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Why are we here? Where do we come from?

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These are the most enduring of questions.

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And it's an essential part of human nature to want to find the answers.

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And we can trace that ancestry back

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hundreds of thousands of years to the dawn of humankind.

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But in reality, our story extends far further back in time.

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Our story starts with the beginning of the universe.

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It began 13.7 billion years ago.

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And today it's filled with over 100 billion galaxies,

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each containing hundreds of billions of stars.

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In the series, I want to tell that story

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because ultimately we are part of the universe.

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So its story is our story.

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It's a story that you couldn't tell without something so fundamental

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that it's impossible to imagine the universe without it.

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It's woven into the very fabric of the cosmos. Time.

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The relentless flow of time has driven the evolution of the universe

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and created many extraordinary wonders.

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These wonders take us from the very first moments in the life

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of the universe to its eventual end.

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This is Chankillo on the north-western coast of Peru.

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And it's one of South America's lesser known archaeological sites.

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But, for me, it is surely one of the most fascinating.

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Around 2,500 years ago,

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a civilisation we know almost nothing about

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built this fortified temple in the desert.

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Its walls were once brilliant white and covered with painted figures.

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Today, all but the smallest fragments

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of the decorations are gone.

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The details of this culture and all traces of its language are lost.

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And yet, if you stand in the right place, you can still experience

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the true purpose of Chankillo,

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in just the same way as you could the day that it was built.

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But, to do that, you have to be here before the sun rises.

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These towers form an ancient solar calendar.

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Now, at different times of year,

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the sunrise point is at a different place on the horizon.

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21st December, which here in the southern hemisphere

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is the summer solstice, the longest day,

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the sun rises just to the right of the right-most tower.

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Then, as the year passes, the sun moves through the towers

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until on 21st June, the winter solstice,

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the shortest day, it rises just to left of the left-most tower.

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Actually just in-between that mountain

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you can see in the distance and the left-most tower.

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So, at any time of year, if you watch the sun rise,

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you can measure its position and you can tell,

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within an accuracy of two or three days, the date.

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Today's date is September the 15th.

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So that means the sun

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will rise between the fifth and the sixth towers.

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Chankillo still works as a calendar

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because the sun still rises in the same place today

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as it did when these stones were first laid down.

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That's a magnificent sight, as the sun burns through the towers.

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You can almost feel the presence of the past here.

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Imagine what it must have been like.

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Thousands of citizens stood here to greet the sun,

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which was almost certainly a deity. Almost certainly their god.

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What a magnificent achievement.

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Probably one of our earliest attempts

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to begin to measure the heavens.

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Over the millennia,

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that desire to measure what's going on in the sky

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has led to modern astronomy

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and the foundations of our modern civilisation.

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I might build one in my garden.

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I want one!

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The 13 towers that line this ridge stand testament to our

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enduring fascination with the clockwork of the heavens.

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And to the direct connection between our lives and the cosmos.

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The rising and setting of the sun provides an epic heartbeat

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that allows us to mark the passage of time.

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A day on Earth is the 24 hours

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it takes our planet to rotate once on its axis.

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Our months are based on the 29-and-a-half days

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it takes the moon to wax and wane in the night sky.

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And a year is the 365-and-a-quarter days

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it takes us to orbit once around the sun.

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These familiar timescales mark the passing of our lives.

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But the life of the universe plays out on a much grander scale.

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When you look up into the night sky, you don't just see stars.

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Those tiny points of light are a million different clocks,

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whose lifespans mark out the passage of time over billions,

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or even trillions, of years.

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This film is about the greatest expanses of time.

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The deep time that shapes the universe.

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From its fiery beginnings, through countless generations of stars,

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planets and galaxies, to its eventual demise,

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the fate of the universe is determined by the passage of time.

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Timescales in the cosmos seem so unimaginably vast,

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it's almost impossible to relate to them.

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Yet there are places on Earth

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where we can begin to encounter time on these universal scales.

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This is Ostional on the northern Pacific coast of Costa Rica.

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I've come here to witness a natural event that's been happening

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long before there were any humans here to see it.

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And I suppose it really is a window

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into the distant past of life on our planet.

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Once the sun has dipped below the horizon

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and the moon conspired to make the tides just right,

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this beach is visited by prehistoric creatures.

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Under the cover of darkness, they emerge from the ocean.

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Playa Ostional is one of the few beaches in the world

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where large numbers of sea turtles make their nests.

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But what makes this truly remarkable is the sheer length of time

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scenes like this have been playing out.

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This is part of one of the oldest life-cycles on Earth.

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On nights like these, for the last 100 million years, turtles like this

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have been hauling themselves out of the ocean to lay their eggs.

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It's an almost incomprehensible timespan.

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100 million years ago, there were dinosaurs roaming the Earth,

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but the Earth itself looked very different.

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South America was not connected to North America.

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North America was somewhere over close to Europe.

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Australia was connected to Antarctica.

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It really is quite...

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wonderful to be so close to such an ancient cycle of life.

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I can hear breathing, actually.

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So, a remarkable experience.

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I mean, it really is beautiful to see that.

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On one night of many hundreds of millions of nights

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stretching back into the past.

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And she's gone.

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To witness a moment like this

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is to open up a connection to the deep past.

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To experience timespans far longer than the history of our own species.

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Yet even the 100-million-year story of the turtles

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only begins to connect us with the vast sweep of cosmic time.

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Our entire solar system is travelling

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on an unimaginably vast orbit,

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spinning around the centre of our galaxy.

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It takes 250 million years to make just one circuit of the Milky Way.

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In the entire history of the human race,

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we've travelled less than a tenth of 1% of that orbit.

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These cycles seem eternal and unchanging,

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but as the story of time unfolds, a fundamental truth is revealed.

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Nothing lasts forever.

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This is the most profound property of time.

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And it plays out just as vividly here on Earth

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as it does in the depths of space.

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Well, this is the Perito Moreno Glacier

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in Patagonia in southern Argentina.

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And it's one of the hundreds of glaciers

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that sweep down the continent from the southern Patagonian ice fields.

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And, you know, if you carry on that way, south about 1,000 kilometres,

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you get to the end of South America.

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From then on, there's nothing to the Antarctic.

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And it feels like that today.

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The glacier is such a massive expanse of ice that, at first sight,

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just like the cycles of the heavens, it appears fixed and unchanging.

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Yet, seen close-up, it's continually on the move.

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As it has been for tens of thousands of years.

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WATER CRASHES

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The whole face of the glacier is moving into the lake

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something like that much every day.

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That means that well over a quarter of a billion tonnes of ice

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drop off the face of the glacier into the lake every year.

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That's about a million tonnes a day. And you can hear it happening.

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Just every now and again, you hear this tremendous cracking sound.

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It really is like the place is alive.

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< CRACKING

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THUNDEROUS CRASH

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You know, it's quite disturbing when these enormous chunks of ice

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fall into the lake.

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Although this thing seems stable and the movement seems glacially slow,

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actually there can be really violent collapses.

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It's an incredibly dynamic place to be.

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The movement of the glacier

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provides an insight into the nature of time.

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It is simply the ordering of events into sequences.

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One step after another.

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As time passes, snow falls, ice forms,

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the glacier gradually inches down the valley

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and huge chunks of ice fall into the lake below.

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But even this simple sequence contains a profound idea.

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Events always happen in the same order.

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They're never jumbled up and they never go backwards.

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RUMBLING

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Now that's something that you would never see in reverse.

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But, interestingly, there's nothing in the laws of physics

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that describe how all those

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water molecules are moving around that prevent them

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from all getting together on the surface of the lake,

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jumping out of the water, sticking together into a block of ice

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and then gluing themselves back on to the surface of the glacier again.

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But, interestingly,

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we do understand why the world doesn't run in reverse.

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There is a reason.

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We have a scientific explanation.

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And it's called the arrow of time.

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We never see waves travelling across lakes,

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coming together and bouncing chunks of ice back onto glaciers.

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We are compelled to travel into the future.

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And that's because the arrow of time dictates that as each moment passes,

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things change.

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And once these changes have happened, they are never undone.

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Permanent change is a fundamental part of what it means to be human.

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We all age as the years pass by.

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People are born, they live, they die.

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I suppose it's part of the joy and tragedy of our lives.

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But out there in the universe,

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those grand and epic cycles appear eternal and unchanging.

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But that's an illusion.

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You see, in the life of the universe, just as in our lives,

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everything is irreversibly changing.

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By building change upon change,

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the arrow of time drives the evolution of the entire universe.

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And as we look out deep into the cosmos,

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we can see that story unfold.

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This is an image of a tiny piece of night sky

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in the constellation of Leo.

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It's actually where the mouth of the lion would be.

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And, despite appearances, it is one of the most interesting images

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taken in recent astronomical history.

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The interesting thing is this little red blob here,

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which looks very unremarkable.

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But what that red blob is is the afterglow

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of an enormous cosmic explosion. It's the death of a star.

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That was about...

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40 or even 50 times the mass of our sun.

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Poetically named GRB 090423, it was once a Wolf-Rayet star.

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Shrouded by rapidly swirling clouds of gas,

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it burned 10,000 times more brightly than our sun.

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But because it burned so brightly, it was extremely short-lived.

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As it died, the giant star collapsed in on itself.

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That caused massive jets of light and stellar material

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to be ejected from its poles,

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in an explosion that shone with the light of 10 million billion suns.

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And it's the afterglow of this catastrophic explosion

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that is just visible from our planet as a faint red dot.

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But that's not what's so interesting about GRB 090423.

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You see, when we look up into the sky, at distant stars and galaxies,

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then we're looking back in time

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because the light takes time to journey from them to us.

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And the light from that red dot has been travelling to us

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for almost the entire history of the universe.

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You see, what we're looking at here is an event that happened

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13 billion years ago.

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That's only about 600 million years after the Big Bang.

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After the universe began.

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So this is something incredibly early in the universe's history.

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In fact, this is the oldest single object that we've ever seen.

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What we're looking at here is the explosive death

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of one of the first stars in the universe.

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As it evolves, the universe passes through distinct eras.

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Vast ages, whose beginnings and endings

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are marked by unique milestones.

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The births and deaths of its wonders.

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The moment the first stars were born is one of the most important changes

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in the evolution of the cosmos.

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It signals the end of the Primordial Era

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and marks the beginning of the second great age of the universe.

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The time in which we live.

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The Stelliferous Era -

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the age of the stars.

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Starlight illuminates the night sky and starlight illuminates our days.

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Our sun is just one of 200 billion stars in our galaxy.

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00:25:01,360 --> 00:25:06,640
Our galaxy is one of 100 billion in the observable universe.

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And countless islands of countless stars.

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00:25:23,680 --> 00:25:27,120
Although the universe is over 13 billion years old,

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we still love close to the stars of the Stelliferous Era.

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It's an age of astonishing beauty and complexity in the universe.

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The cosmos is absolutely awash with stars surrounded by

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nebulae and systems of planets.

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Countless billions of worlds that we've yet to explore.

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00:26:00,120 --> 00:26:04,520
But the cosmos isn't static and unchanging.

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It won't always be this way.

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Because, as the arrow of time plays out,

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it produces a universe that is as dynamic as it's beautiful.

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We've seen stars born and we've seen stars die.

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And we know that tomorrow won't be the same as today

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because the arrow of time says

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00:26:34,880 --> 00:26:38,640
the future will always be different from the past.

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00:26:40,360 --> 00:26:42,720
But what drives this evolution?

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00:26:42,720 --> 00:26:48,440
Why is there a difference between the past and the future?

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00:26:48,440 --> 00:26:51,520
Why is there an arrow of time at all?

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We all have an intuitive understanding of the arrow of time.

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It seems obvious to us that things change and the future

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00:27:25,160 --> 00:27:27,080
will be different to the past.

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00:27:32,160 --> 00:27:35,600
We know that because we see the effects

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00:27:35,600 --> 00:27:38,400
of the passing years all around us.

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00:27:48,680 --> 00:27:51,840
This is Kolmanskop, an abandoned diamond mining town

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00:27:51,840 --> 00:27:53,200
in southern Namibia.

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00:28:02,560 --> 00:28:05,440
This entire town was founded in 1908,

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00:28:05,440 --> 00:28:10,480
when a worker who was building the railway from the port of Luderitz

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00:28:10,480 --> 00:28:14,760
inland into the centre of Namibia found a single diamond

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00:28:14,760 --> 00:28:16,440
here in this desert.

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00:28:32,440 --> 00:28:37,160
For 40 years, this was a thriving community of up to 1,000 people.

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00:28:37,160 --> 00:28:40,560
A place where you could become a millionaire,

291
00:28:40,560 --> 00:28:42,360
picking diamonds out of the sand.

292
00:28:46,040 --> 00:28:49,200
While the money rolled in, they built grand houses

293
00:28:49,200 --> 00:28:52,160
and lived a champagne lifestyle in the desert.

294
00:28:55,320 --> 00:28:59,320
But when the diamonds dried up, the town was abandoned.

295
00:28:59,320 --> 00:29:01,600
And for half a century

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00:29:01,600 --> 00:29:06,320
it's fallen into disrepair as it's slowly reclaimed by the sands.

297
00:29:22,800 --> 00:29:25,200
The processes at play here at Kolmanskop

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00:29:25,200 --> 00:29:28,640
are happening everywhere in the universe.

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00:29:28,640 --> 00:29:31,120
Because it isn't simply permanent change

300
00:29:31,120 --> 00:29:34,000
that's central to the arrow of time.

301
00:29:34,000 --> 00:29:35,360
It's decay.

302
00:29:37,720 --> 00:29:41,920
But the scientific explanation for why that is...

303
00:29:43,080 --> 00:29:45,320
..didn't come from attempting to understand

304
00:29:45,320 --> 00:29:47,120
the effects of time in the universe.

305
00:29:48,640 --> 00:29:51,560
It came from trying to build a faster train.

306
00:29:54,560 --> 00:29:56,480
Back in the 19th century,

307
00:29:56,480 --> 00:30:00,840
engineers were concerned with the efficiency of steam engines.

308
00:30:00,840 --> 00:30:03,280
How hot should the fire be?

309
00:30:03,280 --> 00:30:06,080
What substance should you boil in the steam engine?

310
00:30:06,080 --> 00:30:09,680
Should it be water or something else?

311
00:30:09,680 --> 00:30:11,600
These were profound questions.

312
00:30:11,600 --> 00:30:15,120
And out of those questions arose the science of thermodynamics.

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00:30:15,120 --> 00:30:18,800
It's when concepts like heat and temperature and energy

314
00:30:18,800 --> 00:30:22,040
entered the scientific vocabulary for the first time.

315
00:30:22,040 --> 00:30:26,400
Now, along with that deeper understanding

316
00:30:26,400 --> 00:30:30,320
emerged what is probably the most important law of physics

317
00:30:30,320 --> 00:30:33,960
for understanding the evolution of the universe

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00:30:33,960 --> 00:30:35,760
and the passage of time.

319
00:30:35,760 --> 00:30:39,120
It's called the Second Law of Thermodynamics.

320
00:30:44,480 --> 00:30:47,800
The reason the Second Law of Thermodynamics was so profound

321
00:30:47,800 --> 00:30:53,320
was because, at its heart, it contained a radically new concept.

322
00:30:53,320 --> 00:30:57,080
Something physicists call "entropy".

323
00:30:59,720 --> 00:31:03,640
Entropy explains why, left to the mercy of the elements,

324
00:31:03,640 --> 00:31:08,600
mortar crumbles, glass shatters and buildings collapse.

325
00:31:12,400 --> 00:31:15,120
And a good way to understand how is to think of objects

326
00:31:15,120 --> 00:31:16,680
not as single things,

327
00:31:16,680 --> 00:31:20,120
but as being made up of many constituent parts.

328
00:31:20,120 --> 00:31:24,480
Like the individual grains that make up this pile of sand.

329
00:31:27,600 --> 00:31:31,880
Now, entropy is a measure of how many ways I can rearrange those

330
00:31:31,880 --> 00:31:34,440
grains and still keep the sand pile the same.

331
00:31:34,440 --> 00:31:38,160
And there are trillions and trillions and trillions

332
00:31:38,160 --> 00:31:39,760
of ways of doing that.

333
00:31:39,760 --> 00:31:42,880
I mean, pretty much anything I do to this sand pile,

334
00:31:42,880 --> 00:31:45,560
if I mess the sand around and move it around,

335
00:31:45,560 --> 00:31:48,880
then it doesn't change the shape or the structure at all.

336
00:31:48,880 --> 00:31:50,840
So, in the language of entropy,

337
00:31:50,840 --> 00:31:55,000
this sand pile has high entropy because there are many, many ways

338
00:31:55,000 --> 00:31:59,560
that I can rearrange its constituents and not change it.

339
00:31:59,560 --> 00:32:04,680
Now let me create some order in the universe.

340
00:32:11,080 --> 00:32:15,840
Now, there are approximately as many sand grains in this sand castle

341
00:32:15,840 --> 00:32:18,640
as there are in the sand pile.

342
00:32:18,640 --> 00:32:22,040
But now, virtually anything I do to it will mess it up,

343
00:32:22,040 --> 00:32:26,760
will remove the beautiful order from this structure.

344
00:32:26,760 --> 00:32:30,000
And because of that, the sand castle has a low entropy.

345
00:32:30,000 --> 00:32:32,240
It's a much more ordered state.

346
00:32:32,240 --> 00:32:35,720
So, many ways of rearranging the sand grains

347
00:32:35,720 --> 00:32:39,560
without changing the structure, high entropy.

348
00:32:39,560 --> 00:32:42,480
Very few ways of rearranging the sand grains

349
00:32:42,480 --> 00:32:48,280
without changing the structure, without disordering it, low entropy.

350
00:32:56,880 --> 00:33:00,360
Imagine I was to leave this castle in the desert all day.

351
00:33:00,360 --> 00:33:02,880
Then it's obvious what's going to happen.

352
00:33:02,880 --> 00:33:06,160
The desert winds are going to blow the sand around

353
00:33:06,160 --> 00:33:10,000
and this castle is going to disintegrate.

354
00:33:10,000 --> 00:33:13,040
Is going to become less ordered. It's going to fall to bits.

355
00:33:16,920 --> 00:33:20,240
But think about what's happening on the fundamental level.

356
00:33:20,240 --> 00:33:24,080
I mean, the wind is taking the sand off the castle

357
00:33:24,080 --> 00:33:28,800
and blowing it over there somewhere and making a sand pile.

358
00:33:28,800 --> 00:33:31,720
There's nothing fundamental in the laws of physics

359
00:33:31,720 --> 00:33:36,680
that says that the wind couldn't pick up some sand from over here,

360
00:33:36,680 --> 00:33:38,400
deposit it here

361
00:33:38,400 --> 00:33:42,120
and deposit it in precisely the shape of a sand castle.

362
00:33:42,120 --> 00:33:47,200
In principle, the wind could spontaneously build a sand castle

363
00:33:47,200 --> 00:33:48,920
out of a pile of sand.

364
00:33:59,400 --> 00:34:01,360
There's no reason why that couldn't happen.

365
00:34:01,360 --> 00:34:05,480
It's just extremely, extremely unlikely because there

366
00:34:05,480 --> 00:34:10,480
are very few ways of organising this sand so that it looks like a castle.

367
00:34:17,160 --> 00:34:19,240
It's overwhelmingly more likely

368
00:34:19,240 --> 00:34:21,800
that when the wind blows the sand around

369
00:34:21,800 --> 00:34:25,240
it will take the low entropy structure of the castle

370
00:34:25,240 --> 00:34:29,440
and turn it into a high entropy structure, the sand pile.

371
00:34:37,160 --> 00:34:41,960
So, entropy always increases. Why is that?

372
00:34:41,960 --> 00:34:45,400
Because it's overwhelmingly more likely that it will.

373
00:34:55,800 --> 00:34:58,920
It seems incredible that a law that says that sand castles

374
00:34:58,920 --> 00:35:02,160
don't spontaneously form on the wind

375
00:35:02,160 --> 00:35:05,840
could solve one of the deepest mysteries in physics.

376
00:35:08,760 --> 00:35:11,240
But by saying entropy always increases,

377
00:35:11,240 --> 00:35:15,400
the Second Law of Thermodynamics is able to explain

378
00:35:15,400 --> 00:35:18,720
why time only runs in one direction.

379
00:35:34,640 --> 00:35:38,200
The Second Law of Thermodynamics, for me, demonstrates everything

380
00:35:38,200 --> 00:35:42,800
that's powerful and beautiful and profound about physics.

381
00:35:42,800 --> 00:35:45,000
You see, here's a law that entered science

382
00:35:45,000 --> 00:35:47,480
as a way of talking about how heat moves around

383
00:35:47,480 --> 00:35:49,600
and the efficiency of steam engines.

384
00:35:49,600 --> 00:35:54,760
But it ended up being able to explain one of the great mysteries

385
00:35:54,760 --> 00:35:57,040
in the history of science.

386
00:35:57,040 --> 00:36:00,800
Why is there a difference between the past and the future?

387
00:36:00,800 --> 00:36:03,280
You see, the second law says

388
00:36:03,280 --> 00:36:07,680
that everything tends from order to disorder.

389
00:36:07,680 --> 00:36:10,200
That means that there is a difference

390
00:36:10,200 --> 00:36:12,160
between the past and future.

391
00:36:12,160 --> 00:36:14,640
In the past, the universe was more ordered.

392
00:36:14,640 --> 00:36:18,200
In the future, the universe will be less ordered.

393
00:36:18,200 --> 00:36:21,760
And that means there's a direction to the passage of time.

394
00:36:21,760 --> 00:36:27,320
So the Second Law of Thermodynamics has introduced the concept

395
00:36:27,320 --> 00:36:30,240
of an arrow of time into science.

396
00:36:38,240 --> 00:36:40,400
The arrow of time has been playing out

397
00:36:40,400 --> 00:36:45,320
in Kolmanskop since the mining facility was abandoned in 1954.

398
00:36:48,280 --> 00:36:49,760
But in the universe,

399
00:36:49,760 --> 00:36:53,440
it's been playing out for almost 14 billion years.

400
00:36:53,440 --> 00:36:56,720
And it will have profound consequences.

401
00:37:04,400 --> 00:37:07,880
Because it means stars cannot shine forever.

402
00:37:07,880 --> 00:37:12,760
Including the star at the centre of our solar system.

403
00:37:14,880 --> 00:37:19,920
At the end of its life, the sun won't simply fade away to nothing.

404
00:37:22,640 --> 00:37:27,280
As it begins to run out of fuel, its core will collapse

405
00:37:27,280 --> 00:37:29,400
and the extra heat this generates

406
00:37:29,400 --> 00:37:31,880
will cause its outer layers to expand.

407
00:37:39,120 --> 00:37:41,360
In around a billion years' time,

408
00:37:41,360 --> 00:37:45,600
this will have a catastrophic effect on our fragile world.

409
00:37:52,320 --> 00:37:55,480
Gradually, the Earth will become hotter and hotter.

410
00:37:55,480 --> 00:37:59,480
So there will be one last perfect day on Earth.

411
00:37:59,480 --> 00:38:03,840
But eventually the existence of all life on this planet

412
00:38:03,840 --> 00:38:05,920
will become impossible.

413
00:38:05,920 --> 00:38:09,600
TICKING

414
00:38:09,600 --> 00:38:14,040
Long after a life has disappeared, the sun will have grown so much

415
00:38:14,040 --> 00:38:16,320
it will fill the entire horizon.

416
00:38:23,080 --> 00:38:24,920
It will become a red giant.

417
00:38:24,920 --> 00:38:27,360
The last phase of its life.

418
00:38:37,720 --> 00:38:39,920
Our planet might not survive to this point.

419
00:38:39,920 --> 00:38:45,840
But, if it does, little more than a scorched and barren rock will remain

420
00:38:45,840 --> 00:38:49,080
to witness the final death throes of our star.

421
00:39:00,080 --> 00:39:04,320
In six billion years, our sun will explode.

422
00:39:04,320 --> 00:39:08,520
Throwing vast amounts of gas and dust out into space,

423
00:39:08,520 --> 00:39:10,680
to form a gigantic nebula.

424
00:39:18,480 --> 00:39:22,520
And at its heart will beat a faintly glowing ember.

425
00:39:22,520 --> 00:39:26,040
All that remains of our once magnificent sun.

426
00:39:26,040 --> 00:39:28,800
It will be smaller than the size of the Earth.

427
00:39:28,800 --> 00:39:31,880
Less than a millionth of its current volume

428
00:39:31,880 --> 00:39:34,280
and a fraction of its brightness.

429
00:39:34,280 --> 00:39:37,240
Our sun will have become a white dwarf.

430
00:39:46,760 --> 00:39:49,040
With no fuel left to burn,

431
00:39:49,040 --> 00:39:53,280
a white dwarf's faint glow comes from the last residual heat

432
00:39:53,280 --> 00:39:55,640
from its extinguished furnace.

433
00:39:58,440 --> 00:40:00,280
The sun is now dead.

434
00:40:00,280 --> 00:40:05,880
Its remains slowly cooling in the freezing temperatures of deep space.

435
00:40:10,000 --> 00:40:12,800
Looking at it from where the Earth is now,

436
00:40:12,800 --> 00:40:15,800
it would only generate the same amount of light

437
00:40:15,800 --> 00:40:17,880
as the full moon on a clear night.

438
00:40:23,240 --> 00:40:27,480
The fate of the sun IS the same as for all stars.

439
00:40:27,480 --> 00:40:30,480
One day, they must all eventually die

440
00:40:30,480 --> 00:40:34,480
and the cosmos will be plunged into eternal night.

441
00:40:34,480 --> 00:40:39,280
And this is the most profound consequence of the arrow of time.

442
00:40:39,280 --> 00:40:42,320
Because this structured universe that we inhabit,

443
00:40:42,320 --> 00:40:46,600
and all its wonders - the stars, the planets and the galaxies -

444
00:40:46,600 --> 00:40:49,160
cannot last forever.

445
00:40:49,160 --> 00:40:54,080
The cosmos will eventually fade and die.

446
00:40:57,800 --> 00:41:00,600
First will come the end of the Stelliferous Era.

447
00:41:00,600 --> 00:41:03,800
The end of the age of starlight.

448
00:41:08,000 --> 00:41:11,080
The largest stars are the first to disappear,

449
00:41:11,080 --> 00:41:13,800
violently collapsing into black holes.

450
00:41:13,800 --> 00:41:16,560
Just a few million years after their formation.

451
00:41:19,400 --> 00:41:23,840
But long after they're gone, just one type of star will remain.

452
00:41:27,240 --> 00:41:31,400
This is a picture of the nearest star to our solar system,

453
00:41:31,400 --> 00:41:32,880
Proxima Centauri.

454
00:41:32,880 --> 00:41:36,960
It's only 4.2 light years away. But the reason it doesn't stand out

455
00:41:36,960 --> 00:41:39,880
against the much more distant stars in this photograph

456
00:41:39,880 --> 00:41:43,680
is that Proxima Centauri is incredibly tiny.

457
00:41:43,680 --> 00:41:46,480
It's a kind of star known as a red dwarf star.

458
00:41:46,480 --> 00:41:50,360
It's only about 11-12% the mass of our sun.

459
00:41:50,360 --> 00:41:56,400
But to our eyes it would appear to shine 18,000 times less brightly.

460
00:41:58,360 --> 00:42:01,840
But red dwarves do have one advantage over their much more

461
00:42:01,840 --> 00:42:06,240
luminous and magnificent stellar brethren.

462
00:42:06,240 --> 00:42:09,800
And that's because they're so small,

463
00:42:09,800 --> 00:42:13,200
they burn their nuclear fuel incredibly slowly,

464
00:42:13,200 --> 00:42:16,360
so they have lifespans of trillions of years.

465
00:42:16,360 --> 00:42:20,080
And that means that stars like Proxima Centauri

466
00:42:20,080 --> 00:42:24,160
will be the last living stars in the universe.

467
00:42:29,000 --> 00:42:33,760
If we survive into the far future of the universe,

468
00:42:33,760 --> 00:42:38,440
then it's possible to imagine our distant descendants building

469
00:42:38,440 --> 00:42:41,240
their civilisation around red dwarves

470
00:42:41,240 --> 00:42:46,480
to capture the energy from those last fading embers of stars,

471
00:42:46,480 --> 00:42:51,200
just as our ancestors crowded around campfires

472
00:42:51,200 --> 00:42:54,640
for warmth on cold winter's nights.

473
00:43:13,120 --> 00:43:15,360
The reason why Proxima Centauri

474
00:43:15,360 --> 00:43:19,360
burns so slowly is because its small size and low gravity

475
00:43:19,360 --> 00:43:23,800
mean its core is under much lower pressure than larger stars.

476
00:43:25,880 --> 00:43:29,600
This also means that its interior is constantly churning,

477
00:43:29,600 --> 00:43:32,520
whipping up the surface into a fiery turmoil.

478
00:43:35,960 --> 00:43:39,720
Explosive solar flares occur almost continually,

479
00:43:39,720 --> 00:43:41,960
even though it burns so dimly.

480
00:43:43,480 --> 00:43:48,360
But Proxima Centauri will eventually die.

481
00:43:48,360 --> 00:43:52,040
And like our sun, it too will become a white dwarf.

482
00:43:53,560 --> 00:43:55,680
As the age of starlight ends,

483
00:43:55,680 --> 00:44:00,520
all but the dimmest flicker of light in the universe will go out.

484
00:44:00,520 --> 00:44:05,880
The faint glow of white dwarves will provide the only illumination

485
00:44:05,880 --> 00:44:12,400
in a dark and empty void, littered with dead stars and black holes.

486
00:44:14,240 --> 00:44:19,440
By this point, the universe will be 100 trillion years old.

487
00:44:22,920 --> 00:44:24,440
And yet, even now,

488
00:44:24,440 --> 00:44:29,360
the vast majority of its lifespan still lies ahead of it.

489
00:44:46,760 --> 00:44:49,920
There are few places on Earth where you can get an inkling

490
00:44:49,920 --> 00:44:53,000
of what the far future has in store.

491
00:45:06,560 --> 00:45:08,560
This is Namibia's Skeleton Coast,

492
00:45:08,560 --> 00:45:11,560
where the cold water to the South Atlantic

493
00:45:11,560 --> 00:45:13,880
meet the Namib Desert.

494
00:45:13,880 --> 00:45:16,720
And it is one of the most inhospitable places on Earth.

495
00:45:16,720 --> 00:45:18,280
Back in the 17th century,

496
00:45:18,280 --> 00:45:22,080
Portuguese sailors used to call this place the "gates to hell"

497
00:45:22,080 --> 00:45:23,600
because this dense fog

498
00:45:23,600 --> 00:45:27,520
that you see pretty much every morning along this coast,

499
00:45:27,520 --> 00:45:31,760
coupled with the constantly shifting shape of the sandbanks,

500
00:45:31,760 --> 00:45:34,040
meant that over the years,

501
00:45:34,040 --> 00:45:37,840
literally thousands of ships were wrecked along this coastline.

502
00:45:43,320 --> 00:45:46,320
And even if you made it to shore, that wasn't the end of your problems

503
00:45:46,320 --> 00:45:49,240
because the currents are so strong here

504
00:45:49,240 --> 00:45:52,000
that there is no way of rowing back out to sea.

505
00:45:52,000 --> 00:45:53,400
If you look that way,

506
00:45:53,400 --> 00:45:57,360
there's just hundreds of miles of inhospitable desert.

507
00:45:59,760 --> 00:46:04,480
So, it genuinely was a place of no return.

508
00:46:04,480 --> 00:46:09,000
If you were shipwrecked here, this WAS the end of your universe.

509
00:46:20,400 --> 00:46:22,360
This is the Eduard Bohlen.

510
00:46:22,360 --> 00:46:25,200
She was once an ocean-going steamer,

511
00:46:25,200 --> 00:46:28,200
ferrying passengers and cargo between here and Europe.

512
00:46:32,600 --> 00:46:37,560
On 5th September, 1909, she ran aground in thick fog.

513
00:46:43,920 --> 00:46:47,400
Yet, like all the vessels wrecked along this shoreline,

514
00:46:47,400 --> 00:46:50,240
the time it takes her to decay to nothing

515
00:46:50,240 --> 00:46:52,960
will be far longer than her time at sea.

516
00:46:57,960 --> 00:47:00,920
In the far future of the cosmos,

517
00:47:00,920 --> 00:47:04,600
a similar destiny awaits the remaining white dwarves.

518
00:47:10,760 --> 00:47:14,360
A black dwarf will be the final fate of those last stars.

519
00:47:14,360 --> 00:47:17,160
White dwarves that have become so cold

520
00:47:17,160 --> 00:47:20,600
that they barely emit any more heat or light.

521
00:47:25,000 --> 00:47:30,480
Black dwarves are dark, dense decaying balls of degenerate matter.

522
00:47:30,480 --> 00:47:33,320
Little more than the ashes of stars.

523
00:47:34,840 --> 00:47:39,080
Their constituent atoms are so severely crushed

524
00:47:39,080 --> 00:47:44,160
that black dwarves are a million times denser than our sun.

525
00:47:44,160 --> 00:47:47,120
Stars take so long to reach this point,

526
00:47:47,120 --> 00:47:49,720
that after nearly 14 billion years

527
00:47:49,720 --> 00:47:54,840
we believe there are currently no black dwarves in the universe.

528
00:47:54,840 --> 00:47:57,080
But despite never seeing one,

529
00:47:57,080 --> 00:48:00,920
we can still predict how they will end their days.

530
00:48:00,920 --> 00:48:03,880
Just as the iron than makes up this ship

531
00:48:03,880 --> 00:48:08,480
will eventually rust and be carried away by the desert winds,

532
00:48:08,480 --> 00:48:12,120
so we think that the matter inside black dwarves,

533
00:48:12,120 --> 00:48:14,600
the last matter in the universe,

534
00:48:14,600 --> 00:48:19,160
will eventually evaporate away and be carried off

535
00:48:19,160 --> 00:48:21,720
into the void as radiation,

536
00:48:21,720 --> 00:48:25,320
leaving absolutely nothing behind.

537
00:48:34,560 --> 00:48:36,480
With the black dwarves gone,

538
00:48:36,480 --> 00:48:39,640
there won't be a single atom of matter left.

539
00:48:42,400 --> 00:48:45,840
All that will remain of our once rich cosmos

540
00:48:45,840 --> 00:48:49,240
will be particles of light and black holes.

541
00:48:55,880 --> 00:48:59,080
After an unimaginable length of time,

542
00:48:59,080 --> 00:49:02,560
even the black holes will have evaporated

543
00:49:02,560 --> 00:49:06,720
and the universe will be nothing but a sea of photons,

544
00:49:06,720 --> 00:49:10,200
gradually tending towards the same temperature,

545
00:49:10,200 --> 00:49:14,760
as the expansion of the universe cools them towards absolute zero.

546
00:49:23,160 --> 00:49:26,520
And when I say "unimaginable period of time," I really mean it.

547
00:49:26,520 --> 00:49:28,840
It's 10,000 trillion trillion

548
00:49:28,840 --> 00:49:33,800
trillion trillion trillion trillion trillion trillion years.

549
00:49:33,800 --> 00:49:35,760
How big's that number?

550
00:49:35,760 --> 00:49:41,680
If I were to start counting with a single atom representing one year

551
00:49:41,680 --> 00:49:45,760
then there wouldn't be enough atoms in the entire universe

552
00:49:45,760 --> 00:49:48,160
to get anywhere near that number.

553
00:49:54,840 --> 00:49:56,640
Once the very last remnants

554
00:49:56,640 --> 00:50:00,720
of the very last stars have finally decayed away to nothing,

555
00:50:00,720 --> 00:50:04,680
and everything reaches the same temperature,

556
00:50:04,680 --> 00:50:08,320
the story of the universe finally comes to an end.

557
00:50:12,800 --> 00:50:14,800
For the first time in its life,

558
00:50:14,800 --> 00:50:18,160
the universe will be permanent and unchanging.

559
00:50:18,160 --> 00:50:20,960
Entropy finally stops increasing

560
00:50:20,960 --> 00:50:25,280
because the cosmos cannot get any more disordered.

561
00:50:25,280 --> 00:50:29,680
Nothing happens and it keeps not happening.

562
00:50:29,680 --> 00:50:31,200
Forever.

563
00:50:36,000 --> 00:50:38,920
It's what's known as the heat death of the universe.

564
00:50:38,920 --> 00:50:44,200
An era when the cosmos will remain vast and cold and desolate

565
00:50:44,200 --> 00:50:46,080
for the rest of time.

566
00:50:46,080 --> 00:50:49,880
But that's because there is no difference between the past,

567
00:50:49,880 --> 00:50:51,680
the present and the future.

568
00:50:51,680 --> 00:50:55,080
There's no way of measuring the passage of time

569
00:50:55,080 --> 00:50:58,160
because nothing in the cosmos changes.

570
00:50:58,160 --> 00:51:02,480
The arrow of time has simply ceased to exist.

571
00:51:13,080 --> 00:51:16,320
It's an inescapable fact of the universe,

572
00:51:16,320 --> 00:51:19,880
written into the fundamental laws of physics.

573
00:51:19,880 --> 00:51:22,960
The entire cosmos will die.

574
00:51:27,640 --> 00:51:33,520
Every single one of the 200 billion stars in our galaxy will go out.

575
00:51:36,560 --> 00:51:41,160
And just as the death of the sun means the end of life on our planet,

576
00:51:41,160 --> 00:51:43,120
so the death of every star

577
00:51:43,120 --> 00:51:47,720
will extinguish any possibility of life in the universe.

578
00:51:51,680 --> 00:51:56,280
The fact that the sun will die and it will incinerate the Earth

579
00:51:56,280 --> 00:52:00,160
and obliterate all life on our planet in the process,

580
00:52:00,160 --> 00:52:02,840
might sound a bit depressing to you.

581
00:52:02,840 --> 00:52:04,600
You might legitimately ask,

582
00:52:04,600 --> 00:52:08,360
"Well, surely you could build a universe in a different way?

583
00:52:08,360 --> 00:52:09,960
"Surely you could build it

584
00:52:09,960 --> 00:52:13,240
"so it didn't have to descend from order into chaos?"

585
00:52:13,240 --> 00:52:14,960
Well, the answer is no,

586
00:52:14,960 --> 00:52:18,640
you couldn't, if you wanted life to exist in it.

587
00:52:24,720 --> 00:52:25,880
The arrow of time,

588
00:52:25,880 --> 00:52:30,360
the sequence of changes that slowly leads the universe to its death,

589
00:52:30,360 --> 00:52:33,720
is the very same thing that creates the conditions

590
00:52:33,720 --> 00:52:35,640
for life in the first place.

591
00:52:40,280 --> 00:52:42,560
Because it takes time for matter to form

592
00:52:42,560 --> 00:52:46,320
and it takes time for gravity to pull it together

593
00:52:46,320 --> 00:52:48,240
into stars and planets.

594
00:52:53,280 --> 00:52:55,440
The arrow of time creates a bright window

595
00:52:55,440 --> 00:52:58,080
in the universe's adolescence.

596
00:52:58,080 --> 00:53:00,400
During which, life is possible.

597
00:53:07,880 --> 00:53:11,000
But it's a window that doesn't stay open for long.

598
00:53:14,440 --> 00:53:17,960
As a fraction of the lifespan of the universe,

599
00:53:17,960 --> 00:53:21,760
as measured from its beginning to the evaporation

600
00:53:21,760 --> 00:53:23,520
of the last black hole,

601
00:53:23,520 --> 00:53:28,800
life, as we know it, is only possible for one thousandth

602
00:53:28,800 --> 00:53:32,280
of a billion billion billionth billion billion billionth

603
00:53:32,280 --> 00:53:35,520
billion billion billionth of a per cent.

604
00:53:37,640 --> 00:53:39,600
And that's why, for me,

605
00:53:39,600 --> 00:53:44,640
the most astonishing wonder of the universe isn't a star

606
00:53:44,640 --> 00:53:47,280
or a planet or a galaxy.

607
00:53:47,280 --> 00:53:49,600
It isn't a thing at all.

608
00:53:49,600 --> 00:53:52,520
It's an instant in time.

609
00:53:52,520 --> 00:53:55,000
And that time is now.

610
00:54:03,360 --> 00:54:07,720
Humans have walked the Earth for just the smallest fraction

611
00:54:07,720 --> 00:54:11,560
of that briefest of moments in deep time.

612
00:54:14,040 --> 00:54:16,720
But in our 200,000 years on this planet,

613
00:54:16,720 --> 00:54:19,040
we've made remarkable progress.

614
00:54:22,280 --> 00:54:27,440
It was only 2,500 years ago that we believed that the sun was a god

615
00:54:27,440 --> 00:54:30,280
and measured its orbit with stone towers,

616
00:54:30,280 --> 00:54:32,720
built on the top of a hill.

617
00:54:32,720 --> 00:54:39,400
Today, the language of curiosity is not sun gods, but science.

618
00:54:39,400 --> 00:54:44,240
And we have observatories that are almost infinitely more sophisticated

619
00:54:44,240 --> 00:54:49,640
than the 13 towers, that can gaze out deep into the universe.

620
00:54:52,280 --> 00:54:55,240
And, perhaps even more remarkably,

621
00:54:55,240 --> 00:54:57,880
through theoretical physics and mathematics,

622
00:54:57,880 --> 00:55:01,360
we can calculate what the universe will look like

623
00:55:01,360 --> 00:55:03,040
in the distant future.

624
00:55:03,040 --> 00:55:08,440
And we can even make concrete predictions about its end.

625
00:55:15,200 --> 00:55:17,720
And I believe it's only by continuing

626
00:55:17,720 --> 00:55:19,800
our exploration of the cosmos

627
00:55:19,800 --> 00:55:22,400
and the laws of nature that govern it,

628
00:55:22,400 --> 00:55:26,000
that we can truly understand ourselves

629
00:55:26,000 --> 00:55:29,400
and our place in this universe of wonders.

630
00:55:33,160 --> 00:55:38,240
And that's what we've done in our brief moment on Planet Earth.

631
00:55:41,760 --> 00:55:45,120
In 1977, a space probe called Voyager 1

632
00:55:45,120 --> 00:55:49,280
was launched on a grand tour of the solar system.

633
00:55:49,280 --> 00:55:52,400
And it visited the great gas giant planets -

634
00:55:52,400 --> 00:55:56,680
Jupiter and Saturn - and made some wonderful discoveries

635
00:55:56,680 --> 00:56:00,320
before heading off towards interstellar space.

636
00:56:02,240 --> 00:56:06,760
13 years later, after its mission was almost over,

637
00:56:06,760 --> 00:56:12,920
it turned around and took one last picture of its home solar system.

638
00:56:12,920 --> 00:56:14,640
This is that picture.

639
00:56:17,080 --> 00:56:20,920
And the beautiful thing about this picture

640
00:56:20,920 --> 00:56:24,200
is this single pixel of light

641
00:56:24,200 --> 00:56:26,760
suspended against the blackness of space.

642
00:56:26,760 --> 00:56:31,360
Because that pixel, that point, is Planet Earth.

643
00:56:31,360 --> 00:56:34,440
The most distant picture of our planet ever taken.

644
00:56:34,440 --> 00:56:37,560
6 billion kilometres away.

645
00:56:47,000 --> 00:56:51,000
And whilst I suppose it has very limited scientific value,

646
00:56:51,000 --> 00:56:54,000
for me, this tiny point of light

647
00:56:54,000 --> 00:56:58,120
is the most powerful and profound demonstration

648
00:56:58,120 --> 00:57:01,440
of perhaps the most human of qualities.

649
00:57:01,440 --> 00:57:06,320
Our unique ability to reflect on the universe's existence

650
00:57:06,320 --> 00:57:08,280
and our place within it.

651
00:57:13,160 --> 00:57:18,840
Just as we, and all life on Earth, stand on this tiny speck

652
00:57:18,840 --> 00:57:25,200
adrift in infinite space, so life in the universe will only exist

653
00:57:25,200 --> 00:57:29,000
for a fleeting bright instance in time

654
00:57:29,000 --> 00:57:35,000
because life, just like the stars and the planets and the galaxies,

655
00:57:35,000 --> 00:57:40,400
is just a temporary structure on the long road from order to disorder.

656
00:57:49,960 --> 00:57:52,120
But that doesn't make us insignificant

657
00:57:52,120 --> 00:57:54,920
because we are the cosmos made conscious.

658
00:57:54,920 --> 00:57:59,760
Life is the means by which the universe understands itself.

659
00:58:02,400 --> 00:58:06,400
And, for me, our true significance lies in our ability

660
00:58:06,400 --> 00:58:12,400
and our desire to understand and explore this beautiful universe.

661
00:58:55,440 --> 00:58:58,480
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