0:00:11 > 0:00:14Why are we here? Where do we come from?
0:00:14 > 0:00:17These are the most enduring of questions.
0:00:17 > 0:00:22And it's an essential part of human nature to want to find the answers.
0:00:28 > 0:00:30And we can trace that ancestry back
0:00:30 > 0:00:34hundreds of thousands of years to the dawn of humankind.
0:00:34 > 0:00:39But in reality, our story extends far further back in time.
0:00:39 > 0:00:43Our story starts with the beginning of the universe.
0:00:47 > 0:00:51It began 13.7 billion years ago.
0:00:56 > 0:00:59And today it's filled with over 100 billion galaxies,
0:00:59 > 0:01:03each containing hundreds of billions of stars.
0:01:09 > 0:01:12In the series, I want to tell that story
0:01:12 > 0:01:16because ultimately we are part of the universe.
0:01:16 > 0:01:19So its story is our story.
0:01:24 > 0:01:28It's a story that you couldn't tell without something so fundamental
0:01:28 > 0:01:31that it's impossible to imagine the universe without it.
0:01:31 > 0:01:36It's woven into the very fabric of the cosmos. Time.
0:01:41 > 0:01:46The relentless flow of time has driven the evolution of the universe
0:01:46 > 0:01:49and created many extraordinary wonders.
0:01:51 > 0:01:55These wonders take us from the very first moments in the life
0:01:55 > 0:01:58of the universe to its eventual end.
0:02:27 > 0:02:32This is Chankillo on the north-western coast of Peru.
0:02:32 > 0:02:37And it's one of South America's lesser known archaeological sites.
0:02:37 > 0:02:40But, for me, it is surely one of the most fascinating.
0:02:49 > 0:02:51Around 2,500 years ago,
0:02:51 > 0:02:54a civilisation we know almost nothing about
0:02:54 > 0:02:57built this fortified temple in the desert.
0:03:02 > 0:03:07Its walls were once brilliant white and covered with painted figures.
0:03:11 > 0:03:14Today, all but the smallest fragments
0:03:14 > 0:03:15of the decorations are gone.
0:03:15 > 0:03:21The details of this culture and all traces of its language are lost.
0:03:26 > 0:03:29And yet, if you stand in the right place, you can still experience
0:03:29 > 0:03:32the true purpose of Chankillo,
0:03:32 > 0:03:36in just the same way as you could the day that it was built.
0:03:40 > 0:03:44But, to do that, you have to be here before the sun rises.
0:03:59 > 0:04:02These towers form an ancient solar calendar.
0:04:02 > 0:04:05Now, at different times of year,
0:04:05 > 0:04:09the sunrise point is at a different place on the horizon.
0:04:09 > 0:04:1221st December, which here in the southern hemisphere
0:04:12 > 0:04:15is the summer solstice, the longest day,
0:04:15 > 0:04:20the sun rises just to the right of the right-most tower.
0:04:20 > 0:04:24Then, as the year passes, the sun moves through the towers
0:04:24 > 0:04:28until on 21st June, the winter solstice,
0:04:28 > 0:04:33the shortest day, it rises just to left of the left-most tower.
0:04:33 > 0:04:35Actually just in-between that mountain
0:04:35 > 0:04:38you can see in the distance and the left-most tower.
0:04:38 > 0:04:42So, at any time of year, if you watch the sun rise,
0:04:42 > 0:04:45you can measure its position and you can tell,
0:04:45 > 0:04:49within an accuracy of two or three days, the date.
0:04:49 > 0:04:53Today's date is September the 15th.
0:04:53 > 0:04:54So that means the sun
0:04:54 > 0:04:58will rise between the fifth and the sixth towers.
0:05:10 > 0:05:12Chankillo still works as a calendar
0:05:12 > 0:05:16because the sun still rises in the same place today
0:05:16 > 0:05:19as it did when these stones were first laid down.
0:05:24 > 0:05:29That's a magnificent sight, as the sun burns through the towers.
0:05:36 > 0:05:39You can almost feel the presence of the past here.
0:05:39 > 0:05:41Imagine what it must have been like.
0:05:41 > 0:05:45Thousands of citizens stood here to greet the sun,
0:05:45 > 0:05:48which was almost certainly a deity. Almost certainly their god.
0:05:48 > 0:05:51What a magnificent achievement.
0:05:51 > 0:05:54Probably one of our earliest attempts
0:05:54 > 0:05:57to begin to measure the heavens.
0:06:10 > 0:06:12Over the millennia,
0:06:12 > 0:06:16that desire to measure what's going on in the sky
0:06:16 > 0:06:18has led to modern astronomy
0:06:18 > 0:06:22and the foundations of our modern civilisation.
0:06:31 > 0:06:33I might build one in my garden.
0:06:37 > 0:06:39I want one!
0:06:48 > 0:06:52The 13 towers that line this ridge stand testament to our
0:06:52 > 0:06:57enduring fascination with the clockwork of the heavens.
0:06:57 > 0:07:03And to the direct connection between our lives and the cosmos.
0:07:04 > 0:07:08The rising and setting of the sun provides an epic heartbeat
0:07:08 > 0:07:11that allows us to mark the passage of time.
0:07:19 > 0:07:22A day on Earth is the 24 hours
0:07:22 > 0:07:25it takes our planet to rotate once on its axis.
0:07:34 > 0:07:37Our months are based on the 29-and-a-half days
0:07:37 > 0:07:41it takes the moon to wax and wane in the night sky.
0:07:44 > 0:07:48And a year is the 365-and-a-quarter days
0:07:48 > 0:07:52it takes us to orbit once around the sun.
0:07:54 > 0:07:59These familiar timescales mark the passing of our lives.
0:07:59 > 0:08:04But the life of the universe plays out on a much grander scale.
0:08:08 > 0:08:12When you look up into the night sky, you don't just see stars.
0:08:12 > 0:08:17Those tiny points of light are a million different clocks,
0:08:17 > 0:08:21whose lifespans mark out the passage of time over billions,
0:08:21 > 0:08:23or even trillions, of years.
0:08:32 > 0:08:36This film is about the greatest expanses of time.
0:08:36 > 0:08:40The deep time that shapes the universe.
0:08:42 > 0:08:46From its fiery beginnings, through countless generations of stars,
0:08:46 > 0:08:50planets and galaxies, to its eventual demise,
0:08:50 > 0:08:55the fate of the universe is determined by the passage of time.
0:09:04 > 0:09:08Timescales in the cosmos seem so unimaginably vast,
0:09:08 > 0:09:10it's almost impossible to relate to them.
0:09:12 > 0:09:14Yet there are places on Earth
0:09:14 > 0:09:18where we can begin to encounter time on these universal scales.
0:09:26 > 0:09:32This is Ostional on the northern Pacific coast of Costa Rica.
0:09:32 > 0:09:37I've come here to witness a natural event that's been happening
0:09:37 > 0:09:40long before there were any humans here to see it.
0:09:40 > 0:09:43And I suppose it really is a window
0:09:43 > 0:09:46into the distant past of life on our planet.
0:10:09 > 0:10:11Once the sun has dipped below the horizon
0:10:11 > 0:10:16and the moon conspired to make the tides just right,
0:10:16 > 0:10:20this beach is visited by prehistoric creatures.
0:10:31 > 0:10:34Under the cover of darkness, they emerge from the ocean.
0:10:43 > 0:10:46Playa Ostional is one of the few beaches in the world
0:10:46 > 0:10:49where large numbers of sea turtles make their nests.
0:11:00 > 0:11:04But what makes this truly remarkable is the sheer length of time
0:11:04 > 0:11:07scenes like this have been playing out.
0:11:13 > 0:11:17This is part of one of the oldest life-cycles on Earth.
0:11:17 > 0:11:22On nights like these, for the last 100 million years, turtles like this
0:11:22 > 0:11:27have been hauling themselves out of the ocean to lay their eggs.
0:11:29 > 0:11:32It's an almost incomprehensible timespan.
0:11:32 > 0:11:36100 million years ago, there were dinosaurs roaming the Earth,
0:11:36 > 0:11:39but the Earth itself looked very different.
0:11:39 > 0:11:43South America was not connected to North America.
0:11:43 > 0:11:47North America was somewhere over close to Europe.
0:11:47 > 0:11:50Australia was connected to Antarctica.
0:11:57 > 0:11:59It really is quite...
0:11:59 > 0:12:04wonderful to be so close to such an ancient cycle of life.
0:12:05 > 0:12:08I can hear breathing, actually.
0:12:20 > 0:12:23So, a remarkable experience.
0:12:23 > 0:12:26I mean, it really is beautiful to see that.
0:12:26 > 0:12:30On one night of many hundreds of millions of nights
0:12:30 > 0:12:33stretching back into the past.
0:12:41 > 0:12:43And she's gone.
0:12:53 > 0:12:56To witness a moment like this
0:12:56 > 0:12:59is to open up a connection to the deep past.
0:13:02 > 0:13:07To experience timespans far longer than the history of our own species.
0:13:09 > 0:13:13Yet even the 100-million-year story of the turtles
0:13:13 > 0:13:18only begins to connect us with the vast sweep of cosmic time.
0:13:25 > 0:13:28Our entire solar system is travelling
0:13:28 > 0:13:30on an unimaginably vast orbit,
0:13:30 > 0:13:33spinning around the centre of our galaxy.
0:13:40 > 0:13:46It takes 250 million years to make just one circuit of the Milky Way.
0:13:53 > 0:13:56In the entire history of the human race,
0:13:56 > 0:14:01we've travelled less than a tenth of 1% of that orbit.
0:14:05 > 0:14:10These cycles seem eternal and unchanging,
0:14:10 > 0:14:15but as the story of time unfolds, a fundamental truth is revealed.
0:14:20 > 0:14:22Nothing lasts forever.
0:14:29 > 0:14:32This is the most profound property of time.
0:14:37 > 0:14:40And it plays out just as vividly here on Earth
0:14:40 > 0:14:43as it does in the depths of space.
0:15:01 > 0:15:04Well, this is the Perito Moreno Glacier
0:15:04 > 0:15:06in Patagonia in southern Argentina.
0:15:06 > 0:15:10And it's one of the hundreds of glaciers
0:15:10 > 0:15:14that sweep down the continent from the southern Patagonian ice fields.
0:15:16 > 0:15:21And, you know, if you carry on that way, south about 1,000 kilometres,
0:15:21 > 0:15:24you get to the end of South America.
0:15:24 > 0:15:27From then on, there's nothing to the Antarctic.
0:15:27 > 0:15:30And it feels like that today.
0:15:41 > 0:15:45The glacier is such a massive expanse of ice that, at first sight,
0:15:45 > 0:15:50just like the cycles of the heavens, it appears fixed and unchanging.
0:15:59 > 0:16:03Yet, seen close-up, it's continually on the move.
0:16:03 > 0:16:07As it has been for tens of thousands of years.
0:16:12 > 0:16:14WATER CRASHES
0:16:23 > 0:16:28The whole face of the glacier is moving into the lake
0:16:28 > 0:16:31something like that much every day.
0:16:31 > 0:16:36That means that well over a quarter of a billion tonnes of ice
0:16:36 > 0:16:40drop off the face of the glacier into the lake every year.
0:16:40 > 0:16:43That's about a million tonnes a day. And you can hear it happening.
0:16:43 > 0:16:48Just every now and again, you hear this tremendous cracking sound.
0:16:48 > 0:16:50It really is like the place is alive.
0:16:55 > 0:16:58< CRACKING
0:17:00 > 0:17:02THUNDEROUS CRASH
0:17:03 > 0:17:07You know, it's quite disturbing when these enormous chunks of ice
0:17:07 > 0:17:08fall into the lake.
0:17:08 > 0:17:12Although this thing seems stable and the movement seems glacially slow,
0:17:12 > 0:17:17actually there can be really violent collapses.
0:17:17 > 0:17:21It's an incredibly dynamic place to be.
0:17:32 > 0:17:34The movement of the glacier
0:17:34 > 0:17:38provides an insight into the nature of time.
0:17:38 > 0:17:41It is simply the ordering of events into sequences.
0:17:41 > 0:17:44One step after another.
0:17:47 > 0:17:52As time passes, snow falls, ice forms,
0:17:52 > 0:17:56the glacier gradually inches down the valley
0:17:56 > 0:17:59and huge chunks of ice fall into the lake below.
0:18:01 > 0:18:07But even this simple sequence contains a profound idea.
0:18:09 > 0:18:12Events always happen in the same order.
0:18:12 > 0:18:16They're never jumbled up and they never go backwards.
0:18:26 > 0:18:29RUMBLING
0:18:32 > 0:18:36Now that's something that you would never see in reverse.
0:18:36 > 0:18:39But, interestingly, there's nothing in the laws of physics
0:18:39 > 0:18:41that describe how all those
0:18:41 > 0:18:45water molecules are moving around that prevent them
0:18:45 > 0:18:48from all getting together on the surface of the lake,
0:18:48 > 0:18:52jumping out of the water, sticking together into a block of ice
0:18:52 > 0:18:56and then gluing themselves back on to the surface of the glacier again.
0:18:56 > 0:18:58But, interestingly,
0:18:58 > 0:19:03we do understand why the world doesn't run in reverse.
0:19:03 > 0:19:05There is a reason.
0:19:05 > 0:19:08We have a scientific explanation.
0:19:08 > 0:19:11And it's called the arrow of time.
0:19:21 > 0:19:24We never see waves travelling across lakes,
0:19:24 > 0:19:28coming together and bouncing chunks of ice back onto glaciers.
0:19:31 > 0:19:35We are compelled to travel into the future.
0:19:35 > 0:19:40And that's because the arrow of time dictates that as each moment passes,
0:19:40 > 0:19:42things change.
0:19:42 > 0:19:46And once these changes have happened, they are never undone.
0:19:51 > 0:19:56Permanent change is a fundamental part of what it means to be human.
0:19:56 > 0:20:00We all age as the years pass by.
0:20:00 > 0:20:04People are born, they live, they die.
0:20:04 > 0:20:08I suppose it's part of the joy and tragedy of our lives.
0:20:08 > 0:20:11But out there in the universe,
0:20:11 > 0:20:16those grand and epic cycles appear eternal and unchanging.
0:20:16 > 0:20:18But that's an illusion.
0:20:18 > 0:20:21You see, in the life of the universe, just as in our lives,
0:20:21 > 0:20:26everything is irreversibly changing.
0:20:35 > 0:20:37By building change upon change,
0:20:37 > 0:20:42the arrow of time drives the evolution of the entire universe.
0:20:43 > 0:20:47And as we look out deep into the cosmos,
0:20:47 > 0:20:49we can see that story unfold.
0:20:52 > 0:20:56This is an image of a tiny piece of night sky
0:20:56 > 0:20:59in the constellation of Leo.
0:20:59 > 0:21:02It's actually where the mouth of the lion would be.
0:21:02 > 0:21:07And, despite appearances, it is one of the most interesting images
0:21:07 > 0:21:10taken in recent astronomical history.
0:21:10 > 0:21:14The interesting thing is this little red blob here,
0:21:14 > 0:21:17which looks very unremarkable.
0:21:17 > 0:21:21But what that red blob is is the afterglow
0:21:21 > 0:21:26of an enormous cosmic explosion. It's the death of a star.
0:21:26 > 0:21:28That was about...
0:21:28 > 0:21:3340 or even 50 times the mass of our sun.
0:21:38 > 0:21:45Poetically named GRB 090423, it was once a Wolf-Rayet star.
0:21:50 > 0:21:54Shrouded by rapidly swirling clouds of gas,
0:21:54 > 0:21:58it burned 10,000 times more brightly than our sun.
0:22:00 > 0:22:05But because it burned so brightly, it was extremely short-lived.
0:22:08 > 0:22:11As it died, the giant star collapsed in on itself.
0:22:11 > 0:22:15That caused massive jets of light and stellar material
0:22:15 > 0:22:18to be ejected from its poles,
0:22:18 > 0:22:23in an explosion that shone with the light of 10 million billion suns.
0:22:30 > 0:22:34And it's the afterglow of this catastrophic explosion
0:22:34 > 0:22:38that is just visible from our planet as a faint red dot.
0:22:44 > 0:22:49But that's not what's so interesting about GRB 090423.
0:22:49 > 0:22:54You see, when we look up into the sky, at distant stars and galaxies,
0:22:54 > 0:22:56then we're looking back in time
0:22:56 > 0:23:00because the light takes time to journey from them to us.
0:23:00 > 0:23:05And the light from that red dot has been travelling to us
0:23:05 > 0:23:09for almost the entire history of the universe.
0:23:09 > 0:23:14You see, what we're looking at here is an event that happened
0:23:14 > 0:23:1513 billion years ago.
0:23:15 > 0:23:20That's only about 600 million years after the Big Bang.
0:23:20 > 0:23:22After the universe began.
0:23:22 > 0:23:26So this is something incredibly early in the universe's history.
0:23:26 > 0:23:33In fact, this is the oldest single object that we've ever seen.
0:23:33 > 0:23:37What we're looking at here is the explosive death
0:23:37 > 0:23:40of one of the first stars in the universe.
0:23:53 > 0:23:58As it evolves, the universe passes through distinct eras.
0:24:00 > 0:24:03Vast ages, whose beginnings and endings
0:24:03 > 0:24:06are marked by unique milestones.
0:24:08 > 0:24:12The births and deaths of its wonders.
0:24:18 > 0:24:22The moment the first stars were born is one of the most important changes
0:24:22 > 0:24:24in the evolution of the cosmos.
0:24:27 > 0:24:30It signals the end of the Primordial Era
0:24:30 > 0:24:36and marks the beginning of the second great age of the universe.
0:24:36 > 0:24:39The time in which we live.
0:24:39 > 0:24:42The Stelliferous Era -
0:24:42 > 0:24:44the age of the stars.
0:24:50 > 0:24:56Starlight illuminates the night sky and starlight illuminates our days.
0:24:56 > 0:25:01Our sun is just one of 200 billion stars in our galaxy.
0:25:01 > 0:25:06Our galaxy is one of 100 billion in the observable universe.
0:25:06 > 0:25:11And countless islands of countless stars.
0:25:23 > 0:25:27Although the universe is over 13 billion years old,
0:25:27 > 0:25:31we still love close to the stars of the Stelliferous Era.
0:25:31 > 0:25:36It's an age of astonishing beauty and complexity in the universe.
0:25:40 > 0:25:45The cosmos is absolutely awash with stars surrounded by
0:25:45 > 0:25:48nebulae and systems of planets.
0:25:48 > 0:25:53Countless billions of worlds that we've yet to explore.
0:26:00 > 0:26:04But the cosmos isn't static and unchanging.
0:26:04 > 0:26:07It won't always be this way.
0:26:07 > 0:26:11Because, as the arrow of time plays out,
0:26:11 > 0:26:16it produces a universe that is as dynamic as it's beautiful.
0:26:23 > 0:26:27We've seen stars born and we've seen stars die.
0:26:27 > 0:26:31And we know that tomorrow won't be the same as today
0:26:31 > 0:26:34because the arrow of time says
0:26:34 > 0:26:38the future will always be different from the past.
0:26:40 > 0:26:42But what drives this evolution?
0:26:42 > 0:26:48Why is there a difference between the past and the future?
0:26:48 > 0:26:51Why is there an arrow of time at all?
0:27:13 > 0:27:17We all have an intuitive understanding of the arrow of time.
0:27:21 > 0:27:25It seems obvious to us that things change and the future
0:27:25 > 0:27:27will be different to the past.
0:27:32 > 0:27:35We know that because we see the effects
0:27:35 > 0:27:38of the passing years all around us.
0:27:48 > 0:27:51This is Kolmanskop, an abandoned diamond mining town
0:27:51 > 0:27:53in southern Namibia.
0:28:02 > 0:28:05This entire town was founded in 1908,
0:28:05 > 0:28:10when a worker who was building the railway from the port of Luderitz
0:28:10 > 0:28:14inland into the centre of Namibia found a single diamond
0:28:14 > 0:28:16here in this desert.
0:28:32 > 0:28:37For 40 years, this was a thriving community of up to 1,000 people.
0:28:37 > 0:28:40A place where you could become a millionaire,
0:28:40 > 0:28:42picking diamonds out of the sand.
0:28:46 > 0:28:49While the money rolled in, they built grand houses
0:28:49 > 0:28:52and lived a champagne lifestyle in the desert.
0:28:55 > 0:28:59But when the diamonds dried up, the town was abandoned.
0:28:59 > 0:29:01And for half a century
0:29:01 > 0:29:06it's fallen into disrepair as it's slowly reclaimed by the sands.
0:29:22 > 0:29:25The processes at play here at Kolmanskop
0:29:25 > 0:29:28are happening everywhere in the universe.
0:29:28 > 0:29:31Because it isn't simply permanent change
0:29:31 > 0:29:34that's central to the arrow of time.
0:29:34 > 0:29:35It's decay.
0:29:37 > 0:29:41But the scientific explanation for why that is...
0:29:43 > 0:29:45..didn't come from attempting to understand
0:29:45 > 0:29:47the effects of time in the universe.
0:29:48 > 0:29:51It came from trying to build a faster train.
0:29:54 > 0:29:56Back in the 19th century,
0:29:56 > 0:30:00engineers were concerned with the efficiency of steam engines.
0:30:00 > 0:30:03How hot should the fire be?
0:30:03 > 0:30:06What substance should you boil in the steam engine?
0:30:06 > 0:30:09Should it be water or something else?
0:30:09 > 0:30:11These were profound questions.
0:30:11 > 0:30:15And out of those questions arose the science of thermodynamics.
0:30:15 > 0:30:18It's when concepts like heat and temperature and energy
0:30:18 > 0:30:22entered the scientific vocabulary for the first time.
0:30:22 > 0:30:26Now, along with that deeper understanding
0:30:26 > 0:30:30emerged what is probably the most important law of physics
0:30:30 > 0:30:33for understanding the evolution of the universe
0:30:33 > 0:30:35and the passage of time.
0:30:35 > 0:30:39It's called the Second Law of Thermodynamics.
0:30:44 > 0:30:47The reason the Second Law of Thermodynamics was so profound
0:30:47 > 0:30:53was because, at its heart, it contained a radically new concept.
0:30:53 > 0:30:57Something physicists call "entropy".
0:30:59 > 0:31:03Entropy explains why, left to the mercy of the elements,
0:31:03 > 0:31:08mortar crumbles, glass shatters and buildings collapse.
0:31:12 > 0:31:15And a good way to understand how is to think of objects
0:31:15 > 0:31:16not as single things,
0:31:16 > 0:31:20but as being made up of many constituent parts.
0:31:20 > 0:31:24Like the individual grains that make up this pile of sand.
0:31:27 > 0:31:31Now, entropy is a measure of how many ways I can rearrange those
0:31:31 > 0:31:34grains and still keep the sand pile the same.
0:31:34 > 0:31:38And there are trillions and trillions and trillions
0:31:38 > 0:31:39of ways of doing that.
0:31:39 > 0:31:42I mean, pretty much anything I do to this sand pile,
0:31:42 > 0:31:45if I mess the sand around and move it around,
0:31:45 > 0:31:48then it doesn't change the shape or the structure at all.
0:31:48 > 0:31:50So, in the language of entropy,
0:31:50 > 0:31:55this sand pile has high entropy because there are many, many ways
0:31:55 > 0:31:59that I can rearrange its constituents and not change it.
0:31:59 > 0:32:04Now let me create some order in the universe.
0:32:11 > 0:32:15Now, there are approximately as many sand grains in this sand castle
0:32:15 > 0:32:18as there are in the sand pile.
0:32:18 > 0:32:22But now, virtually anything I do to it will mess it up,
0:32:22 > 0:32:26will remove the beautiful order from this structure.
0:32:26 > 0:32:30And because of that, the sand castle has a low entropy.
0:32:30 > 0:32:32It's a much more ordered state.
0:32:32 > 0:32:35So, many ways of rearranging the sand grains
0:32:35 > 0:32:39without changing the structure, high entropy.
0:32:39 > 0:32:42Very few ways of rearranging the sand grains
0:32:42 > 0:32:48without changing the structure, without disordering it, low entropy.
0:32:56 > 0:33:00Imagine I was to leave this castle in the desert all day.
0:33:00 > 0:33:02Then it's obvious what's going to happen.
0:33:02 > 0:33:06The desert winds are going to blow the sand around
0:33:06 > 0:33:10and this castle is going to disintegrate.
0:33:10 > 0:33:13Is going to become less ordered. It's going to fall to bits.
0:33:16 > 0:33:20But think about what's happening on the fundamental level.
0:33:20 > 0:33:24I mean, the wind is taking the sand off the castle
0:33:24 > 0:33:28and blowing it over there somewhere and making a sand pile.
0:33:28 > 0:33:31There's nothing fundamental in the laws of physics
0:33:31 > 0:33:36that says that the wind couldn't pick up some sand from over here,
0:33:36 > 0:33:38deposit it here
0:33:38 > 0:33:42and deposit it in precisely the shape of a sand castle.
0:33:42 > 0:33:47In principle, the wind could spontaneously build a sand castle
0:33:47 > 0:33:48out of a pile of sand.
0:33:59 > 0:34:01There's no reason why that couldn't happen.
0:34:01 > 0:34:05It's just extremely, extremely unlikely because there
0:34:05 > 0:34:10are very few ways of organising this sand so that it looks like a castle.
0:34:17 > 0:34:19It's overwhelmingly more likely
0:34:19 > 0:34:21that when the wind blows the sand around
0:34:21 > 0:34:25it will take the low entropy structure of the castle
0:34:25 > 0:34:29and turn it into a high entropy structure, the sand pile.
0:34:37 > 0:34:41So, entropy always increases. Why is that?
0:34:41 > 0:34:45Because it's overwhelmingly more likely that it will.
0:34:55 > 0:34:58It seems incredible that a law that says that sand castles
0:34:58 > 0:35:02don't spontaneously form on the wind
0:35:02 > 0:35:05could solve one of the deepest mysteries in physics.
0:35:08 > 0:35:11But by saying entropy always increases,
0:35:11 > 0:35:15the Second Law of Thermodynamics is able to explain
0:35:15 > 0:35:18why time only runs in one direction.
0:35:34 > 0:35:38The Second Law of Thermodynamics, for me, demonstrates everything
0:35:38 > 0:35:42that's powerful and beautiful and profound about physics.
0:35:42 > 0:35:45You see, here's a law that entered science
0:35:45 > 0:35:47as a way of talking about how heat moves around
0:35:47 > 0:35:49and the efficiency of steam engines.
0:35:49 > 0:35:54But it ended up being able to explain one of the great mysteries
0:35:54 > 0:35:57in the history of science.
0:35:57 > 0:36:00Why is there a difference between the past and the future?
0:36:00 > 0:36:03You see, the second law says
0:36:03 > 0:36:07that everything tends from order to disorder.
0:36:07 > 0:36:10That means that there is a difference
0:36:10 > 0:36:12between the past and future.
0:36:12 > 0:36:14In the past, the universe was more ordered.
0:36:14 > 0:36:18In the future, the universe will be less ordered.
0:36:18 > 0:36:21And that means there's a direction to the passage of time.
0:36:21 > 0:36:27So the Second Law of Thermodynamics has introduced the concept
0:36:27 > 0:36:30of an arrow of time into science.
0:36:38 > 0:36:40The arrow of time has been playing out
0:36:40 > 0:36:45in Kolmanskop since the mining facility was abandoned in 1954.
0:36:48 > 0:36:49But in the universe,
0:36:49 > 0:36:53it's been playing out for almost 14 billion years.
0:36:53 > 0:36:56And it will have profound consequences.
0:37:04 > 0:37:07Because it means stars cannot shine forever.
0:37:07 > 0:37:12Including the star at the centre of our solar system.
0:37:14 > 0:37:19At the end of its life, the sun won't simply fade away to nothing.
0:37:22 > 0:37:27As it begins to run out of fuel, its core will collapse
0:37:27 > 0:37:29and the extra heat this generates
0:37:29 > 0:37:31will cause its outer layers to expand.
0:37:39 > 0:37:41In around a billion years' time,
0:37:41 > 0:37:45this will have a catastrophic effect on our fragile world.
0:37:52 > 0:37:55Gradually, the Earth will become hotter and hotter.
0:37:55 > 0:37:59So there will be one last perfect day on Earth.
0:37:59 > 0:38:03But eventually the existence of all life on this planet
0:38:03 > 0:38:05will become impossible.
0:38:05 > 0:38:09TICKING
0:38:09 > 0:38:14Long after a life has disappeared, the sun will have grown so much
0:38:14 > 0:38:16it will fill the entire horizon.
0:38:23 > 0:38:24It will become a red giant.
0:38:24 > 0:38:27The last phase of its life.
0:38:37 > 0:38:39Our planet might not survive to this point.
0:38:39 > 0:38:45But, if it does, little more than a scorched and barren rock will remain
0:38:45 > 0:38:49to witness the final death throes of our star.
0:39:00 > 0:39:04In six billion years, our sun will explode.
0:39:04 > 0:39:08Throwing vast amounts of gas and dust out into space,
0:39:08 > 0:39:10to form a gigantic nebula.
0:39:18 > 0:39:22And at its heart will beat a faintly glowing ember.
0:39:22 > 0:39:26All that remains of our once magnificent sun.
0:39:26 > 0:39:28It will be smaller than the size of the Earth.
0:39:28 > 0:39:31Less than a millionth of its current volume
0:39:31 > 0:39:34and a fraction of its brightness.
0:39:34 > 0:39:37Our sun will have become a white dwarf.
0:39:46 > 0:39:49With no fuel left to burn,
0:39:49 > 0:39:53a white dwarf's faint glow comes from the last residual heat
0:39:53 > 0:39:55from its extinguished furnace.
0:39:58 > 0:40:00The sun is now dead.
0:40:00 > 0:40:05Its remains slowly cooling in the freezing temperatures of deep space.
0:40:10 > 0:40:12Looking at it from where the Earth is now,
0:40:12 > 0:40:15it would only generate the same amount of light
0:40:15 > 0:40:17as the full moon on a clear night.
0:40:23 > 0:40:27The fate of the sun IS the same as for all stars.
0:40:27 > 0:40:30One day, they must all eventually die
0:40:30 > 0:40:34and the cosmos will be plunged into eternal night.
0:40:34 > 0:40:39And this is the most profound consequence of the arrow of time.
0:40:39 > 0:40:42Because this structured universe that we inhabit,
0:40:42 > 0:40:46and all its wonders - the stars, the planets and the galaxies -
0:40:46 > 0:40:49cannot last forever.
0:40:49 > 0:40:54The cosmos will eventually fade and die.
0:40:57 > 0:41:00First will come the end of the Stelliferous Era.
0:41:00 > 0:41:03The end of the age of starlight.
0:41:08 > 0:41:11The largest stars are the first to disappear,
0:41:11 > 0:41:13violently collapsing into black holes.
0:41:13 > 0:41:16Just a few million years after their formation.
0:41:19 > 0:41:23But long after they're gone, just one type of star will remain.
0:41:27 > 0:41:31This is a picture of the nearest star to our solar system,
0:41:31 > 0:41:32Proxima Centauri.
0:41:32 > 0:41:36It's only 4.2 light years away. But the reason it doesn't stand out
0:41:36 > 0:41:39against the much more distant stars in this photograph
0:41:39 > 0:41:43is that Proxima Centauri is incredibly tiny.
0:41:43 > 0:41:46It's a kind of star known as a red dwarf star.
0:41:46 > 0:41:50It's only about 11-12% the mass of our sun.
0:41:50 > 0:41:56But to our eyes it would appear to shine 18,000 times less brightly.
0:41:58 > 0:42:01But red dwarves do have one advantage over their much more
0:42:01 > 0:42:06luminous and magnificent stellar brethren.
0:42:06 > 0:42:09And that's because they're so small,
0:42:09 > 0:42:13they burn their nuclear fuel incredibly slowly,
0:42:13 > 0:42:16so they have lifespans of trillions of years.
0:42:16 > 0:42:20And that means that stars like Proxima Centauri
0:42:20 > 0:42:24will be the last living stars in the universe.
0:42:29 > 0:42:33If we survive into the far future of the universe,
0:42:33 > 0:42:38then it's possible to imagine our distant descendants building
0:42:38 > 0:42:41their civilisation around red dwarves
0:42:41 > 0:42:46to capture the energy from those last fading embers of stars,
0:42:46 > 0:42:51just as our ancestors crowded around campfires
0:42:51 > 0:42:54for warmth on cold winter's nights.
0:43:13 > 0:43:15The reason why Proxima Centauri
0:43:15 > 0:43:19burns so slowly is because its small size and low gravity
0:43:19 > 0:43:23mean its core is under much lower pressure than larger stars.
0:43:25 > 0:43:29This also means that its interior is constantly churning,
0:43:29 > 0:43:32whipping up the surface into a fiery turmoil.
0:43:35 > 0:43:39Explosive solar flares occur almost continually,
0:43:39 > 0:43:41even though it burns so dimly.
0:43:43 > 0:43:48But Proxima Centauri will eventually die.
0:43:48 > 0:43:52And like our sun, it too will become a white dwarf.
0:43:53 > 0:43:55As the age of starlight ends,
0:43:55 > 0:44:00all but the dimmest flicker of light in the universe will go out.
0:44:00 > 0:44:05The faint glow of white dwarves will provide the only illumination
0:44:05 > 0:44:12in a dark and empty void, littered with dead stars and black holes.
0:44:14 > 0:44:19By this point, the universe will be 100 trillion years old.
0:44:22 > 0:44:24And yet, even now,
0:44:24 > 0:44:29the vast majority of its lifespan still lies ahead of it.
0:44:46 > 0:44:49There are few places on Earth where you can get an inkling
0:44:49 > 0:44:53of what the far future has in store.
0:45:06 > 0:45:08This is Namibia's Skeleton Coast,
0:45:08 > 0:45:11where the cold water to the South Atlantic
0:45:11 > 0:45:13meet the Namib Desert.
0:45:13 > 0:45:16And it is one of the most inhospitable places on Earth.
0:45:16 > 0:45:18Back in the 17th century,
0:45:18 > 0:45:22Portuguese sailors used to call this place the "gates to hell"
0:45:22 > 0:45:23because this dense fog
0:45:23 > 0:45:27that you see pretty much every morning along this coast,
0:45:27 > 0:45:31coupled with the constantly shifting shape of the sandbanks,
0:45:31 > 0:45:34meant that over the years,
0:45:34 > 0:45:37literally thousands of ships were wrecked along this coastline.
0:45:43 > 0:45:46And even if you made it to shore, that wasn't the end of your problems
0:45:46 > 0:45:49because the currents are so strong here
0:45:49 > 0:45:52that there is no way of rowing back out to sea.
0:45:52 > 0:45:53If you look that way,
0:45:53 > 0:45:57there's just hundreds of miles of inhospitable desert.
0:45:59 > 0:46:04So, it genuinely was a place of no return.
0:46:04 > 0:46:09If you were shipwrecked here, this WAS the end of your universe.
0:46:20 > 0:46:22This is the Eduard Bohlen.
0:46:22 > 0:46:25She was once an ocean-going steamer,
0:46:25 > 0:46:28ferrying passengers and cargo between here and Europe.
0:46:32 > 0:46:37On 5th September, 1909, she ran aground in thick fog.
0:46:43 > 0:46:47Yet, like all the vessels wrecked along this shoreline,
0:46:47 > 0:46:50the time it takes her to decay to nothing
0:46:50 > 0:46:52will be far longer than her time at sea.
0:46:57 > 0:47:00In the far future of the cosmos,
0:47:00 > 0:47:04a similar destiny awaits the remaining white dwarves.
0:47:10 > 0:47:14A black dwarf will be the final fate of those last stars.
0:47:14 > 0:47:17White dwarves that have become so cold
0:47:17 > 0:47:20that they barely emit any more heat or light.
0:47:25 > 0:47:30Black dwarves are dark, dense decaying balls of degenerate matter.
0:47:30 > 0:47:33Little more than the ashes of stars.
0:47:34 > 0:47:39Their constituent atoms are so severely crushed
0:47:39 > 0:47:44that black dwarves are a million times denser than our sun.
0:47:44 > 0:47:47Stars take so long to reach this point,
0:47:47 > 0:47:49that after nearly 14 billion years
0:47:49 > 0:47:54we believe there are currently no black dwarves in the universe.
0:47:54 > 0:47:57But despite never seeing one,
0:47:57 > 0:48:00we can still predict how they will end their days.
0:48:00 > 0:48:03Just as the iron than makes up this ship
0:48:03 > 0:48:08will eventually rust and be carried away by the desert winds,
0:48:08 > 0:48:12so we think that the matter inside black dwarves,
0:48:12 > 0:48:14the last matter in the universe,
0:48:14 > 0:48:19will eventually evaporate away and be carried off
0:48:19 > 0:48:21into the void as radiation,
0:48:21 > 0:48:25leaving absolutely nothing behind.
0:48:34 > 0:48:36With the black dwarves gone,
0:48:36 > 0:48:39there won't be a single atom of matter left.
0:48:42 > 0:48:45All that will remain of our once rich cosmos
0:48:45 > 0:48:49will be particles of light and black holes.
0:48:55 > 0:48:59After an unimaginable length of time,
0:48:59 > 0:49:02even the black holes will have evaporated
0:49:02 > 0:49:06and the universe will be nothing but a sea of photons,
0:49:06 > 0:49:10gradually tending towards the same temperature,
0:49:10 > 0:49:14as the expansion of the universe cools them towards absolute zero.
0:49:23 > 0:49:26And when I say "unimaginable period of time," I really mean it.
0:49:26 > 0:49:28It's 10,000 trillion trillion
0:49:28 > 0:49:33trillion trillion trillion trillion trillion trillion years.
0:49:33 > 0:49:35How big's that number?
0:49:35 > 0:49:41If I were to start counting with a single atom representing one year
0:49:41 > 0:49:45then there wouldn't be enough atoms in the entire universe
0:49:45 > 0:49:48to get anywhere near that number.
0:49:54 > 0:49:56Once the very last remnants
0:49:56 > 0:50:00of the very last stars have finally decayed away to nothing,
0:50:00 > 0:50:04and everything reaches the same temperature,
0:50:04 > 0:50:08the story of the universe finally comes to an end.
0:50:12 > 0:50:14For the first time in its life,
0:50:14 > 0:50:18the universe will be permanent and unchanging.
0:50:18 > 0:50:20Entropy finally stops increasing
0:50:20 > 0:50:25because the cosmos cannot get any more disordered.
0:50:25 > 0:50:29Nothing happens and it keeps not happening.
0:50:29 > 0:50:31Forever.
0:50:36 > 0:50:38It's what's known as the heat death of the universe.
0:50:38 > 0:50:44An era when the cosmos will remain vast and cold and desolate
0:50:44 > 0:50:46for the rest of time.
0:50:46 > 0:50:49But that's because there is no difference between the past,
0:50:49 > 0:50:51the present and the future.
0:50:51 > 0:50:55There's no way of measuring the passage of time
0:50:55 > 0:50:58because nothing in the cosmos changes.
0:50:58 > 0:51:02The arrow of time has simply ceased to exist.
0:51:13 > 0:51:16It's an inescapable fact of the universe,
0:51:16 > 0:51:19written into the fundamental laws of physics.
0:51:19 > 0:51:22The entire cosmos will die.
0:51:27 > 0:51:33Every single one of the 200 billion stars in our galaxy will go out.
0:51:36 > 0:51:41And just as the death of the sun means the end of life on our planet,
0:51:41 > 0:51:43so the death of every star
0:51:43 > 0:51:47will extinguish any possibility of life in the universe.
0:51:51 > 0:51:56The fact that the sun will die and it will incinerate the Earth
0:51:56 > 0:52:00and obliterate all life on our planet in the process,
0:52:00 > 0:52:02might sound a bit depressing to you.
0:52:02 > 0:52:04You might legitimately ask,
0:52:04 > 0:52:08"Well, surely you could build a universe in a different way?
0:52:08 > 0:52:09"Surely you could build it
0:52:09 > 0:52:13"so it didn't have to descend from order into chaos?"
0:52:13 > 0:52:14Well, the answer is no,
0:52:14 > 0:52:18you couldn't, if you wanted life to exist in it.
0:52:24 > 0:52:25The arrow of time,
0:52:25 > 0:52:30the sequence of changes that slowly leads the universe to its death,
0:52:30 > 0:52:33is the very same thing that creates the conditions
0:52:33 > 0:52:35for life in the first place.
0:52:40 > 0:52:42Because it takes time for matter to form
0:52:42 > 0:52:46and it takes time for gravity to pull it together
0:52:46 > 0:52:48into stars and planets.
0:52:53 > 0:52:55The arrow of time creates a bright window
0:52:55 > 0:52:58in the universe's adolescence.
0:52:58 > 0:53:00During which, life is possible.
0:53:07 > 0:53:11But it's a window that doesn't stay open for long.
0:53:14 > 0:53:17As a fraction of the lifespan of the universe,
0:53:17 > 0:53:21as measured from its beginning to the evaporation
0:53:21 > 0:53:23of the last black hole,
0:53:23 > 0:53:28life, as we know it, is only possible for one thousandth
0:53:28 > 0:53:32of a billion billion billionth billion billion billionth
0:53:32 > 0:53:35billion billion billionth of a per cent.
0:53:37 > 0:53:39And that's why, for me,
0:53:39 > 0:53:44the most astonishing wonder of the universe isn't a star
0:53:44 > 0:53:47or a planet or a galaxy.
0:53:47 > 0:53:49It isn't a thing at all.
0:53:49 > 0:53:52It's an instant in time.
0:53:52 > 0:53:55And that time is now.
0:54:03 > 0:54:07Humans have walked the Earth for just the smallest fraction
0:54:07 > 0:54:11of that briefest of moments in deep time.
0:54:14 > 0:54:16But in our 200,000 years on this planet,
0:54:16 > 0:54:19we've made remarkable progress.
0:54:22 > 0:54:27It was only 2,500 years ago that we believed that the sun was a god
0:54:27 > 0:54:30and measured its orbit with stone towers,
0:54:30 > 0:54:32built on the top of a hill.
0:54:32 > 0:54:39Today, the language of curiosity is not sun gods, but science.
0:54:39 > 0:54:44And we have observatories that are almost infinitely more sophisticated
0:54:44 > 0:54:49than the 13 towers, that can gaze out deep into the universe.
0:54:52 > 0:54:55And, perhaps even more remarkably,
0:54:55 > 0:54:57through theoretical physics and mathematics,
0:54:57 > 0:55:01we can calculate what the universe will look like
0:55:01 > 0:55:03in the distant future.
0:55:03 > 0:55:08And we can even make concrete predictions about its end.
0:55:15 > 0:55:17And I believe it's only by continuing
0:55:17 > 0:55:19our exploration of the cosmos
0:55:19 > 0:55:22and the laws of nature that govern it,
0:55:22 > 0:55:26that we can truly understand ourselves
0:55:26 > 0:55:29and our place in this universe of wonders.
0:55:33 > 0:55:38And that's what we've done in our brief moment on Planet Earth.
0:55:41 > 0:55:45In 1977, a space probe called Voyager 1
0:55:45 > 0:55:49was launched on a grand tour of the solar system.
0:55:49 > 0:55:52And it visited the great gas giant planets -
0:55:52 > 0:55:56Jupiter and Saturn - and made some wonderful discoveries
0:55:56 > 0:56:00before heading off towards interstellar space.
0:56:02 > 0:56:0613 years later, after its mission was almost over,
0:56:06 > 0:56:12it turned around and took one last picture of its home solar system.
0:56:12 > 0:56:14This is that picture.
0:56:17 > 0:56:20And the beautiful thing about this picture
0:56:20 > 0:56:24is this single pixel of light
0:56:24 > 0:56:26suspended against the blackness of space.
0:56:26 > 0:56:31Because that pixel, that point, is Planet Earth.
0:56:31 > 0:56:34The most distant picture of our planet ever taken.
0:56:34 > 0:56:376 billion kilometres away.
0:56:47 > 0:56:51And whilst I suppose it has very limited scientific value,
0:56:51 > 0:56:54for me, this tiny point of light
0:56:54 > 0:56:58is the most powerful and profound demonstration
0:56:58 > 0:57:01of perhaps the most human of qualities.
0:57:01 > 0:57:06Our unique ability to reflect on the universe's existence
0:57:06 > 0:57:08and our place within it.
0:57:13 > 0:57:18Just as we, and all life on Earth, stand on this tiny speck
0:57:18 > 0:57:25adrift in infinite space, so life in the universe will only exist
0:57:25 > 0:57:29for a fleeting bright instance in time
0:57:29 > 0:57:35because life, just like the stars and the planets and the galaxies,
0:57:35 > 0:57:40is just a temporary structure on the long road from order to disorder.
0:57:49 > 0:57:52But that doesn't make us insignificant
0:57:52 > 0:57:54because we are the cosmos made conscious.
0:57:54 > 0:57:59Life is the means by which the universe understands itself.
0:58:02 > 0:58:06And, for me, our true significance lies in our ability
0:58:06 > 0:58:12and our desire to understand and explore this beautiful universe.
0:58:55 > 0:58:58Subtitles by Red Bee Media Ltd