Original subtitles

PROFESSOR BRIAN COX: Why are we here?

Where do we come from?

These are the most enduring of questions.

And it's an essential part of human nature

to want to find the answers.

Now, we can trace our ancestry back hundreds of thousands of years

to the dawn of humankind.

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

Our story starts with the beginning of the universe.

It began 13.7 billion years ago.

And today it's filled with over a hundred billion galaxies,

each containing hundreds of billions of stars.

In this series, I want to tell that story.

Because ultimately, we are part of the universe.

So its story is our story.

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

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

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

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

and created many extraordinary wonders.

These wonders take us from the very first moments

in the life of the universe to its eventual end.

This is Chankillo on the northwestern coast of Peru.

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

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

Around two and a half thousand years ago,

a civilisation we know almost nothing about

built this fortified temple in the desert.

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

Today, all but the smallest fragments of the decorations are gone.

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

And yet, if you stand in the right place,

you can still experience the true purpose of Chankillo

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

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

These towers form an ancient solar calendar.

Now, at different times of year, the sunrise point

is at a different place on the horizon.

Actually, December 21st,

which here in the Southern Hemisphere is the summer solstice, the longest day,

and the sun rises just to the right of the rightmost tower.

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

until, on June 21st, which is the winter solstice,

the shortest day, it rises just to the left

of the leftmost tower.

Actually, just in between that mountain you can see in the distance

and the leftmost tower.

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

you can measure its position and you can tell,

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

Today's date is September the 15th,

and so that means that the sun will rise between the fifth and the sixth towers.

Chankillo still works as a calendar,

because the sun still rises in the same place today

as it did when these stones were first laid down.

Now, that is a magnificent sight, as the sun burns through the towers.

You can almost feel the presence of the past here.

I mean, imagine what it must have been like.

Thousands of citizens stood here to greet the sun,

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

What a magnificent achievement.

I mean, it's probably one of our earliest attempts

to begin to measure the heavens.

Over the millennia,

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

has led to modern astronomy

and the foundations of our modern civilisation.

I might build one in my garden.

(LAUGHS) I want one.

The 13 towers that line this ridge stand testament

to our enduring fascination with the clockwork of the heavens

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

The rising and setting of the sun provides an epic heartbeat

that allows us to mark the passage of time.

A day on Earth is the 24 hours it takes our planet

to rotate once on its axis.

Our months are based on the 29 and a half days

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

And a year is the 365 and a quarter days

it takes us to orbit once around the sun.

These familiar time scales mark the passing of our lives,

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

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

those tiny points of light are a million different clocks,

whose life spans mark out the passage of time

over billions or even trillions of years.

This film is about the greatest expanses of time.

The deep time that shapes the universe.

From its fiery beginnings

through countless generations of stars, planets and galaxies

to its eventual demise,

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

Time scales in the cosmos seem so unimaginably vast

it's almost impossible to relate to them.

Yet there are places on Earth where we can begin to encounter time

on these universal scales.

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

and I've come here to witness a natural event

that's been happening long before there were any humans here to see it.

And I suppose it really is a window

into the distant past of life on our planet.

(INDISTINCT)

Once the sun has dipped below the horizon

and the moon conspired to make the tides just right,

this beach is visited by prehistoric creatures.

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

Playa Ostional is one of the few beaches in the world

where large numbers of sea turtles make their nests.

But what makes this truly remarkable

is the sheer length of time scenes like this have been playing out.

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

On nights like these for the last hundred million years,

turtles like this have been hauling themselves out of the ocean

to lay their eggs.

It's an almost incomprehensible time span.

I mean, a hundred million years ago there were dinosaurs roaming the Earth

but the Earth itself looked very different.

I mean, South America was not connected to North America.

North America was somewhere over close to Europe.

Australia was connected to Antarctica.

It really is quite wonderful to be so close

to such an ancient cycle of life.

I can hear her breathing, actually.

(BREATHING HEAVILY)

It's a remarkable experience.

I mean, it really is beautiful to see that

on one night of many hundreds of millions of nights

stretching back into the past.

And she's gone.

To witness a moment like this

is to open up a connection to the deep past,

to experience time spans far longer than the history of our own species.

Yet even the hundred million years story of the turtles

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

Our entire solar system is travelling on an unimaginably vast orbit,

spinning around the centre of our galaxy.

It takes 250 million years

to make just one circuit of the Milky Way.

In the entire history of the human race,

we've travelled less than a tenth of one percent of that orbit.

These cycles seem eternal and unchanging,

but as the story of time unfolds,

a fundamental truth is revealed.

Nothing lasts forever.

This is the most profound property of time.

And it plays out just as vividly here on Earth

as it does in the depths of space.

This is the Perito Moreno Glacier in Patagonia in Southern Argentina,

and it's one of the hundreds of glaciers

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

And you know, if you carry on that way,

so south, about... I don't know about 1000 kilometres

you get to the end of South America

and from then on there's nothing till the Antarctic.

(LAUGHS) And it feels like that today.

The glacier is such a massive expanse of ice,

but at first sight, just like the cycles of the heavens,

it appears fixed and unchanging.

(CRACKING)

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

as it has been for tens of thousands of years.

The whole face of the glacier is moving into the lake,

about something like that much every day.

And that means that well over a quarter of a billion tons of ice

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

It's about a million tons a day.

And you can hear it happening.

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

It really is like the place is alive.

(RUMBLING)

You know, it's quite disturbing

when these enormous chunks of ice fall into the lake.

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

actually, there can be really violent collapses.

It's an incredibly dynamic place to be.

This movement of the glacier provides an insight into the nature of time.

It is simply the ordering of events into sequences, one step after another.

As time passes, snow falls, ice forms,

the glacier gradually inches down the valley

and huge chunks of ice fall into the lake below.

But even this simple sequence contains a profound idea.

Events always happen in the same order.

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

Now, that is something that you would never see in reverse.

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

that describe how all those water molecules are moving around

that prevent them from all getting together on the surface of the lake,

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

and then gluing themselves back onto the surface of the glacier again.

But interestingly, we do understand

why the world doesn't run in reverse.

There is a reason, we have a scientific explanation,

and it's called the arrow of time.

We never see waves travelling across lakes,

coming together and bouncing chunks of ice back onto glaciers.

We are compelled to travel into the future.

And that's because the arrow of time dictates

that as each moment passes, things change.

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

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

You know, we all age as the years pass by.

People are born, they live and they die.

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

But out there in the universe, those grand and epic cycles

appear eternal and unchanging, but that's an illusion.

See, in the life of the universe, just as in our lives,

everything is irreversibly changing.

By building change upon change,

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

And as we look out deep in to the cosmos,

we can see that story unfold.

This is an image of a tiny piece of night sky

in the constellation of Leo.

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

And despite appearances,

it is one of the most interesting images taken in recent astronomical history.

The interesting thing is this little red blob here,

which looks very unremarkable.

But what that red blob is

is the afterglow of an enormous cosmic explosion.

It's the death of a star that was about, what,

40 or even 50 times the mass of our sun.

Poetically named GRB 090423,

it was once a Wolf-Rayet star.

Shrouded by rapidly swirling clouds of gas,

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

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

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

That caused massive jets of light and stellar material

to be ejected from its poles,

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

And it's the afterglow of this catastrophic explosion

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

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

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

then we're looking back in time,

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

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

for almost the entire history of the universe.

You see, what we're looking at here

is an event that happened 13 billion years ago.

I mean, that's only about 600 million years

after the Big Bang, after the universe began.

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

In fact, this is the oldest single object

that we've ever seen.

What we're looking at here is the explosive death

of one of the first stars in the universe.

As it evolves, the universe passes through distinct eras.

Vast ages, whose beginnings and endings

are marked by unique milestones.

The births and deaths of its wonders.

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

in the evolution of the cosmos.

It signals the end of the Primordial Era

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

The time in which we live,

the Stelliferous Era,

the age of the stars.

Starlight illuminates the night sky

and starlight illuminates our days.

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

And our galaxy is one of 100 billion in the observable universe.

Countless islands of countless stars.

Although the universe is over 13 billion years old,

we still live close to the start of the Stelliferous Era.

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

The cosmos is absolutely awash with stars,

surrounded by nebulae and systems of planets.

There are countless billions of worlds that we've yet to explore.

But the cosmos isn't static and unchanging

and it won't always be this way.

Because as the arrow of time plays out,

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

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

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

Because the arrow of time says the future

will always be different from the past.

But what drives this evolution?

Why is there a difference between the past and the future?

Why is there an arrow of time at all?

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

It seems obvious to us that things change

and the future will be different to the past.

We know that because we see the effects

of the passing years all around us.

This is Kolmanskop, an abandoned diamond-mining town in southern Namibia.

Now, this entire town was founded in 1908,

when a worker, who was building the railway

from the Port of Lüderitz inland, into the centre of Namibia,

found a single diamond here in this desert.

For 40 years, this was a thriving community of up to 1000 people,

a place where you could become a millionaire

picking diamonds out of the sand.

While the money rolled in, they built grand houses

and lived a champagne lifestyle in the desert.

But when the diamonds dried up the town was abandoned

and for half a century, it's fallen into disrepair

as it's slowly reclaimed by the sands.

The processes at play here at Kolmanskop

are happening everywhere in the universe.

Because it isn't simply permanent change

that's central to the arrow of time,

it's decay.

But the scientific explanation for why that is

didn't come from attempting to understand

the effects of time in the universe.

It came from trying to build a faster train.

Back in the 19th century,

engineers were concerned with the efficiency of steam engines.

You know, how hot should the fire be?

What substance should you boil in the steam engine?

Should it be water or should it be something else?

These were profound questions.

And out of those questions arose the science of thermodynamics.

It's when concepts like heat and temperature and energy

entered the scientific vocabulary for the first time.

Now, along with that deeper understanding

emerged what is probably the most important law of physics

for understanding the evolution of the universe

and the passage of time.

It's called the second law of thermodynamics.

The reason the second law of thermodynamics was so profound

was because at its heart it contained a radically new concept,

something physicists call "entropy".

Entropy explains why, left to the mercy of the elements,

mortar crumbles, glass shatters and buildings collapse.

And a good way to understand how is to think of objects

not as single things, but as being made up of many constituent parts,

like the individual grains that make up this pile of sand.

Now, entropy is a measure

of how many ways I can rearrange those grains

and still keep the sand pile the same.

And there are trillions and trillions and trillions of ways of doing that.

I mean, pretty much anything I do to this sand pile,

if I mess the sand around and move it around,

then it doesn't change the shape or the structure at all.

So, in the language of entropy,

this sand pile has high entropy,

because there are many, many ways that I can rearrange

its constituents and not change it.

But now let me create some order in the universe.

Now, there are approximately as many sand grains

in this sandcastle as there are in the sand pile.

But now, virtually anything I do to it will mess it up,

will remove the beautiful order from this structure.

And because of that, the sandcastle has a low entropy.

It's a much more ordered state.

So many ways of rearranging the sand grains

without changing the structure, high entropy.

Very few ways of rearranging the sand grains

without changing the structure, without disordering it, low entropy.

Now, imagine I was to leave this castle in the desert all day,

then it's obvious what's going to happen.

The desert winds are going to blow the sand around

and this castle is going to disintegrate.

It's going to become less ordered,

it's going to fall to bits.

But think about what's happening on a fundamental level.

I mean, the wind is taking the sand off the castle

and blowing it over there somewhere, and making a sand pile.

There's nothing fundamental in the laws of physics

that says that the wind couldn't pick up some sand from over here,

deposit it here and deposit it in precisely the shape of a sandcastle.

You know, in principle, the wind could spontaneously build

a sandcastle out of a pile of sand.

There's no reason why that couldn't happen.

It's just extremely, extremely unlikely,

because there are very few ways of organising this sand

so that it looks like a castle.

It's overwhelmingly more likely

that when the wind blows the sand around,

it will take the low entropy structure, the castle,

and turn it into a high entropy structure, the sand pile.

So entropy always increases.

Why is that? Because it's overwhelmingly more likely that it will.

It seems incredible that a law that says that sandcastles

don't spontaneously form on the wind

could solve one of the deepest mysteries in physics.

But by saying entropy always increases,

the second law of thermodynamics is able to explain

why time only runs in one direction.

The second law of thermodynamics, for me, demonstrates everything

that is powerful and beautiful and profound about physics.

You see, here's a law that entered science as a way of talking

about how heat moves around and the efficiency of steam engines,

but it ended up being able to explain

one of the great mysteries in the history of science.

Why is there a difference between the past and the future?

You see, the second law says

that everything tends from order to disorder.

That means that there is a difference between the past and the future.

In the past, the universe was more ordered,

and in the future, the universe will be less ordered.

And that means that there's a direction to the passage of time.

So the second law of thermodynamics

has introduced the concept

of an arrow of time into science.

The arrow of time has been playing out in Kolmanskop

since the mining facility was abandoned in 1954.

But in the universe, it's been playing out

for almost 14 billion years,

and it will have profound consequences.

Because it means stars cannot shine forever,

including the star at the centre of our solar system.

At the end of its life, the sun won't simply fade away to nothing.

As it begins to run out of fuel, its core will collapse,

and the extra heat this generates will cause its outer layers to expand.

In around a billion years' time,

this will have a catastrophic effect on our fragile world.

Gradually, the Earth will become hotter and hotter.

So there will be one last perfect day on Earth,

but eventually the existence of all life on this planet will become impossible.

Long after life has disappeared,

the sun will have grown so much, it will fill the entire horizon.

It will have become a red giant, the last phase of its life.

Our planet might not survive to this point,

but if it does, little more than a scorched and barren rock

will remain to witness the final death throes of our star.

(RUMBLING)

In six billion years, our sun will explode,

throwing vast amounts of gas and dust

out into space to form a gigantic nebula.

And at its heart will be a faintly glowing ember,

all that remains of our once-magnificent sun.

It will be smaller than the size of the Earth,

less than a millionth of its current volume and a fraction of its brightness.

Our sun will have become a white dwarf.

With no fuel left to burn, a white dwarf's faint glow

comes from the last residual heat from its extinguished furnace.

The sun is now dead,

its remains slowly cooling

in the freezing temperatures of deep space.

Looking at it from where the Earth is now,

it would only generate the same amount of light

as the full moon on a clear night.

The fate of the sun is the same as for all stars.

One day, they must all eventually die

and the cosmos will be plunged into eternal night.

And this is the most profound consequence

of the arrow of time.

Because this structured universe that we inhabit

and all its wonders, the stars and the planets and the galaxies,

cannot last forever.

The cosmos will eventually fade and die.

First will come the end of the Stelliferous Era,

the end of the age of starlight.

The largest stars are the first to disappear,

violently collapsing into black holes,

just a few million years after their formation.

But long after they're gone, just one type of star will remain.

This is a picture of the nearest star to our solar system, Proxima Centauri.

Now, it's only 4.2 light years away, but the reason it doesn't stand out

against the much more distant stars in this photograph

is that Proxima Centauri is incredibly tiny.

It's the kind of star known as a red dwarf star,

and it's only about 11 to 12 percent the mass of our sun.

But to our eyes, it would appear to shine 18,000 times less brightly.

But red dwarves do have one advantage

over their much more luminous and magnificent stellar brethren.

And that's because they're so small,

they burn their nuclear fuel incredibly slowly,

so they have life spans of trillions of years.

And that means that stars like Proxima Centauri

will be the last living stars in the universe.

If we survive into the far future of the universe,

then it's possible to imagine our distant descendents

building their civilisation around red dwarves,

to capture the energy from those last fading embers of stars.

Just as our ancestors crowded around campfires

for warmth on cold winters' nights.

The reason why Proxima Centauri burns so slowly

is because its small size and low gravity

mean its core is under much lower pressure than larger stars.

This also means that its interior is constantly churning,

whipping up the surface into a fiery turmoil.

Explosive solar flares occur almost continually,

even though it burns so dimly.

But Proxima Centauri will eventually die.

And like our sun, it too will become a white dwarf.

As the age of starlight ends,

all but the dimmest flicker of light in the universe will go out.

The faint glow of white dwarves will provide the only illumination

in a dark and empty void littered with dead stars and black holes.

By this point, the universe will be 100 trillion years old.

And yet, even now, the vast majority of its lifespan

still lies ahead of it.

There are few places on Earth

where you can get an inkling of what the far future has in store.

Well, this is Namibia's Skeleton Coast,

where the cold waters of the South Atlantic meet the Namib Desert.

And it is one of the most inhospitable places on Earth.

I mean, back in the 17th century,

Portuguese sailors used to call this place

"the gates to hell", because this dense fog that you see

pretty much every morning along this coast,

coupled with the constantly shifting shape of the sandbanks

meant that over the years,

literally thousands of ships were wrecked along this coastline.

And even if you made it to shore, that wasn't the end of your problems,

because the currents are so strong here

that there is no way of rowing back out to sea.

And if you look that way, there's just hundreds of miles

of inhospitable desert.

So, it genuinely was a place of no return.

If you were shipwrecked here, this was the end of your universe.

This is the Eduard Bohlen.

She was once an ocean-going steamer,

ferrying passengers and cargo between here and Europe.

On the 5th of September, 1909, she ran aground in thick fog.

Yet, like all the vessels wrecked along this shoreline,

the time it takes her to decay to nothing

will be far longer than her time at sea.

And in the far future of the cosmos,

a similar destiny awaits the remaining white dwarves.

A black dwarf will be the final fate of those last stars,

white dwarves that have become so cold

that they barely emit any more heat or light.

Black dwarves are dark, dense, decaying balls

of degenerate matter,

little more than the ashes of stars.

Their constituent atoms are so severely crushed

that black dwarves are a million times denser than our sun.

Stars take so long to reach this point that after nearly 14 billion years,

we believe there are currently no black dwarves in the universe.

But despite never seeing one,

we can still predict how they will end their days.

Just as the iron that makes up this ship will eventually rust

and be carried away by the desert winds,

so we think that the matter inside black dwarves,

the last matter in the universe,

will eventually evaporate away and be carried off

into the void as radiation, leaving absolutely nothing behind.

With the black dwarves gone,

there won't be a single atom of matter left.

All that will remain of our once-rich cosmos

will be particles of light and black holes.

After an unimaginable length of time,

even the black holes will have evaporated

and the universe will be nothing but a sea

of photons gradually tending towards the same temperature,

as the expansion of the universe cools them towards absolute zero.

And when I say unimaginable period of time,

I really mean it.

It's 10,000 trillion trillion trillion trillion

trillion trillion trillion trillion years.

Now, how big is that number?

Well, if I were to start counting with a single atom

representing one year, then there wouldn't be enough atoms

in the entire universe to get anywhere near that number.

Once the very last remnants of the very last stars

have finally decayed away to nothing

and everything reaches the same temperature,

the story of the universe finally comes to an end.

For the first time in its life,

the universe will be permanent and unchanging.

Entropy finally stops increasing,

because the cosmos cannot get any more disordered.

Nothing happens and it keeps not happening,

forever.

It's what's known as the heat death of the universe,

an era when the cosmos will remain

vast and cold and desolate for the rest of time.

And that's because there is no difference

between the past, the present and the future.

There's no way of measuring the passage of time,

because nothing in the cosmos changes.

The arrow of time has simply ceased to exist.

It's an inescapable fact of the universe,

written into the fundamental laws of physics.

The entire cosmos will die.

Every single one of the 200 billion stars in our galaxy

will go out.

And just as the death of the sun

means the end of life on our planet,

so the death of every star

will extinguish any possibility of life in the universe.

The fact that the sun will die,

and it will incinerate the Earth and obliterate all life

on our planet in the process, might sound a bit depressing to you.

You might legitimately ask,

"Well, surely you could build a universe in a different way,

"surely you could build it so it didn't have to descend

"from order into chaos?"

Well, the answer is, "No, you couldn't, if you wanted life to exist in it."

The arrow of time, the sequence of changes

that slowly leads the universe to its death,

is the very same thing that creates

the conditions for life in the first place.

Because it takes time for matter to form

and it takes time for gravity

to pull it together into stars and planets.

The arrow of time creates a bright window

in the universe's adolescence, during which life is possible.

But it's a window that doesn't stay open for long.

As a fraction of the life span of the universe,

as measured from its beginning

to the evaporation of the last black hole,

life as we know it is only possible

for one thousandth of a billion billion billionth,

billion billion billionth, billion billion billionth of a percent.

And that's why, for me,

the most astonishing wonder of the universe

isn't a star or a planet or a galaxy.

It isn't a thing at all. It's an instant in time.

And that time is now.

Humans have walked the Earth for just the smallest fraction

of that briefest of moments in deep time.

But in our 200000 years on this planet,

we've made remarkable progress.

It was only two and a half thousand years ago

that we believed that the sun was a god

and measured its orbit with stone towers

built on the top of a hill.

Today, the language of curiosity is not sun gods but science.

And we have observatories that are almost infinitely

more sophisticated than the 13 towers,

that can gaze out deep into the universe.

And perhaps even more remarkably,

through theoretical physics and mathematics,

we can calculate what the universe will look like in the distant future

and we can even make concrete predictions about its end.

And I believe it's only by continuing our exploration of the cosmos

and the laws of nature that govern it

that we can truly understand ourselves

and our place in this universe of wonders.

And that's what we've done in our brief moment on planet Earth.

In 1977, a space probe called Voyager 1

was launched on a grand tour of the solar system.

And it visited the great gas-giant planets Jupiter and Saturn

and made some wonderful discoveries

before heading off towards interstellar space.

13 years later, after its mission was almost over,

it turned around and took one last picture

of its home solar system.

And this is that picture.

And the beautiful thing about this picture

is this single pixel of light

suspended against the blackness of space.

Because that pixel, that point, is planet Earth,

the most distant picture of our planet ever taken

at six billion kilometres away.

And whilst I suppose it has very limited scientific value,

for me, this tiny point of light

is the most powerful and profound demonstration

of perhaps the most human of qualities,

our unique ability to reflect

on the universe's existence and our place within it.

Just as we, and all life on Earth,

stand on this tiny speck adrift in infinite space,

so life in the universe will only exist

for a fleeting, bright instant in time.

Because life, just like the stars and the planets and the galaxies,

is just a temporary structure on the long road from order to disorder.

But that doesn't make us insignificant,

because we are the cosmos made conscious.

Life is the means by which the universe understands itself.

And for me, our true significance lies in our ability

and our desire to understand and explore this beautiful universe.

(WE HAVE ALL THE TIME IN THE WORLD BY LOUIS ARMSTRONG PLAYING)