The natural world is beautiful, but complex.
The skies dance with colour.
Yaaaay-ya!
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 telltale 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 and drove the coastline back.
That left the Skoga River to tumble over the old sea cliffs
to form that -
Skogafoss, one of Iceland's great waterfalls.
When the sun, moon and Earth align,
moonlight interacts with the spray at the foot of the falls...
...to form a moonbow.
Light leaves the sun,
travels 93 million miles across space,
and reflects off the surface of the moon.
And it enters the Earth's atmosphere,
bounces in and out of water droplets in the waterfall,
and enters my eye.
That sends a signal to my brain,
and I reconstruct the signal as something beautiful.
But the moonbow 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.
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 the 31st August, 2014,
a vast chasm opened up in central Iceland.
At its peak, the Bardabunga 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 many 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'd 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.
Cos we're blissfully unaware of it, usually,
but when it breaks through the surface, you see what it can do.
Today the lava has cooled into matt-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.
You see there that this lava
is glowing brighter than the background.
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 a light 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 droplets.
They reflect off the rear face,
come 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 Vasquez
heads out onto the high seas...
...at the port bow, over.
11, 11 o'clock.
...in search of these waters' most awe-inspiring seasonal visitors.
Humpback whales.
Every year, they make the longest migration of any mammal on Earth,
travelling up to 8,000km
from their feeding grounds in the northern Atlantic
to the striking blue waters of the Silver Bank 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 the species.
The whales' 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.
Yep, yep, yep.
Slow down. Slow down.
Slow down, stay down.
The reason humpbacks come here to give birth
is because the shallow waters around Silver Bank are exceptionally warm.
Whales are warm-blooded animals,
and when they give birth,
they have a very skinny calf with no blubber.
They need a warm environment, as they had inside Mother.
So here, Silver Bank, is warm and is 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 a balmy 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 tonne. 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 Thingvellir.
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.
How'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 come 50km,
sometimes going deep underground,
and being filtered by these rocks.
So it's some of 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 drysuit
hints at the process by which
the oceans acquire their distinctive blue.
Well, now we're down at about, erm...
...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 vibrate.
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.
Parakapuni 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.
At the end of their long journeys,
Maasai warriors from across 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 Nomipuni 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.
With the cows away,
the family must survive on their meagre stores
of maize flour and water.
Now, after four long months apart,
things are about to change.
The coming of the rains means the grass can grow again.
Revived, 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 molecule.
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 Parakapuni can begin the long journey to rejoin his family.
After months apart, the greening of the Serengeti
transforms the harshest of environments
into a place the Maasai can call home.
Animals eat 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 off
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,
shining on our planet,
and our planet is casting a 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 blank.
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 the rain 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,
which 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.
Salma lives with her seven-year-old son Majidil
on the banks of the Gorai River in Bangladesh.
This is a vibrant place to grow up.
But unlike the other children, Majidil can't experience it.
For us to see, photons must travel into our eyes,
but Majidil was born with severe cataracts.
The lenses of his eyes are so clouded,
he's been effectively blind since birth.
But today, Majidil's life is about to change.
His family are making the eight-hour journey to the capital, Dhaka.
Because this afternoon,
Majidil 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 Majidil'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 colour.
For the first time in his life,
photons are able to enter Majidil's eyes unimpeded.
For the first time in his life, he can see.
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.
Cos it's starting to make fire on the world.
Now, let's just do them there.
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.
I'm Aurora Chaser!
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 is 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.
Our eldest, Aurora, she's 11.
She's fascinated by,
why do those different gases
make those different colours?
She's quite fascinated by it. Oce...
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.
Here we go!
Clear dark skies must combine with strong solar winds.
You're going to burn it.
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!
Yay! Yes!
Now that's what I'm talking about.
Look. Ooh!
Look, look, look, look, look! Corona, corona!
- Look at it. - Look at the reds at the top!
- Stunning. - Wow.
Oh, oh, oh!
Oh, look at that.
- And on the left, on the left, look, there's some pink. - Yay!
Look, look, look, look, look!
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 green comes from oxygen atoms,
pink from nitrogen.
Yes!
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 the universe beyond.
In the shadow of the Eiger, that is the Sphinx.
And despite appearances,
it isn't the lair of some Alpine super-villain.
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.
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.
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.
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 found that the atmosphere is 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 earthlike 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 began 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 world with hemispheres
in perpetual day and permanent night.
Dreams of world 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.