:53,724 --> 00:23:58,187 As it evolves, the universe passes through distinct eras. 250 00:24:00,314 --> 00:24:03,859 Vast ages, whose beginnings and endings 251 00:24:03,901 --> 00:24:06,862 are marked by unigue milestones. 252 00:24:08,948 --> 00:24:12,242 The births and deaths of its wonders. 253 00:24:18,582 --> 00:24:23,462 The moment the first stars were born is one of the most important changes 254 00:24:23,545 --> 00:24:25,839 in the evolution of the cosmos. 255 00:24:28,300 --> 00:24:31,178 It signals the end of the Primordial Era 256 00:24:31,887 --> 00:24:36,517 and marks the beginning of the second great age of the universe. 257 00:24:38,060 --> 00:24:40,938 The time in which we live, 258 00:24:40,980 --> 00:24:43,107 the Stelliferous Era, 259 00:24:43,190 --> 00:24:45,943 the age of the stars. 260 00:24:50,698 --> 00:24:53,701 Starlight illuminates the night sky 261 00:24:53,742 --> 00:24:57,121 and starlight illuminates our days. 262 00:24:57,204 --> 00:25:02,209 Our sun is just one of 200 billion stars in our galaxy. 263 00:25:02,292 --> 00:25:07,172 And our galaxy is one of 100 billion in the observable universe. 264 00:25:07,715 --> 00:25:11,802 Countless islands of countless stars. 265 00:25:24,648 --> 00:25:28,068 Although the universe is over 13 billion years old, 266 00:25:28,152 --> 00:25:32,406 we still live close to the start of the Stelliferous Era. 267 00:25:32,489 --> 00:25:37,703 And it's an age of astonishing beauty and complexity in the universe. 268 00:25:41,290 --> 00:25:45,377 The cosmos is absolutely awash with stars, 269 00:25:45,461 --> 00:25:48,547 surrounded by nebulae and systems of planets. 270 00:25:49,131 --> 00:25:54,303 There are countless billions of worlds that we've yet to explore. 271 00:26:00,934 --> 00:26:05,439 But the cosmos isn't static and unchanging 272 00:26:05,522 --> 00:26:08,567 and it won't always be this way. 273 00:26:08,650 --> 00:26:12,112 Because as the arrow of time plays out, 274 00:26:12,196 --> 00:26:16,825 it produces a universe that is as dynamic as it's beautiful. 275 00:26:24,458 --> 00:26:28,420 We've seen stars born and we've seen stars die. 276 00:26:29,088 --> 00:26:33,258 And we know that tomorrow won't be the same as today. 277 00:26:33,425 --> 00:26:35,761 Because the arrow of time says the future 278 00:26:35,844 --> 00:26:38,680 will always be different from the past. 279 00:26:41,058 --> 00:26:43,268 But what drives this evolution? 280 00:26:44,061 --> 00:26:48,190 Why is there a difference between the past and the future? 281 00:26:49,066 --> 00:26:52,069 MWhy is there an arrow of time at all? 282 00:27:13,757 --> 00:27:17,261 We all have an intuitive understanding of the arrow of time. 283 00:27:22,391 --> 00:27:25,102 İt seems obvious to us that things change 284 00:27:25,185 --> 00:27:27,980 and the future will be different to the past. 285 00:27:33,861 --> 00:27:36,530 We know that because we see the effects 286 00:27:36,613 --> 00:27:39,449 of the passing years all around us. 287 00:27:49,459 --> 00:27:54,131 This is Kolmanskop, an abandoned diamond-mining town in southern Namibia. 288 00:28:03,140 --> 00:28:06,351 Now, this entire town was founded in 1908, 289 00:28:06,435 --> 00:28:08,729 when a worker, who was building the railway 290 00:28:08,812 --> 00:28:12,691 from the Port of Lüderitz inland, into the centre of Namibia, 291 00:28:12,774 --> 00:28:17,112 found a single diamond here in this desert. 292 00:28:33,170 --> 00:28:37,841 For 40 years, this was a thriving community of up to 1000 people, 293 00:28:37,925 --> 00:28:40,469 a place where you could become a millionaire 294 00:28:40,594 --> 00:28:42,930 picking diamonds out of the sand. 295 00:28:47,142 --> 00:28:50,437 While the money rolled in, they built grand houses 296 00:28:50,520 --> 00:28:53,523 and lived a champagne lifestyle in the desert. 297 00:28:56,860 --> 00:29:00,530 But when the diamonds dried up the town was abandoned 298 00:29:00,614 --> 00:29:04,076 and for half a century, it's fallen into disrepair 299 00:29:04,159 --> 00:29:07,037 as it's slowly reclaimed by the sands. 300 00:29:23,553 --> 00:29:26,181 The processes at play here at Kolmanskop 301 00:29:26,265 --> 00:29:29,059 are happening everywhere in the universe. 302 00:29:29,935 --> 00:29:32,229 Because it isn't simply permanent change 303 00:29:32,312 --> 00:29:34,481 that's central to the arrow of time, 304 00:29:34,940 --> 00:29:36,275 it's decay. 305 00:29:39,569 --> 00:29:42,698 But the scientific explanation for why that is 306 00:29:43,532 --> 00:29:45,575 didn't come from attempting to understand 307 00:29:45,659 --> 00:29:48,161 the effects of time in the universe. 308 00:29:49,788 --> 00:29:52,666 It came from trying to build a faster train. 309 00:29:55,585 --> 00:29:57,379 Back in the 19th century, 310 00:29:57,421 --> 00:30:01,758 engineers were concerned with the efficiency of steam engines. 311 00:30:01,883 --> 00:30:04,886 You know, how hot should the fire be? 312 00:30:04,970 --> 00:30:07,180 What substance should you boil in the steam engine? 313 00:30:07,222 --> 00:30:09,599 Should it be water or should it be something else? 314 00:30:09,683 --> 00:30:12,060 These were profound guestions. 315 00:30:12,102 --> 00:30:16,064 And out of those guestions arose the science of thermodynamics. 316 00:30:16,189 --> 00:30:19,776 It's when concepts like heat and temperature and energy 317 00:30:19,860 --> 00:30:22,863 entered the scientific vocabulary for the first time. 318 00:30:23,447 --> 00:30:27,075 Now, along with that deeper understanding 319 00:30:27,159 --> 00:30:31,330 emerged what is probably the most important law of physics 320 00:30:31,413 --> 00:30:34,499 for understanding the evolution of the universe 321 00:30:34,541 --> 00:30:36,668 and the passage of time. 322 00:30:36,752 --> 00:30:39,796 It's called the second law of thermodynamics. 323 00:30:45,093 --> 00:30:48,764 The reason the second law of thermodynamics was so profound 324 00:30:48,847 --> 00:30:53,393 was because at its heart it contained a radically new concept, 325 00:30:54,561 --> 00:30:57,064 something physicists call "entropy”". 326 00:31:00,609 --> 00:31:04,529 Entropy explains why, left to the mercy of the elemenits, 327 00:31:04,613 --> 00:31:09,201 mortar crumbles, glass shatters and buildings collapse. 328 00:31:12,621 --> 00:31:15,916 And a good way to üunderstand how is to think of objects 329 00:31:15,999 --> 00:31:20,629 not as single things, but as being made up of many constituent parts, 330 00:31:21,296 --> 00:31:25,258 like the individual grains that make up this pile of sand. 331 00:31:28,303 --> 00:31:30,180 Now, entropy is a measure 332 00:31:30,222 --> 00:31:32,682 of how many ways I can rearrange those grains 333 00:31:32,766 --> 00:31:35,435 and still keep the sand pile the same. 334 00:31:35,519 --> 00:31:40,440 And there are trillions and trillions and trillions of ways of doing that. 335 00:31:40,565 --> 00:31:43,193 I mean, pretty much anything I do to this sand pile, 336 00:31:43,276 --> 00:31:46,029 if I mess the sand around and move it around, 337 00:31:46,113 --> 00:31:48,824 then it doesn't change the shape or the structure at all. 338 00:31:49,324 --> 00:31:51,952 So, in the language of entropy, 339 00:31:52,035 --> 00:31:54,204 this sand pile has high entropy, 340 00:31:54,329 --> 00:31:57,082 because there are many, many ways that I can rearrange 341 00:31:57,165 --> 00:31:59,751 its constituents and not change it. 342 00:32:00,794 --> 00:32:04,506 But now let me create some order in the universe. 343 00:32:12,055 --> 00:32:14,975 Now, there are approximately as many sand grains 344 00:32:15,058 --> 00:32:18,770 in this sandcastle as there are in the sand pile. 345 00:32:18,854 --> 00:32:22,941 But now, virtually anything I do to it will mess it up, 346 00:32:23,024 --> 00:32:26,820 will remove the beautiful order from this structure. 347 00:32:26,862 --> 00:32:30,699 And because of that, the sandcastle has a low entropy. 348 00:32:30,782 --> 00:32:33,118 Iİt's a much more ordered state. 349 00:32:33,368 --> 00:32:36,830 So many ways of rearranging the sand grains 350 00:32:36,913 --> 00:32:40,500 without changing the structure, high entropy. 351 00:32:40,542 --> 00:32:43,420 Very few ways of rearranging the sand grains 352 00:32:43,503 --> 00:32:48,717 without changing the structure, without disordering it, low entropy. 353 00:32:57,893 --> 00:33:01,146 Now, imagine I was to leave this castle in the desert all day, 354 00:33:01,229 --> 00:33:03,648 then it's obvious what's going to happen. 355 00:33:03,732 --> 00:33:06,485 The desert winds are going to blow the sand around 356 00:33:06,568 --> 00:33:10,113 and this castle is going to disintegrate. 357 00:33:10,238 --> 00:33:12,199 It's going to become less ordered, 358 00:33:12,616 --> 00:33:14,367 it's going to fall to bits. 359 00:33:17,913 --> 00:33:21,249 But think about what's happening on a fundamental level. 360 00:33:21,333 --> 00:33:24,920 I mean, the wind is taking the sand off the castle 361 00:33:25,045 --> 00:33:28,882 and blowing it over there somewhere, and making a sand pile. 362 00:33:29,007 --> 00:33:32,344 There's nothing fundamental in the laws of physics 363 00:33:32,427 --> 00:33:36,806 that says that the wind couldn't pick up some sand from over here, 364 00:33:36,890 --> 00:33:42,437 deposit it here and deposit it in precisely the shape of a sandcastle. 365 00:33:43,146 --> 00:33:46,900 You know, in principle, the wimd could spontaneously build 366 00:33:46,983 --> 00:33:49,819 a sandcastle out of a pile of sand. 367 00:34:00,664 --> 00:34:02,541 There's no reason why that couldn't happen. 368 00:34:02,624 --> 00:34:05,752 İt's just extremely, extremely unlikely, 369 00:34:05,835 --> 00:34:09,339 because there are very few ways of organising this sand 370 00:34:09,381 --> 00:34:11,341 so that it looks like a castle. 371 00:34:18,014 --> 00:34:20,267 It's overwhelmingly more likely 372 00:34:20,350 --> 00:34:22,602 that when the wind blows the sand around, 373 00:34:22,686 --> 00:34:26,147 it will take the low entropy structure, the castle, 374 00:34:26,273 --> 00:34:30,277 and turn it into a high entropy structure, the sand pile. 375 00:34:37,701 --> 00:34:41,162 So entropy always increases. 376 00:34:41,580 --> 00:34:46,960 Why is that? Because it's overwhelmingly more likely that it will. 377 00:34:56,386 --> 00:34:59,931 It seems incredible that a law that says that sandcastles 378 00:35:00,015 --> 00:35:02,267 don't spontaneously form on the wind 379 00:35:03,226 --> 00:35:06,354 could solve one of the deepest mysteries in physics. 380 00:35:09,232 --> 00:35:12,485 But by saying entropy always increases, 381 00:35:12,569 --> 00:35:16,615 the second law of thermodynamics is able to explain 382 00:35:16,698 --> 00:35:19,534 why time only runs in öone direction. 383 00:35:35,175 --> 00:35:38,595 The second law of thermodynamics, for me, demonstrates everything 384 00:35:38,678 --> 00:35:43,600 that is powerful and beautiful and profound about physics. 385 00:35:43,683 --> 00:35:46,728 You see, here's a law that entered science as a way of talking 386 00:35:46,811 --> 00:35:50,357 about how heat moves around and the efficiency of steam engines, 387 00:35:50,899 --> 00:35:54,527 but it ended up being able to explain 388 00:35:54,611 --> 00:35:57,989 one of the great mysteries in the history of science. 389 00:35:58,114 --> 00:36:01,660 Why is there a difference between the past anıd the future? 390 00:36:02,285 --> 00:36:04,204 You see, the second law says 391 00:36:04,287 --> 00:36:08,041 that everything tends from order to disorder. 392 00:36:08,583 --> 00:36:12,879 That means that there is a difference between the past and the future. 393 00:36:12,962 --> 00:36:15,465 In the past, the universe was more ordered, 394 00:36:15,548 --> 00:36:19,052 and in the future, the universe will be less ordered. 395 00:36:19,469 --> 00:36:22,806 And that means that there's a direction to the passage of time. 396 00:36:22,889 --> 00:36:25,558 So the second law of thermodynamics 397 00:36:25,642 --> 00:36:27,727 has introduced the concept 398 00:36:27,811 --> 00:36:31,022 of an arrow of time into science. 399 00:36:38,988 --> 00:36:42,659 The arrow of time has been playing out in Kolmanskop 400 00:36:42,701 --> 00:36:45,912 since the mining facility Was abandoned in 1954. 401 00:36:48,707 --> 00:36:51,459 But in the universe, it's been playing out 402 00:36:51,501 --> 00:36:53,753 for almost 14 billion years, 403 00:36:55,004 --> 00:36:57,632 and it will have profound conseguences. 404 00:37:05,056 --> 00:37:08,727 Because it means stars cannot shine forever, 405 00:37:09,686 --> 00:37:13,523 including the star at the centre of our solar system. 406 00:37:16,401 --> 00:37:20,989 At the end of its life, the sun won't simply fade away to nothing. 407 00:37:23,950 --> 00:37:27,871 As it begins to run out of fuel, its core will collapse, 408 00:37:28,329 --> 00:37:32,751 and the extra heat this generates Will cause its outer layers to expand. 409 00:37:39,674 --> 00:37:41,843 In around a billion years' time, 410 00:37:41,926 --> 00:37:46,389 this will have a catastrophic effect on our fragile world. 411 00:37:53,062 --> 00:37:56,316 Gradually, the Farth will become hotter and hotter. 412 00:37:56,399 --> 00:38:00,695 So there will be one last perfect day on Earth, 413 00:38:00,779 --> 00:38:05,784 but eventually the existence of all life on this planet will become impossible. 414 00:38:10,371 --> 00:38:12,624 Long after life has disappeared, 415 00:38:12,707 --> 00:38:16,961 the sun will have grown so much, it will fill the entire horizon. 416 00:38:23,843 --> 00:38:28,139 It will have become a red giant, the last phase of its life. 417 00:38:38,316 --> 00:38:40,777 Our planet might not survive to this pornt, 418 00:38:41,277 --> 00:38:45,573 but if it does, little more than a scorched and barren rock 419 00:38:45,782 --> 00:38:49,744 will remain to witness the final death throes of our star. 420 00:38:50,537 --> 00:38:52,121 (RUMBLING) 421 00:39:00,672 --> 00:39:05,468 In six billion years, our sun will explode, 422 00:39:05,552 --> 00:39:08,513 throwing vast amounts of gas and dust 423 00:39:08,596 --> 00:39:11,432 out into space to form a gigantic nebula. 424 00:39:18,982 --> 00:39:23,570 And at its heart will be a faintly glowing ember, 425 00:39:23,695 --> 00:39:27,156 all that remains of our once-magnificent sun. 426 00:39:27,240 --> 00:39:29,868 It will be smaller than the size of the Earth, 427 00:39:29,951 --> 00:39:34,581 less than a millionth of its current volume and a fraction of its brightness. 428 00:39:35,290 --> 00:39:38,001 Our sun will have become a white dwarf. 429 00:39:47,552 --> 00:39:52,015 With no fuel left to burn, a white dwarf's faint glow 430 00:39:52,098 --> 00:39:56,311 comes from the last residual heat from its extinguished furnace. 431 00:39:59,105 --> 00:40:01,065 The sun is now dead, 432 00:40:01,441 --> 00:40:03,610 its remarns slowly cooling 433 00:40:03,693 --> 00:40:06,613 in the freezing temperatures of deep space. 434 00:40:10,700 --> 00:40:13,119 Looking at it from where the Farth is now, 435 00:40:13,202 --> 00:40:15,955 it would only generate the same amount of light 436 00:40:16,039 --> 00:40:18,708 as the full moon on a clear night. 437 00:40:24,005 --> 00:40:28,217 The fate of the sun is the same as for all stars. 438 00:40:28,259 --> 00:40:31,471 One day, they must all eventually die 439 00:40:31,554 --> 00:40:35,099 and the cosmos will be plunged into eternal night. 440 00:40:35,767 --> 00:40:38,519 And this is the most profound conseguence 441 00:40:38,603 --> 00:40:40,188 of the arrow of time. 442 00:40:40,271 --> 00:40:43,399 Because this structured universe that we inhabit 443 00:40:43,483 --> 00:40:47,403 and all its wonders, the stars and the planets and the galaxies, 444 00:40:47,779 --> 00:40:50,114 carınot last forever. 445 00:40:50,198 --> 00:40:54,369 The cosmos will eventually fade and die. 446 00:40:58,831 --> 00:41:02,126 First will come the end of the Stelliferocus Era, 447 00:41:02,210 --> 00:41:04,712 the end of the age of starliğht. 448 00:41:08,800 --> 00:41:11,552 The largest stars are the first to disappear, 449 00:41:11,636 --> 00:41:14,555 vrolentiy collapsing into black holes, 450 00:41:14,639 --> 00:41:17,350 Just a few million years after their formation. 451 00:41:20,228 --> 00:41:24,565 But long after they're göne, Just one iype of star will remailn. 452 00:41:28,569 --> 00:41:33,908 This is a picture of the nearest star to our solar system, Proxima Centauri. 453 00:41:33,992 --> 00:41:37,829 Now, it's only 4.2 light years away, but the reason it doesn't stand out 454 00:41:37,912 --> 00:41:40,707 against the much more distant stars in this photograph 455 00:41:41,082 --> 00:41:44,252 is that Proxima Centauri is incredibly tiny. 456 00:41:44,711 --> 00:41:47,380 It's the kind of star known as a red dwarf star, 457 00:41:47,463 --> 00:41:51,175 and it's only about 11 to 12 percent the mass of our sun. 458 00:41:51,634 --> 00:41:57,181 But to our eyes, İt would appear to shine 18,000 times less brightly. 459 00:41:59,809 --> 00:42:02,770 But red dwarves do have one advantage 460 00:42:02,854 --> 00:42:06,774 over their much more luminous and magnificent stellar brethren. 461 00:42:06,858 --> 00:42:09,569 And that's because they're so small, 462 00:42:09,736 --> 00:42:13,072 they burn their nuclear fuel incredibly slowly, 463 00:42:13,156 --> 00:42:16,743 so they have life spans of trillions of years. 464 00:42:17,160 --> 00:42:20,663 And that means that stars like Proxima Centauri 465 00:42:20,747 --> 00:42:24,375 will be the last living stars in the universe. 466 00:42:30,089 --> 00:42:34,093 If we survive into the far future of the universe, 467 00:42:34,177 --> 00:42:37,638 then it's possible to imagine our distant descendents 468 00:42:37,722 --> 00:42:41,309 building their civilisation around red dwarves, 469 00:42:41,392 --> 00:42:47,482 to capture the energy from those last fading embers of stars. 470 00:42:48,024 --> 00:42:52,028 Just as our ancestors crowded around campfires 471 00:42:52,111 --> 00:42:55,573 for warmth on cold winters' nights. 472 00:43:13,966 --> 00:43:17,720 The reason why Proxima Centauri burns so slowly 473 00:43:17,804 --> 00:43:20,890 is because its small size and low gravity 474 00:43:20,973 --> 00:43:24,769 meanı its core is under much lower pressure than larger stars. 475 00:43:26,437 --> 00:43:30,233 This also means that its interior is constantly churning, 476 00:43:30,316 --> 00:43:33,361 whipping up the surface into a fiery turmoll. 477 00:43:36,489 --> 00:43:40,034 Explosive solar flares occur almost continually, 478 00:43:40,118 --> 00:43:42,703 even though it burms so dimly. 479 00:43:44,789 --> 00:43:48,543 But Proxima Centauri will eventually die. 480 00:43:49,210 --> 00:43:53,047 And like our sun, it too will become a white dwarf. 481 00:43:54,006 --> 00:43:56,300 As the age of starlight ends, 482 00:43:56,342 --> 00:44:00,763 all but the dimmest flicker of light in the universe will go out. 483 00:44:01,806 --> 00:44:06,269 The faint glow of white dwarves will provide the only illumination 484 00:44:07,019 --> 00:44:13,067 in a dark and empty vord littered with dead stars and black holes. 485 00:44:15,403 --> 00:44:20,366 By this point, the universe will be 100 trillion years old. 486 00:44:24,287 --> 00:44:28,666 And yet, even now, the vast mafjority of its lifespan 487 00:44:28,749 --> 00:44:30,459 still lies ahead of it. 488 00:44:47,393 --> 00:44:49,478 There are few places on Earth 489 00:44:49,562 --> 00:44:53,608 where you can get an inkling of what the far future has in store. 490 00:45:06,913 --> 00:45:09,498 Well, this is Namibia's Skeleton Coast, 491 00:45:09,540 --> 00:45:13,669 where the cold waters of the South Atlantic meet the Namib Desert. 492 00:45:13,753 --> 00:45:17,006 And it is one of the most inhospitable places on Earth. 493 00:45:17,089 --> 00:45:18,799 I mean, back in the 17th century, 494 00:45:18,883 --> 00:45:20,801 Portuguese sailors used to call this place 495 00:45:20,885 --> 00:45:25,139 "the gates to hell", because this dense fog that you see 496 00:45:25,181 --> 00:45:28,100 pretty much every morning along this coast, 497 00:45:28,184 --> 00:45:32,897 coupled with the constantly shifting shape of the sandbanks 498 00:45:32,980 --> 00:45:34,941 meant that over the years, 499 00:45:35,024 --> 00:45:38,819 literally thousands of ships were wrecked along this coastline. 500 00:45:43,950 --> 00:45:47,286 And even if you made it to shore, that wasn't the end of your problems, 501 00:45:47,370 --> 00:45:49,580 because the currents are so strong here 502 00:45:49,664 --> 00:45:53,125 that there is no way of rowing back out to sea. 503 00:45:53,209 --> 00:45:56,629 And if you look that way, there's just hundreds of miles 504 00:45:56,712 --> 00:45:58,506 of inhospitable desert. 505 00:46:01,175 --> 00:46:05,304 So, it genuinely was a place of no return. 506 00:46:05,388 --> 00:46:09,892 If you were shipwrecked here, this was the end of your universe. 507 00:46:21,112 --> 00:46:23,197 This is the Eduard Bohlen. 508 00:46:23,656 --> 00:46:25,700 She was öonce an öcean-gorng steamer, 509 00:46:25,783 --> 00:46:29,078 ferrying passengers and cargo between here and Furope. 510 00:46:33,916 --> 00:46:38,212 On the 5th of September, 1909, she ran aground in thick fog. 511 00:46:44,343 --> 00:46:47,847 Yek, like all the vessels wrecked along this shoreline, 512 00:46:47,930 --> 00:46:50,683 the time it takes her to decay to nothing 513 00:46:50,766 --> 00:46:53,644 will be far longer than her time at sea. 514 00:46:58,941 --> 00:47:01,235 And in the far future of the cosmos, 515 00:47:01,360 --> 00:47:04,488 a similar destiny awaits the remaining white dwarves. 516 00:47:11,162 --> 00:47:15,291 A black dwarf will be the final fate of those last stars, 517 00:47:15,374 --> 00:47:17,877 white dwarves that have become so cold 518 00:47:17,960 --> 00:47:21,005 that they barely emit any more heat or light. 519 00:47:25,176 --> 00:47:29,180 Black dwarves are dark, dense, decaying balls 520 00:47:29,263 --> 00:47:31,015 of degenerate matter, 521 00:47:31,724 --> 00:47:34,268 little more than the ashes of stars. 522 00:47:36,103 --> 00:47:39,732 Their constituent atoms are so severely crushed 523 00:47:39,815 --> 00:47:43,861 that black dwarves are a million times denser than our sun. 524 00:47:45,404 --> 00:47:50,826 Stars take so long to reach this point that after nearly 14 billion years, 525 00:47:50,910 --> 00:47:55,247 we believe there are currentiy no black dwarves in the universe. 526 00:47:56,082 --> 00:47:58,000 But despite never seeing one, 527 00:47:58,084 --> 00:48:00,711 we can still predict how they will end their days. 528 00:48:02,213 --> 00:48:06,050 Just as the iron that makes up this ship will eventually rust 529 00:48:06,133 --> 00:48:09,345 and be carried away by the desert winds, 530 00:48:09,512 --> 00:48:12,723 so we thirik that the matter inside black dwarves, 531 00:48:12,848 --> 00:48:15,559 the last matter in the universe, 532 00:48:15,768 --> 00:48:19,897 will eventually evaporate away and be carried off 533 00:48:19,980 --> 00:48:25,820 into the void as radiation, leaving absolutely nothing behind. 534 00:48:35,329 --> 00:48:37,540 With the black dwarves gone, 535 00:48:37,623 --> 00:48:40,543 there won't be a single atom of matter left. 536 00:48:43,003 --> 00:48:46,257 All that will remain of our önce-rich cosmos 537 00:48:46,340 --> 00:48:49,593 will be particles of light and black holes. 538 00:48:56,600 --> 00:48:59,854 After an unimaginable length of time, 539 00:48:59,937 --> 00:49:02,940 even the black holes will have evaporated 540 00:49:02,982 --> 00:49:06,652 and the universe will be nothing but a sea 541 00:49:06,735 --> 00:49:10,739 of photons gradually tending towards the same temperature, 542 00:49:10,906 --> 00:49:15,369 as the expansion of the universe cools them towards absolute zero. 543 00:49:23,919 --> 00:49:26,380 And when I say unimaginable period of time, 544 00:49:26,422 --> 00:49:27,506 I really mean it. 545 00:49:27,590 --> 00:49:30,801 It's 10,000 trillion trillion trillion trillion 546 00:49:30,885 --> 00:49:34,263 trillion trillion trillion trillion years. 547 00:49:34,847 --> 00:49:36,432 Now, how big is that number? 548 00:49:36,515 --> 00:49:40,227 Well, if I were to start counting with a single atom 549 00:49:40,311 --> 00:49:44,023 representing one year, then there wouldn't be enough atoms 550 00:49:44,106 --> 00:49:48,819 in the entire universe to get anywhere near that number. 551 00:49:55,493 --> 00:49:59,205 Once the very last remnants of the very last stars 552 00:49:59,288 --> 00:50:01,665 have finally decayed away to nothing 553 00:50:01,749 --> 00:50:05,044 and everything reaches the same temperature, 554 00:50:05,127 --> 00:50:09,215 the story of the universe finally comes to an end. 555 00:50:12,927 --> 00:50:15,054 For the first time in its life, 556 00:50:15,137 --> 00:50:18,390 the universe will be permanent and unchanging. 557 00:50:19,433 --> 00:50:22,269 Entropy finally stops increasing, 558 00:50:22,353 --> 00:50:25,731 because the cosmos carınot get any more disordered. 559 00:50:26,524 --> 00:50:30,069 Nothing happens and it keeps not happening, 560 00:50:30,945 --> 00:50:32,279 forever. 561 00:50:36,700 --> 00:50:39,912 It's what's known as the heat death of the universe, 562 00:50:39,995 --> 00:50:42,331 an era when the cosmos will remain 563 00:50:42,414 --> 00:50:46,585 vast and cold and desolate for the rest of time. 564 00:50:47,419 --> 00:50:49,672 And that's because there is no difference 565 00:50:49,755 --> 00:50:52,466 between the past, the present and the future. 566 00:50:52,550 --> 00:50:55,261 There's no way of measuring the passage of time, 567 00:50:55,344 --> 00:50:58,639 because nothing in the cosmos changes. 568 00:50:59,390 --> 00:51:03,310 The arrow of time has simply ceased to exist. 569 00:51:14,029 --> 00:51:17,366 İt's an inescapable fact of the universe, 570 00:51:17,449 --> 00:51:20,995 written into the fundamental laws of physics. 571 00:51:21,078 --> 00:51:23,872 The entire cosmos will die. 572 00:51:28,294 --> 00:51:33,465 Every single öone of the 200 billion stars in our galaxy 573 00:51:33,549 --> 00:51:35,009 will go out. 574 00:51:37,052 --> 00:51:39,138 And just as the death of the sun 575 00:51:39,179 --> 00:51:41,640 means the end of life on our planet, 576 00:51:41,724 --> 00:51:44,059 so the death of every star 577 00:51:44,143 --> 00:51:48,314 will extingursh any possibility of life in the universe. 578 00:51:52,276 --> 00:51:54,945 The fact that the sun will die, 579 00:51:55,029 --> 00:51:58,866 and it will incinerate the Earth and obliterate all life 580 00:51:58,949 --> 00:52:03,454 on our planet in the process, might sound a bit depressing to you. 581 00:52:03,871 --> 00:52:05,331 You might legitimately ask, 582 00:52:05,414 --> 00:52:08,792 "Well, surely you could build a universe in a different way, 583 00:52:08,876 --> 00:52:12,379 "surely you could build it so it didn't have to descend 584 00:52:12,421 --> 00:52:14,173 "from order into chaos?" 585 00:52:14,923 --> 00:52:19,386 Well, the answer is, "No, you couldn't, if you wanted life to exist in it." 586 00:52:25,225 --> 00:52:28,270 The arrow of time, the seguence of changes 587 00:52:28,354 --> 00:52:31,357 that slowly leads the universe to its death, 588 00:52:31,440 --> 00:52:33,817 is the very same thing that creates 589 00:52:33,901 --> 00:52:36,737 the conditions for life in the first place. 590 00:52:41,033 --> 00:52:43,452 Because it takes time for matter to form 591 00:52:44,078 --> 00:52:46,205 and it takes time for gravity 592 00:52:46,288 --> 00:52:49,249 to pull it together into stars and planets. 593 00:52:54,254 --> 00:52:57,091 The arrow of time creates a bright window 594 00:52:57,174 --> 00:53:01,512 in the universe's adolescence, during which life is possible. 595 00:53:08,894 --> 00:53:12,022 But it's a window that doesn't stay open for long. 596 00:53:15,484 --> 00:53:18,987 As a fraction of the life span of the universe, 597 00:53:19,071 --> 00:53:21,115 as measured from its beginning 598 00:53:21,198 --> 00:53:24,076 to the evaporation of the last black hole, 599 00:53:24,159 --> 00:53:27,579 life as we know it is only possible 600 00:53:27,621 --> 00:53:31,083 for one thousandth of a billion billion billionth, 601 00:53:31,166 --> 00:53:35,796 billion billion billionth, billion billion billionth of a percent. 602 00:53:38,841 --> 00:53:40,801 And that's why, for me, 603 00:53:40,884 --> 00:53:43,887 the most astonishing wonder of the universe 604 00:53:43,971 --> 00:53:47,099 isn'ta star or a planet or a galaxy. 605 00:53:48,475 --> 00:53:52,563 Itisn'tathing at all. İt's an instant in time. 606 00:53:53,814 --> 00:53:56,150 And that time is now. 607 00:54:04,199 --> 00:54:07,953 Humans have walked the Farth for just the smallest fraction 608 00:54:08,078 --> 00:54:11,415 of that briefest of moments in deep time. 609 00:54:14,334 --> 00:54:17,463 But in our 200,000 years on this planet, 610 00:54:17,546 --> 00:54:19,840 we've made remarkable progress. 611 00:54:22,676 --> 00:54:25,679 İt was only two and a half thousand years ago 612 00:54:25,763 --> 00:54:28,557 that we believed that the sun was a god 613 00:54:28,640 --> 00:54:31,351 and measured its orbit with stone towers 614 00:54:31,435 --> 00:54:33,979 built on the top of a hill. 615 00:54:35,731 --> 00:54:40,611 Today, the language of curiosity is not sun gods but science. 616 00:54:40,903 --> 00:54:44,031 And we have observatories that are almost infinitely 617 00:54:44,114 --> 00:54:46,450 more sophisticated than the 13 towers, 618 00:54:46,533 --> 00:54:49,495 that can gaze out deep into the universe. 619 00:54:53,248 --> 00:54:56,168 And perhaps even more remarkably, 620 00:54:56,251 --> 00:54:58,253 through theoretical physics and mathematics, 621 00:54:58,337 --> 00:55:03,592 we can calculate what the universe will look like in the distant future 622 00:55:03,926 --> 00:55:08,347 and we can even make concrete predictions about its end. 623 00:55:15,979 --> 00:55:20,943 And I believe it's only by continuing our exploration of the cosmos 624 00:55:21,026 --> 00:55:23,570 and the laws of nature that gövern it 625 00:55:23,654 --> 00:55:26,448 that we can truly understand ourselves 626 00:55:26,532 --> 00:55:30,160 and our place in this universe of wonders. 627 00:55:34,373 --> 00:55:38,919 And that's what we've done in our brief moment on planet Farth. 628 00:55:42,756 --> 00:55:46,134 In 1977, a space probe called Voyager 7 629 00:55:46,218 --> 00:55:49,513 was launched on a grand tour of the solar system. 630 00:55:50,514 --> 00:55:55,185 And it visited the great gas-giant planets Jupiter and Saturn 631 00:55:55,269 --> 00:55:57,521 and made some wonderful discoveries 632 00:55:57,604 --> 00:56:01,024 before heading off towards interstellar space. 633 00:56:03,318 --> 00:56:07,698 13 years later, after its mission was almost over, 634 00:56:07,739 --> 00:56:11,869 it turned around and took one last picture 635 00:56:11,952 --> 00:56:13,745 of its home solar system. 636 00:56:14,162 --> 00:56:16,039 And this is that picture. 637 00:56:18,125 --> 00:56:20,794 And the beautiful thing about this picture 638 00:56:20,919 --> 00:56:24,590 is this single pixel of light 639 00:56:25,382 --> 00:56:27,759 suspended against the blackness of space. 640 00:56:27,843 --> 00:56:32,180 Because that pixel, that point, is planet Earth, 641 00:56:32,598 --> 00:56:35,267 the most distant picture of our planet ever taken 642 00:56:35,350 --> 00:56:38,228 at six billion kilometres away. 643 00:56:47,613 --> 00:56:52,075 And whilst 1 suppose it has very limited scientific value, 644 00:56:52,159 --> 00:56:54,786 for me, this tiny pornt of light 645 00:56:54,870 --> 00:56:58,248 is the most powerful and profound demonstration 646 00:56:58,373 --> 00:57:01,335 of perhaps the most human of guüalities, 647 00:57:02,628 --> 00:57:04,963 our unigue ability to reflect 648 00:57:05,047 --> 00:57:08,842 on the universe's existence and our place within it. 649 00:57:13,972 --> 00:57:16,975 Just as we, and all life on Earth, 650 00:57:17,059 --> 00:57:21,313 stand on this tiny speck adrift in infinite space, 651 00:57:21,980 --> 00:57:25,317 so life in the universe will only exist 652 00:57:25,400 --> 00:57:30,155 for a fleeting, bright instant in time. 653 00:57:30,697 --> 00:57:35,327 Because life, just like the stars and the planets and the galaxies, 654 00:57:35,410 --> 00:57:40,958 is just a temporary structure on the lomg road from order to disorder. 655 00:57:50,467 --> 00:57:52,260 But that doesn't make us insignificant, 656 00:57:52,302 --> 00:57:55,013 because we are the cosmos made conscious. 657 00:57:55,514 --> 00:58:00,352 Life is the means by which the universe understands itself. 658 00:58:02,896 --> 00:58:06,858 And for me, our true significance lies in our ability 659 00:58:07,651 --> 00:58:12,906 and our desire to understand and explore this beautiful universe. 660 00:58:17,285 --> 00:58:20,956 (WE HAVE ALL THE TIME IN THE WORLD BY LOUIS ARMSTRONG PLAYING) 57389

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