Original subtitles

is beautiful but complex.

The skies dance with colour.

Shapes of great geometrical beauty form and disappear.

And the planet itself is constantly transformed.

But this seemingly infinite complexity is just a shadow of something deeper.

The underlying laws of nature.

The world we live in is beautiful to look at.

But it's even more beautiful to understand.

Light is our window on the universe.

By understanding how light is emitted by the sun.

And how it interacts with the oceans, atmosphere and life on our planet.

We can explore worlds beyond our solar system.

And even search for the tell-tale signatures of life.

Amongst the stars.

Night on our planet seems eerie.

Other.

As if all the colour has drained away.

But in a few places on Earth.

On just a handful of nights in the year.

Colour bursts through.

10,000 years ago at the end of the last ice age.

The ice sheets melted.

And this part of Iceland rose up.

Grew the coastline back.

That left the Sköa River to tumble over the old sea cliffs to form that.

Sköafoss.

One of Iceland's great waterfalls.

When the sun, moon and the earth align.

Moonlight interacts with the spray at the foot of the falls.

But the Federiges are using a revolution.

Let's perform a moon brain.

Light leaves the sun, travels 93 million miles across

space and reflects off the surface of the moon.

It enters the Earth's atmosphere, bounces in

and out of water droplets in the waterfall.

It enters my eye.

That sends a signal to my brain and I

reconstruct that signal as something beautiful.

But the moon, though, isn't just beautiful.

It's physics.

Understanding why something's the colour it is tells you something.

It tells you about its structure, about the processes going on inside,

about its history even.

And because light travels freely across the universe,

we can explore distant worlds using light alone.

And we can tell their stories too.

The reflected light from the moon has its origin in our star, the sun.

Stars illuminate the universe.

They are the source of the light that bathes the planets.

But the processes by which light is emitted by the stars can be explored here on Earth.

On 31st August 2014, a vast chasm opened up.

In central Iceland.

At its peak, the Badabunga volcano spewed out

350 cubic metres of molten rock every second.

Producing light so bright it

could be seen from space.

And just like the sun, lava glows because it's hot.

Just look at that.

It's been months since the eruption, but it's steaming away back there.

We're actually fortunate that it erupted here.

The helicopter pilot told me that if it erupted on the glacier, then we would have

been plunged into a perpetual nuclear winter

and civilisation would have been destroyed.

That's not actually quite right, but it would have been significantly worse.

The amount of ash that went up into the

atmosphere would have been really considerable.

You get a sense of the power and violence of the earth.

Because we're blissfully unaware of it usually.

When it breaks through the surface, you see what it can do.

Today, the lava has cooled into matte black rock.

But it's still giving off light.

The thing is, everything's hot.

Everything has a temperature.

But if it's cold, it just emits light that we can't see.

This camera can see it, though.

It's called infrared light.

And you see there that this lava is glowing brighter than the background.

It means it's hotter.

So, although we can't see it, everything shines.

Light fills the universe.

Radio waves are light.

X-rays are light.

Visible light is simply the part of the spectrum we can see.

Matter, like lava, or you and me, is made up of electrically charged particles.

Here's one, moving along.

Say it's an electron.

Temperature is a measure of how fast those particles

are moving around, how fast they're jiggling.

So, in something hot, these particles are

always bouncing around and changing direction.

Now, here's a fundamental law of the universe.

When a charged particle changes direction, it emits light.

A particle called a photon.

Light can be thought of as a stream of photons,

particles whose energy corresponds to their colour.

Cooler things, like solid black lava, emit lower energy photons, infrared light,

which our eyes can't detect.

As things get hotter, they can radiate higher energy photons.

At 1,000 degrees Celsius, molten lava shines

with mainly red light, which our eyes can detect.

The surface of the sun is 5,500 degrees Celsius.

So, it can also produce higher energy green and blue photons alongside the red.

The white light of the sun is made up of all the colours of the rainbow.

And when it reaches the Earth, those individual colours are revealed.

Photons from the sun rain down and enter water.

They reflect off the rear face, out of the front again, and into your eye.

But the blue photons, the higher energy ones, behave differently

to the green ones and the yellow ones and the red ones.

They come out at a shallower angle.

And that's why you get the full spectrum of colours from the white light of the sun.

And it's this light that paints the Earth.

The colours of our planet arise because of the way photons from

the sun interact with the matter from which the Earth is made.

From space, our planet is a blue world.

And that blue colour arises because of the way light interacts with water.

And the process is linked to one of the planet's greatest migrations.

Every spring, marine biologist Osvaldo Vázquez heads out onto the high seas.

In search of these waters' most awe-inspiring seasonal visitors.

Right there, coming up.

Amazing.

Humpback whales.

Humpback whales.

Humpback whales.

Humpback whales.

Every year, they make the longest migration of any mammal on Earth.

Travelling up to 8,000 kilometres from their feeding grounds in the Northern

Atlantic to the striking blue waters of the Silverbank Marine Reserve.

This is the greatest nursery of humpback whales in the world.

85% of the North Atlantic population comes here for breeding, mating and giving birth.

So, from this place depends the survival of this species.

The whale's life cycle is intricately linked to the interaction between the

light of the sun and the water of the ocean.

In order to understand their behaviour, we need to go

beneath and to see with our own eyes what is happening there.

Not just on the surface.

Underwater.

Turn left.

Up.

There's one.

There's a half.

Right there.

Right there.

Up again.

You saw it?

Yep, yep, yep.

We became part of a group of this group now.

From the bottom, it looks like a whale.

The

reason humpbacks come here to give birth is because the

shallow waters around Silverbank are exceptionally warm.

Whales are warm-blood animals.

And when they give birth, they have a very skinny calf with no blubber.

They need a warm environment as they had inside the mother.

So here at Silverbank, it's warm and it's protected.

So it's suitable for giving birth.

As photons of light rain down onto the ocean, they strike water molecules.

And some of this energy goes into making them jiggle around, increasing their speed

and, therefore, their temperature.

But it takes an awful lot of energy to heat the oceans.

And so it's only here, exposed to the full glare of the equatorial sun, that enough

energy is absorbed to raise the water temperature to about 26 degrees.

The perfect conditions for mothers to raise their calves.

The calves are really very cute because they are like puppies.

But underwater, it weighs one ton.

So it's a big puppy.

But not all of the photons that enter the water are absorbed.

Some of them bounce straight back out again.

And it's these photons that reach our eyes and create the blue of the oceans.

To understand why it's mainly red photons that are absorbed rather than the blue

ones, we need to take a closer look at the structure of water.

This is called Þingvelli.

That's a rough pronunciation.

But in Icelandic, it means Parliament Valley.

Because the Vikings used to meet here over a thousand

years ago and had the world's first parliament.

The thing is, it's not actually a valley in the

sense that it wasn't cut by a river or a glacier.

It's actually formed by the continents themselves splitting apart.

So that is the American continental plate and that is the Eurasian continental plate.

And if you just go about a mile down the road, this place is flooded.

And you can dive in it.

The rift is filled with glacial meltwater that

seeps down from Iceland's frozen interior.

The sun's rays are much weaker here.

But wherever sunlight hits water molecules, it always produces the same colour.

What's that feel?

That's good.

Quite nice in here actually.

Look how blue this water is.

You can see for miles.

I've never seen anything like it.

The reason for that purity is that the water's got 50 kilometres.

Sometimes it's going deep underground and being filtered by these rocks.

So it's almost the coldest, purest water you can imagine.

Really, really spectacular.

Look at that.

You can see forever.

Descending deeper into the fissure, the effect on the colour of my drysuits

hints at the process by which the oceans acquire their distinctive blue.

Well, now we're down at about...

well, 12 metres.

And you see that down here, my beautiful red

diving suit is no longer quite as red as it was.

In fact, it's looking quite black.

And the reason for that is the structure of water molecules themselves.

See, a water molecule is an oxygen with two hydrogens attached.

And when the light streams into the water, the red light is

just the right energy to start those molecules vibrating.

And they do.

They start going like that...

and like that...

and even like that.

So all the energy in the red light is taken...

making the water molecules wobble and vibrating.

The blue light doesn't do that.

So the blue light can scatter around in the water relatively unimpeded.

And that's why water is blue.

Cool, isn't it?

Vibrations inside the water molecules themselves give the oceans their colour.

The weaker bonds between water molecules also absorb sunlight.

Together, these processes absorb so much energy that

there isn't a lot left to increase the sea temperature.

Which is why humpbacks must travel so far to find water warm enough to give birth.

Because water is hard to heat up, the temperature of the oceans remains very

stable, creating the perfect conditions for life to thrive.

Liquid water is the essential ingredient for any life-supporting world.

Oceans cover 70% of the Earth's surface.

Their distinctive blue, broadcasting to the universe

the message that Earth is a possible home for life.

And beyond the waves on the land, we see life's

other signature colour, a blanket of green plants.

As with all living things, they depend on water to survive.

And when the seasonal rains recede, the land

reverts to the arid colours of the naked Earth.

The intense African sun delivers 1,000 watts of solar power per square metre,

baking the plains of the Serengeti to dust.

Scorched brown, barely a single blade of green grass survives.

For the Maasai, the dry season means months of hunger and hardship.

Paracapuni and his son and brother must say goodbye to their home valley.

And drive their cows across the plains in search of enough

grass and water to keep them alive until the rains return.

If you are looking for a place to live, you will find a place to live.

They spend 7 days in the 방법 styling in the area including their houses.

At the end

of their long journeys, Maasai warriors from across the

Serengeti The Serengeti gather on the banks of Lake Masek.

But

survival on the plains of the Serengeti forces

Parakapuni and his family to spend the dry season apart.

Because while he tends the cattle, his wife, Nomi Puni, must stay behind to

look after those too young or too old to make the trip.

And eke out an existence on the brown, lifeless plains.

The family now has the YEAH family on a farm.

We look after our cattle and we raise them.

I want to see my children.

With

the cows away, the family must survive on

their meagre stores of maize flour and water.

The cows are the best.

We are proud of them.

Now, after four long months apart, things are about to change.

The

coming of the rains means the grass can grow again.

New shoots emerge, packed with the green pigment chlorophyll,

which allows plants to harvest the sun's energy.

And the plains are transformed from dusty brown to vibrant green.

In just a few days, the landscape is reborn.

I suppose we think of green as the colour of life, but actually it's the colour that

life throws away, waste green photons reflected back into our eyes.

Plants absorb most of the rest of the rainbow, the blue and red photons,

and use their energy to power photosynthesis.

Photosynthesis is the process by which plants harness light.

They are the bridge between nuclear reactions

93 million miles away and life on Earth.

Energy released from nuclear fusion reactions in the sun's core heats

everything up and shakes electrons around, and those electrons will emit photons,

which travel across space for eight minutes

and then hit an electron in a chlorophyll mass.

But instead of that energy being dissipated away as heat, chlorophyll is clever.

And ultimately, the energy imparted to that electron is used to do all sorts of

clever things through an intricate piece of machinery, split water up, force

electrons onto carbon dioxide, and ultimately

build sugars, which allows the plant to grow.

On the plains of the Serengeti, the intricate process of photosynthesis,

means Paracapuni can begin the long journey to rejoin his family.

I'm going to plant more trees.

I want to grow more plants, and I want to grow more trees.

After months apart, the greening of the Serengeti transforms the harshest of

environments into a place the Maasai can call home.

I'm going to plant more trees and the plants will grow on top of that.

Plants that feed on sunlight.

In this way, the sun's energy powers the entire food chain.

But photosynthesis does more than provide life with energy.

Around two and a half billion years ago, it began to transform the composition of

the Earth's atmosphere itself by filling it with a waste product, oxygen.

The gas upon which all complex life depends.

Without air to breathe, there would be no intelligent life on Earth.

We rely absolutely on the oxygen that forms a fifth

of the thin blue line that envelops our planet.

And perhaps paradoxically, why the sky is

blue is best explained in the dead of night.

It's a beautiful crisp autumn night here in the south

of England, rolling hills illuminated by moonlight.

Which is the light from the sun, reflected off the surface of the moon.

It's quite silent actually, almost eerie.

But out there, the Earth's shadow is racing through space.

And actually, it's just beginning to cut in.

We're at the top, the top left hand corner of the Moon.

On a few nights every decade, the Sun, Moon and Earth line up to create one of

the wonders of the night sky, a total lunar eclipse.

You get a real sense of the celestial mechanics in action during an eclipse.

The Sun is somewhere over there behind me, it's shining on our planet and our planet

is cast in the shadow through space which falls on the surface of our satellite,

the Moon.

As the Earth moves between the Moon and the Sun, it

blocks light from falling directly onto the lunar surface.

Now the Moon is completely covered by the Earth's shadow.

But it's not completely black, it's glowing a deep blood red.

At the height of the eclipse, the only photons reaching

the Moon have passed through the atmosphere of our planet.

And it's red because of the way that light interacts with the Earth's atmosphere.

Imagine that rain of photons passing through the

atmosphere from the Sun, all the colours of the rainbow.

Well, the blue photons, the higher energy photons, have a higher probability of

bouncing off the molecules in the Earth's atmosphere.

So they scatter around.

That just leaves the lower energy red photons.

They have a much lower probability of bouncing around and therefore can pass

through the Earth's atmosphere relatively unscathed.

So they are the ones that make it onto the

Moon's surface and reflect back into my eye.

And that's why in a lunar eclipse you get a blood red Moon.

And by day, the blue photons the atmosphere scatters are what paint the skies blue.

So when you look up into the sky on a summer's day and see that deep blue,

what you're actually seeing is blue photons

scattering around off the molecules in the atmosphere.

Red, green, blue.

The primary colours of our planet.

The Earth is painted by the photons that rain down from the surface of the Sun.

But for us to see them, those photons must take one final journey.

My name is Salma.

I live in Ramanath.

My home is very beautiful.

Salma lives with her seven-year-old son, Majidul,

on the banks of the Gorai River in Bangladesh.

The sky is so blue.

The plants are so beautiful.

It's so red and blue.

There are so many people.

I feel so good when I think about it.

This is a vibrant place to grow up.

But unlike the other children, Majidul can't experience it.

No one can see my children.

They're walking around on their feet.

I'm walking on my own.

I've been walking for a long time.

I'm walking outside.

I feel so much pain in my heart.

For us to see, photons must travel into our eyes.

But Majidul was born with severe cataracts.

The lenses of his eyes are so clouded, he's been effectively blind since birth.

When my son was born, he played around here.

He was able to do a lot of things.

And I feel so much pain in my heart.

My son feels so much pain.

But today, Majidul's life is about to change.

His family are making the eight-hour journey to the capital, Dhaka.

Because this afternoon, Majidul will undergo

surgery designed to restore his sight.

If the surgery is successful, replacing his faulty lenses with artificial ones

will allow light to travel directly into Majidul's eyes.

It's at the back of the eye that photons collide with cells in the retina sensitive

to red, green and blue light, and trigger the

nerve impulses that allow us to see in color.

Who do you want to see?

Who do you want to see now?

You?

Open your eyes.

Open, open.

Look at my eyes.

Who are you looking at?

Why don't you see?

Grip your fingers.

There.

What?

For the first time in his life, photons are able to enter Majidul's eyes unimpeded.

For the first time in his life, he can see.

For the first time in his life, he can see.

Our eyes reveal to us the colours of the earth.

Our

eyes reveal to us the colours of the earth.

The first step towards understanding their meaning.

The blue that signals the presence of liquid water.

The green that marks out our planet as a home for complex life.

And in this most distant image of the earth ever taken

from six billion kilometres away by the Voyager spacecraft.

The sunlight bouncing off our atmosphere reveals our planet as a pale blue dot

hanging in the void of space.

And even from this distance, anyone who happened to be looking

could tell our planet contained the ingredients for life.

Because in the most delicate colour signals, we have measured with precision

what the universe is made of.

And those signals can be glimpsed at the very edge of our own atmosphere.

What are we drawing?

A sun dragon.

What's a sun dragon?

It's just like a sun.

And it's got smoke coming out of its mouth.

Because it's starting to make fire on the world.

Now, let's just do the thing.

We need to get ready to go, don't we?

Push!

For seven years, the Nation family have been consumed by an obsession.

Ready to drop everything at a moment's notice

in pursuit of a light show like no other.

They've left their home on the Sussex coast for the cold climes of Northern Norway.

73, 98, 97.

69, 98, 100.

Spending their weekends chasing one of the planet's most extraordinary phenomena.

The Northern Lights.

Aurora Borealis.

It's an addiction.

It's an absolute full-blown addiction.

It's a buzz and it's a rush of adrenaline.

And it's a holding of your breath and...

Wow.

The aurora was once thought to be caused by dragons fighting in the sky.

But today, we know its ethereal light is created

by charged particles known as the solar wind.

Interacting with the gases of the upper atmosphere.

The oldest aurora.

She's 11.

She's fascinated by why do those different gases make those different colours.

She's quite fascinated by it.

Oh, she likes the pink because she likes pink.

It's her favourite colour.

So pink's cool.

Anything else, just wake me up if it goes pink.

And Lyrica is quite a fact box on everything.

She can explain to you what causes an aurora,

where it comes from, the technical names.

And, you know, what the magnetic field does and

how it stops the solar wind from harming us.

But the aurora is elusive.

To see it, conditions have to be just right.

Clear, dark skies must combine with strong solar winds.

All the family can do is wait.

Oh, oh, oh, oh.

I can see some uprights there.

Oh, wow.

Oh, yeah.

Here we go, look at it.

Beautiful.

Look at it.

Look how bright that is up there, look.

Ah, yes!

Now that's what I'm talking about.

Look at the reds at the top.

Stunning.

Wow.

On

The left, look, there's some pink.

Yeah, it's going to take a while.

But the aurora isn't simply beautiful.

It contains information.

As charged particles strike gas atoms, a photon of light is released.

And because the atoms of every element have a unique

structure, each one gives off a photon of a unique colour.

The

colours dancing across the sky reveal the composition of our atmosphere.

And it's these fingerprints, encoded in photons

of light, that have shown us the entire universe.

In the last few hundred years, astronomers have

gazed out from the edges of our home world.

And dreamt of a universe beyond.

In the shadow of the Eiger, that is the Sphinx.

And despite appearances, it isn't the lair of some alpine supervillain.

It's an observatory.

It was built in 1937.

And since then, astronomers have been coming here

from all over the world to look at the night sky.

And by decoding messages hidden in the light reaching our telescopes,

we've discovered the true nature of the cosmos.

That's an aurora in a box.

It's a tube full of hydrogen gas that's heated up.

And what you're seeing in that pinky-red colour are

the fundamental laws of nature themselves in action.

They tell you that a single proton, which is the nucleus of hydrogen,

and a single electron can only behave in certain ways.

The electrons can only go in certain places around the proton.

So when you heat them up, they can only jump high up.

And then when they cool down, they can only fall back in a specific way,

releasing light of a particular energy which equals a particular colour.

And those laws don't just apply here on Earth.

They apply everywhere across the universe.

So anywhere that you see glowing hydrogen gas, let's say in a nebula heated up by

young stars, then you'll see that specific colour.

Vast clouds of glowing hydrogen create some

of the most spectacular sights in the cosmos.

Their pink hue revealing the identity of the gas from which they're made.

And what's true for nebulae is true for everything in the universe.

By analysing the light from the stars, we know

that they're not just twinkling lights in the sky.

They're other suns made out of the same stuff as ours.

And if there are other suns, then there must be other planets.

Maybe even other Earths.

By understanding the origin of the colours of our world, the blue of its oceans,

the green of its life, and the colours dancing in its atmosphere, we've uncovered

the colour signature of a life-supporting planet.

And because we understand the signature of our own world,

we can look for worlds like ours elsewhere in the cosmos.

But here's what I think is the most wonderful thought of all.

When you look out, to those distant star

systems and planets, you're connected to them.

See, an electron somewhere out there radiated

a photon which hit an electron in your eye.

Alien photons crossing light-years of space, entering

your retina, and carrying stories of distant worlds.

And in the constellation of Pegasus, around a

sun-like star, there's a planet called Osiris.

It's way closer to its parent star than Mercury is to

our sun, but it's a massive gas giant, a hot Jupiter.

And by measuring the light that shines through its atmosphere, we've found that

the atmosphere's full of hydrogen, and methane, and water.

And just 24 light-years away in the constellation of Scorpius, there's a

small, rocky world, one of the most Earth-like worlds we know.

It orbits around a red dwarf star, which is one of three stars in the system.

A planet with three suns in the sky.

In the last 20 years, we've found hundreds of planets around distant suns.

We've begun to analyse their atmospheres and search for life-giving water.

Science is about gathering data information about

the natural world and trying to understand it.

And put like that, it seems a bit dry.

But when that information carries stories of alien worlds around distant suns,

hundreds of light-years away, that's a whole different thing.

Astronomy turns data into dreams.

Dreams of worlds of ice and snow.

Dreams of worlds with hemispheres in perpetual day and permanent night.

Dreams of worlds with moons and moonbows.

And perhaps, just perhaps, alien astronomers observing the

light passing through the atmosphere of our blue world.

And dreaming of us.