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The natural world
is beautiful, but complex.

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The skies dance with colour.

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Yaaaay-ya!

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Shapes of great geometrical beauty
form and disappear.

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And the planet itself
is constantly transformed.

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But this seemingly infinite
complexity...

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..is just a shadow of
something deeper.

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The underlying laws of nature.

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The world we live in
is beautiful to look at,

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but it's even more beautiful
to understand.

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Light is our window on the universe.

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By understanding
how light is emitted by the sun,

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and how it interacts with
the oceans, atmosphere,

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and life on our planet,

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we can explore worlds
beyond our solar system...

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..and even search for
the telltale signatures of life

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amongst the stars.

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Night on our planet seems eerie,

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other,

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as if all the colour
has drained away.

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But in a few places on Earth,

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on just a handful of nights
in the year,

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colour bursts through.

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

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the ice sheets melted

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and this part of Iceland rose up
and drove the coastline back.

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That left the Skoga River to
tumble over the old sea cliffs

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to form that -

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Skogafoss,
one of Iceland's great waterfalls.

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When the sun, moon and Earth align,

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

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..to form a moonbow.

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Light leaves the sun,

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travels 93 million miles
across space,

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and reflects off the surface
of the moon.

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And it enters
the Earth's atmosphere,

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bounces in and out of water droplets
in the waterfall,

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and enters my eye.

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That sends a signal to my brain,

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and I reconstruct the signal
as something beautiful.

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But the moonbow
isn't just beautiful.

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It's physics.

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

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It tells you about its structure,

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about the processes going on inside,
about its history, even.

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And because light travels freely
across the universe,

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we can explore distant worlds
using light alone.

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We can tell their stories, too.

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

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Stars illuminate the universe.

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

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But the processes by which
light is emitted by the stars

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can be explored here on Earth.

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On the 31st August, 2014,

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a vast chasm opened up
in central Iceland.

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At its peak, the Bardabunga volcano

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spewed out 350 cubic metres
of molten rock every second...

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..producing light so bright...

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..it could be seen from space.

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

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Just look at that.

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

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We're actually fortunate
that it erupted here.

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The helicopter pilot told me that
if it'd erupted on the glacier,

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then we would have been plunged into
a perpetual nuclear winter

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and civilisation
would have been destroyed.

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That's not actually quite right,

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but it would have been
significantly worse.

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The amount of ash
that went up into the atmosphere

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would have been really considerable.

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

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Cos we're blissfully unaware of it,
usually,

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

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Today the lava
has cooled into matt-black rock,

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but it's still giving off light.

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The thing is, everything's hot.
Everything has a temperature.

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

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This camera can see it, though.

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It's called infrared light.

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You see there that this lava

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is glowing brighter
than the background.

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Means it's hotter.

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So, although we can't see it,

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everything shines.

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Light fills the universe.

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Radio waves are light,
X-rays are light.

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

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Matter, like lava,
or you and me,

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is made up of
electrically charged particles.

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Here's one moving along -

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say it's an electron.

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Temperature is a measure of how fast
those particles are moving around,

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how fast they're jiggling.

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So, in something hot,

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these particles
are always bouncing around

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and changing direction.

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

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When a charged particle
changes direction,

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it emits a light particle

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called a photon.

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Light can be thought of
as a stream of photons -

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particles whose energy
corresponds to their colour.

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Cooler things,
like solid black lava,

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emit lower energy photons - infrared
light, which our eyes can't detect.

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

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At 1,000 degrees Celsius, molten
lava shines with mainly red light,

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which our eyes can detect.

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

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so it can also produce
higher-energy green and blue photons

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alongside the red.

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The white light of the sun

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is made up of
all the colours of the rainbow.

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And when it reaches the Earth,

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those individual colours
are revealed.

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Photons from the sun rain down
and enter water droplets.

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They reflect off the rear face,

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come out of the front again
and into your eye.

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But the blue photons,
the higher-energy ones,

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behave differently to the green ones

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and the yellow ones
and the red ones.

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They come out at a shallower angle,

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and that's why you get
the full spectrum of colours

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from the white light of the sun.

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And it's this light
that paints the Earth.

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The colours of our planet arise
because of the way photons

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

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From space,
our planet is a blue world.

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And that blue colour arises

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because of the way light
interacts with water.

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

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Every spring, marine biologist
Osvaldo Vasquez

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heads out onto the high seas...

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MAN ON RADIO: ...at the port bow,
over.

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11, 11 o'clock.

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

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Humpback whales.

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

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travelling up to 8,000km

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from their feeding grounds
in the northern Atlantic

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to the striking blue waters
of the Silver Bank Marine Reserve.

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

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85% of the North Atlantic population
comes here for breeding,

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mating, and giving birth.

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

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The whales' life cycle
is intricately linked

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to the interaction
between the light of the sun

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and the water of the ocean.

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In order to understand
their behaviour,

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we need to go beneath, and to see,

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with our own eyes,
what is happening there.

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Not just on the surface -
underwater.

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Yep, yep, yep.

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Slow down. Slow down.

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Slow down, stay down.

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WHALES SING SOFTLY

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The reason humpbacks
come here to give birth

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is because the shallow waters around
Silver Bank are exceptionally warm.

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Whales are warm-blooded animals,

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and when they give birth,

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they have a very skinny calf
with no blubber.

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They need a warm environment,
as they had inside Mother.

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So here, Silver Bank,
is warm and is protected.

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So it's suitable for giving birth.

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As photons of light
rain down onto the ocean,

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they strike water molecules,

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and some of this energy goes into
making them jiggle around,

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increasing their speed
and, therefore, their temperature.

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

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and so it's only here,

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exposed to the full glare
of the equatorial sun,

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that enough energy is absorbed

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to raise the water temperature
to a balmy 26 degrees.

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The perfect conditions for mothers
to raise their calves.

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The calves are really very cute

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because they are like puppies.

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But underwater, it weighs one tonne.
So...it's a big puppy.

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

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Some of them bounce
straight back out again,

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and it's these photons
that reach our eyes

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and create the blue of the oceans.

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To understand why it's mainly
red photons that are absorbed

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rather than the blue ones,

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

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This is called Thingvellir.

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That's a rough pronunciation.

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But in Icelandic,
it means "parliament valley",

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because the Vikings used to meet
here over a thousand years ago

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and had the world's first
parliament.

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

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

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It's actually formed

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by the continents themselves
splitting apart.

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So that is the American
continental plate

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and that is the Eurasian
continental plate.

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And if you just go about a mile
down the road,

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this place is flooded,
and you can dive in it.

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The rift is filled with glacial
meltwater that seeps down

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from Iceland's frozen interior.

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The sun's rays are much weaker here.

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But, wherever sunlight
hits water molecules,

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it always produces the same colour.

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How's that feel?

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That's good.

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Quite nice in here, actually.

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Look how blue this water is.

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You can see for miles.

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00:19:01,090 --> 00:19:03,970
I've never seen anything like it.

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00:19:03,970 --> 00:19:10,130
The reason for that purity
is that the water's come 50km,

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sometimes going deep underground,

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00:19:13,370 --> 00:19:16,330
and being filtered by these rocks.

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00:19:16,330 --> 00:19:20,730
So it's some of the coldest,
purest water you can imagine.

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00:19:21,930 --> 00:19:26,730
Really, really spectacular -
look at that.

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00:19:26,730 --> 00:19:28,210
You can see forever.

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00:19:35,810 --> 00:19:38,210
Descending deeper into the fissure,

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00:19:38,210 --> 00:19:40,970
the effect on the colour of
my drysuit

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00:19:40,970 --> 00:19:43,090
hints at the process by which

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the oceans acquire
their distinctive blue.

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

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..well, 12 metres.

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And you see that down here,
my beautiful red diving suit

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is no longer quite as red as it was.
HE LAUGHS

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In fact, it's looking quite black.

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00:20:10,690 --> 00:20:12,130
And the reason for that...

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00:20:13,210 --> 00:20:16,490
..is the structure of
water molecules themselves.

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00:20:18,410 --> 00:20:23,850
See, a water molecule is an oxygen
with two hydrogens attached.

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

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is just the right energy
to start those molecules vibrating,

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and they do.
They start going like that...

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and like that....

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and even like that.

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So all the energy
in the red light is taken,

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making the water molecules
wobble and vibrate.

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00:20:51,490 --> 00:20:53,530
The blue light doesn't do that.

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00:20:53,530 --> 00:20:56,970
So the blue light
can scatter around in the water

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relatively unimpeded.

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And that's why water is blue.

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Cool, isn't it?

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00:21:16,530 --> 00:21:20,010
Vibrations inside
the water molecules themselves

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give the oceans their colour.

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The weaker bonds between water
molecules also absorb sunlight.

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Together, these processes absorb
so much energy

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

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WHALE SQUEAKS

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00:21:37,450 --> 00:21:40,330
Which is why humpbacks
must travel so far

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to find water warm enough
to give birth.

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00:21:52,770 --> 00:21:54,730
WHALE SINGS

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Because water is hard to heat up,

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00:22:04,810 --> 00:22:09,010
the temperature of the oceans
remains very stable,

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creating the perfect conditions
for life to thrive.

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Liquid water
is the essential ingredient

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for any life-supporting world.

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Oceans cover 70% of
the Earth's surface,

253
00:22:37,370 --> 00:22:41,530
their distinctive blue
broadcasting to the universe

254
00:22:41,530 --> 00:22:45,650
the message that Earth
is a possible home for life.

255
00:22:50,370 --> 00:22:53,370
And beyond the waves, on the land,

256
00:22:53,370 --> 00:22:56,450
we see life's other
signature colour -

257
00:22:56,450 --> 00:22:58,810
a blanket of green plants.

258
00:23:00,490 --> 00:23:05,010
As with all living things,
they depend on water to survive.

259
00:23:05,010 --> 00:23:07,810
And when the seasonal rains recede,

260
00:23:07,810 --> 00:23:11,890
the land reverts to the arid colours
of the naked earth.

261
00:23:24,170 --> 00:23:26,290
The intense African sun

262
00:23:26,290 --> 00:23:30,810
delivers 1,000 watts of solar power
per square metre,

263
00:23:30,810 --> 00:23:34,330
baking the plains of
the Serengeti to dust.

264
00:23:38,730 --> 00:23:40,690
Scorched brown -

265
00:23:40,690 --> 00:23:44,010
barely a single blade
of green grass survives.

266
00:23:48,210 --> 00:23:53,130
For the Maasai, the dry season
means months of hunger and hardship.

267
00:23:57,450 --> 00:24:00,330
Parakapuni and his son
and brother

268
00:24:00,330 --> 00:24:02,810
must say goodbye
to their home valley...

269
00:24:04,770 --> 00:24:08,210
..and drive their cows
across the plains

270
00:24:08,210 --> 00:24:10,250
in search of enough grass and water

271
00:24:10,250 --> 00:24:13,010
to keep them alive
until the rains return.

272
00:24:52,330 --> 00:24:54,490
At the end of their long journeys,

273
00:24:54,490 --> 00:24:57,290
Maasai warriors
from across the Serengeti

274
00:24:57,290 --> 00:25:00,250
gather on the banks of Lake Masek.

275
00:25:33,850 --> 00:25:36,970
But survival on the plains of
the Serengeti

276
00:25:36,970 --> 00:25:41,330
forces Parakapuni and his family
to spend the dry season apart.

277
00:25:44,890 --> 00:25:48,130
Because while he tends the cattle,

278
00:25:48,130 --> 00:25:50,090
his wife Nomipuni must stay behind

279
00:25:50,090 --> 00:25:54,570
to look after those too young
or too old to make the trip...

280
00:25:55,970 --> 00:26:00,130
..and eke out an existence
on the brown, lifeless plains.

281
00:26:26,250 --> 00:26:27,970
With the cows away,

282
00:26:27,970 --> 00:26:31,130
the family must survive
on their meagre stores

283
00:26:31,130 --> 00:26:32,890
of maize flour and water.

284
00:26:35,490 --> 00:26:37,010
YOUNGER CHILD WAILS

285
00:27:27,290 --> 00:27:28,970
THUNDER RUMBLES

286
00:27:28,970 --> 00:27:32,050
Now, after four long months apart,

287
00:27:32,050 --> 00:27:34,490
things are about to change.

288
00:27:45,010 --> 00:27:46,850
NOMIPUNI:

289
00:28:04,810 --> 00:28:08,210
The coming of the rains
means the grass can grow again.

290
00:28:17,050 --> 00:28:20,410
Revived, new shoots emerge,

291
00:28:20,410 --> 00:28:23,970
packed with the green pigment
chlorophyll...

292
00:28:27,610 --> 00:28:31,530
..which allows plants
to harvest the sun's energy.

293
00:28:36,970 --> 00:28:42,570
And the plains are transformed from
dusty brown to vibrant green.

294
00:28:47,690 --> 00:28:52,210
In just a few days,
the landscape is reborn.

295
00:29:12,290 --> 00:29:15,090
I suppose we think of green
as the colour of life,

296
00:29:15,090 --> 00:29:18,610
but actually, it's the colour
that life throws away,

297
00:29:18,610 --> 00:29:22,690
waste green photons
reflected back into our eyes.

298
00:29:22,690 --> 00:29:25,290
Plants absorb
most of the rest of the rainbow,

299
00:29:25,290 --> 00:29:27,970
the blue and red photons,

300
00:29:27,970 --> 00:29:30,650
and use their energy
to power photosynthesis.

301
00:29:34,170 --> 00:29:39,490
Photosynthesis is the process
by which plants harness light.

302
00:29:39,490 --> 00:29:44,690
They are the bridge between nuclear
reactions 93 million miles away

303
00:29:44,690 --> 00:29:46,290
and life on Earth.

304
00:29:49,410 --> 00:29:53,730
Energy released from nuclear fusion
reactions in the sun's core

305
00:29:53,730 --> 00:29:57,050
heats everything up
and shakes electrons around,

306
00:29:57,050 --> 00:30:00,690
and those electrons
will emit photons,

307
00:30:00,690 --> 00:30:03,770
which travel across space
for eight minutes,

308
00:30:03,770 --> 00:30:08,850
and then hit an electron
in a chlorophyll molecule.

309
00:30:08,850 --> 00:30:12,850
But instead of that energy
being dissipated away as heat,

310
00:30:12,850 --> 00:30:14,970
chlorophyll is clever,

311
00:30:14,970 --> 00:30:18,850
and ultimately the energy
imparted to that electron

312
00:30:18,850 --> 00:30:21,090
is used to do
all sorts of clever things

313
00:30:21,090 --> 00:30:23,570
through an intricate piece
of machinery.

314
00:30:23,570 --> 00:30:27,130
Split water up, force electrons
onto carbon dioxide,

315
00:30:27,130 --> 00:30:31,290
and ultimately, build sugars,
which allows the plant to grow.

316
00:30:36,010 --> 00:30:41,050
On the plains of the Serengeti, the
intricate process of photosynthesis

317
00:30:41,050 --> 00:30:45,970
means Parakapuni can begin the
long journey to rejoin his family.

318
00:30:57,530 --> 00:31:01,250
After months apart,
the greening of the Serengeti

319
00:31:01,250 --> 00:31:03,970
transforms the harshest
of environments

320
00:31:03,970 --> 00:31:08,090
into a place
the Maasai can call home.

321
00:32:09,610 --> 00:32:12,330
Animals eat plants
that feed on sunlight.

322
00:32:18,850 --> 00:32:22,650
In this way, the sun's energy
powers the entire food chain.

323
00:32:28,570 --> 00:32:32,330
But photosynthesis does more than
provide life with energy.

324
00:32:36,330 --> 00:32:40,210
Around two and a half billion
years ago, it began to transform

325
00:32:40,210 --> 00:32:43,450
the composition of
the Earth's atmosphere itself,

326
00:32:43,450 --> 00:32:47,570
by filling it with a waste product -
oxygen.

327
00:32:47,570 --> 00:32:51,850
The gas upon which
all complex life depends.

328
00:33:01,570 --> 00:33:05,810
Without air to breathe, there would
be no intelligent life on Earth.

329
00:33:12,370 --> 00:33:15,930
We rely absolutely
on the oxygen that forms

330
00:33:15,930 --> 00:33:20,010
a fifth of the thin blue line
that envelops our planet.

331
00:33:27,050 --> 00:33:29,930
And, perhaps paradoxically,

332
00:33:29,930 --> 00:33:32,690
why the sky is blue
is best explained

333
00:33:32,690 --> 00:33:34,850
in the dead of night.

334
00:33:47,330 --> 00:33:50,850
It's a beautiful, crisp autumn night
here in the south of England.

335
00:33:50,850 --> 00:33:53,970
Rolling hills
illuminated by moonlight.

336
00:33:55,650 --> 00:33:58,210
Which is the light from the sun

337
00:33:58,210 --> 00:34:00,010
reflected off
the surface of the moon.

338
00:34:02,690 --> 00:34:05,610
It's quite silent, actually,
almost eerie.

339
00:34:05,610 --> 00:34:09,890
But out there, the Earth's shadow
is racing through space,

340
00:34:09,890 --> 00:34:13,010
and actually
it's just beginning to cut off

341
00:34:13,010 --> 00:34:15,730
the top left-hand corner
of the moon.

342
00:34:19,850 --> 00:34:22,130
On a few nights every decade,

343
00:34:22,130 --> 00:34:25,290
the sun, moon and Earth line up

344
00:34:25,290 --> 00:34:29,050
to create one of the wonders
of the night sky.

345
00:34:29,050 --> 00:34:30,850
A total lunar eclipse.

346
00:34:33,970 --> 00:34:37,970
You get a real sense of
the celestial mechanics in action

347
00:34:37,970 --> 00:34:39,970
during an eclipse.

348
00:34:39,970 --> 00:34:42,810
The sun is somewhere over there
behind me,

349
00:34:42,810 --> 00:34:44,930
shining on our planet,

350
00:34:44,930 --> 00:34:48,690
and our planet is casting a shadow
through space,

351
00:34:48,690 --> 00:34:52,890
which falls on the surface
of our satellite, the moon.

352
00:34:57,850 --> 00:35:01,130
As the Earth moves between
the moon and the sun,

353
00:35:01,130 --> 00:35:04,890
it blocks light from falling
directly onto the lunar surface.

354
00:35:12,490 --> 00:35:15,530
Now the moon is completely
covered by the Earth's shadow.

355
00:35:16,690 --> 00:35:18,650
But it's not completely blank.

356
00:35:18,650 --> 00:35:21,770
It's glowing a deep blood red.

357
00:35:25,970 --> 00:35:29,730
At the height of the eclipse,
the only photons reaching the moon

358
00:35:29,730 --> 00:35:33,170
have passed through
the atmosphere of our planet.

359
00:35:36,370 --> 00:35:39,450
And it's red
because of the way that light

360
00:35:39,450 --> 00:35:42,370
interacts with
the Earth's atmosphere.

361
00:35:42,370 --> 00:35:46,970
Imagine the rain photons
passing through the atmosphere

362
00:35:46,970 --> 00:35:50,090
from the sun,
all the colours of the rainbow.

363
00:35:50,090 --> 00:35:53,530
Well, the blue photons,
the higher-energy photons,

364
00:35:53,530 --> 00:35:57,250
have a higher probability of
bouncing off the molecules

365
00:35:57,250 --> 00:36:00,570
in the Earth's atmosphere
so they scatter around.

366
00:36:02,970 --> 00:36:05,850
That just leaves the lower-energy
red photons,

367
00:36:05,850 --> 00:36:09,410
which have a much lower probability
of bouncing around,

368
00:36:09,410 --> 00:36:12,930
and therefore can pass through
the Earth's atmosphere

369
00:36:12,930 --> 00:36:14,490
relatively unscathed.

370
00:36:14,490 --> 00:36:19,450
So they are the ones that make it
onto the moon's surface,

371
00:36:19,450 --> 00:36:22,890
and reflect back into my eye,
and that's why,

372
00:36:22,890 --> 00:36:26,170
in a lunar eclipse,
you get a blood-red moon.

373
00:36:37,570 --> 00:36:41,770
And by day, the blue photons
the atmosphere scatters

374
00:36:41,770 --> 00:36:44,050
are what paint the skies blue.

375
00:36:47,450 --> 00:36:50,170
So when you look up into the sky
on a summer's day

376
00:36:50,170 --> 00:36:52,050
and see that deep blue,

377
00:36:52,050 --> 00:36:55,810
what you're actually seeing
is blue photons

378
00:36:55,810 --> 00:36:59,090
scattering around off the molecules
in the atmosphere.

379
00:37:05,410 --> 00:37:09,370
Red, green, blue -

380
00:37:09,370 --> 00:37:12,450
the primary colours of our planet.

381
00:37:12,450 --> 00:37:15,770
The Earth is painted by the photons
that rain down

382
00:37:15,770 --> 00:37:17,530
from the surface of the sun.

383
00:37:19,570 --> 00:37:25,290
But for us to see them, those
photons must take one final journey.

384
00:37:50,370 --> 00:37:53,930
Salma lives with
her seven-year-old son Majidil

385
00:37:53,930 --> 00:37:56,850
on the banks of the Gorai River
in Bangladesh.

386
00:38:16,170 --> 00:38:18,290
This is a vibrant place to grow up.

387
00:38:20,130 --> 00:38:24,250
But unlike the other children,
Majidil can't experience it.

388
00:38:47,170 --> 00:38:51,330
For us to see,
photons must travel into our eyes,

389
00:38:51,330 --> 00:38:54,810
but Majidil was born
with severe cataracts.

390
00:38:56,530 --> 00:38:59,090
The lenses of his eyes
are so clouded,

391
00:38:59,090 --> 00:39:02,290
he's been effectively blind
since birth.

392
00:39:24,210 --> 00:39:27,650
But today,
Majidil's life is about to change.

393
00:39:35,810 --> 00:39:39,650
His family are making the eight-hour
journey to the capital, Dhaka.

394
00:39:47,330 --> 00:39:49,130
Because this afternoon,

395
00:39:49,130 --> 00:39:53,170
Majidil will undergo surgery
designed to restore his sight.

396
00:40:12,170 --> 00:40:14,090
If the surgery is successful,

397
00:40:14,090 --> 00:40:17,410
replacing his faulty lenses
with artificial ones

398
00:40:17,410 --> 00:40:21,210
will allow light to travel
directly into Majidil's eyes.

399
00:40:33,530 --> 00:40:37,130
It's at the back of the eye
that photons collide with cells

400
00:40:37,130 --> 00:40:41,010
in the retina, sensitive to red,
green, and blue light...

401
00:40:43,610 --> 00:40:47,530
..and trigger the nerve impulses
that allow us to see in colour.

402
00:41:02,010 --> 00:41:05,530
BICYCLE BELL RINGS

403
00:41:23,290 --> 00:41:26,610
THEY SPEAK ENCOURAGINGLY TO HIM

404
00:41:37,450 --> 00:41:40,050
For the first time in his life,

405
00:41:40,050 --> 00:41:43,090
photons are able to
enter Majidil's eyes unimpeded.

406
00:41:50,490 --> 00:41:54,490
For the first time in his life,
he can see.

407
00:42:43,970 --> 00:42:46,730
Our eyes reveal to us
the colours of the Earth.

408
00:42:46,730 --> 00:42:49,210
MAJIDIL LAUGHS

409
00:42:51,730 --> 00:42:55,490
The first step towards
understanding their meaning.

410
00:43:01,530 --> 00:43:05,090
The blue that signals the presence
of liquid water.

411
00:43:07,730 --> 00:43:12,650
The green that marks out our
planet as a home for complex life.

412
00:43:15,490 --> 00:43:19,290
And, in this most distant image of
the Earth ever taken,

413
00:43:19,290 --> 00:43:22,450
from six billion kilometres away,

414
00:43:22,450 --> 00:43:24,290
by the Voyager spacecraft,

415
00:43:24,290 --> 00:43:27,370
the sunlight
bouncing off our atmosphere

416
00:43:27,370 --> 00:43:31,050
reveals our planet
as a pale blue dot

417
00:43:31,050 --> 00:43:33,810
hanging in the void of space.

418
00:43:36,090 --> 00:43:40,570
And even from this distance,
anyone who happened to be looking

419
00:43:40,570 --> 00:43:45,450
could tell our planet contained
the ingredients for life.

420
00:43:46,650 --> 00:43:49,770
Because in the most delicate
colour signals,

421
00:43:49,770 --> 00:43:54,010
we have measured with precision
what the universe is made of.

422
00:43:59,610 --> 00:44:04,890
And those signals can be glimpsed at
the very edge of our own atmosphere.

423
00:44:20,090 --> 00:44:21,810
What are we drawing?

424
00:44:21,810 --> 00:44:23,290
A sun dragon.

425
00:44:23,290 --> 00:44:25,010
What's a sun dragon?

426
00:44:25,010 --> 00:44:26,730
It's just like a sun.

427
00:44:28,050 --> 00:44:31,650
And it's got smoke
coming out of its mouth.

428
00:44:33,490 --> 00:44:37,290
Cos it's starting to make fire
on the world.

429
00:44:37,290 --> 00:44:38,810
SHE PRETENDS TO BREATHE FIRE

430
00:44:40,170 --> 00:44:43,530
Now, let's just do them there.

431
00:44:43,530 --> 00:44:46,130
We need to get ready to go,
don't we?

432
00:44:47,810 --> 00:44:48,970
Push.

433
00:44:48,970 --> 00:44:53,050
For seven years, the Nation family
have been consumed by an obsession.

434
00:44:56,210 --> 00:44:58,970
Ready to drop everything
at a moment's notice,

435
00:44:58,970 --> 00:45:01,570
in pursuit of a light show
like no other.

436
00:45:05,410 --> 00:45:07,850
I'm Aurora Chaser!

437
00:45:12,730 --> 00:45:15,410
They've left their home
on the Sussex coast

438
00:45:15,410 --> 00:45:18,690
for the cold climes
of northern Norway...

439
00:45:18,690 --> 00:45:21,090
73, 98, 97.

440
00:45:22,290 --> 00:45:24,850
69, 98, 100.

441
00:45:24,850 --> 00:45:26,610
..spending their weekends

442
00:45:26,610 --> 00:45:30,490
chasing one of the planet's
most extraordinary phenomena -

443
00:45:30,490 --> 00:45:32,250
the Northern lights.

444
00:45:32,250 --> 00:45:34,130
Aurora borealis.

445
00:45:37,130 --> 00:45:40,530
It's an addiction. It's an absolute
full-blown addiction.

446
00:45:40,530 --> 00:45:43,330
It's a buzz
and it's a rush of adrenaline.

447
00:45:43,330 --> 00:45:47,410
And it is a holding
of your breath and...wow.

448
00:45:49,530 --> 00:45:51,690
The aurora was once thought to
be caused

449
00:45:51,690 --> 00:45:53,850
by dragons fighting in the sky.

450
00:45:56,530 --> 00:45:59,050
But today, we know
its ethereal light

451
00:45:59,050 --> 00:46:03,130
is created by charged particles
known as the solar wind

452
00:46:03,130 --> 00:46:06,450
interacting with the gases of
the upper atmosphere.

453
00:46:11,330 --> 00:46:13,290
Our eldest, Aurora, she's 11.

454
00:46:13,290 --> 00:46:14,450
She's fascinated by,

455
00:46:14,450 --> 00:46:16,130
why do those different gases

456
00:46:16,130 --> 00:46:17,970
make those different colours?

457
00:46:17,970 --> 00:46:20,810
She's quite fascinated by it. Oce...

458
00:46:20,810 --> 00:46:22,770
she likes the pink,
because she likes pink.

459
00:46:22,770 --> 00:46:25,010
It's her favourite colour,
so pink's cool.

460
00:46:25,010 --> 00:46:27,610
Anything else,
just wake me up if it goes pink.

461
00:46:27,610 --> 00:46:30,890
And Lyrica is
quite a fact box on everything.

462
00:46:30,890 --> 00:46:33,450
She can explain to you
what causes an aurora,

463
00:46:33,450 --> 00:46:35,610
where it comes from,
the technical names,

464
00:46:35,610 --> 00:46:38,330
and, you know,
what the magnetic field does

465
00:46:38,330 --> 00:46:41,290
and how it stops the solar
wind from harming us.

466
00:46:46,370 --> 00:46:49,010
But the aurora is elusive.

467
00:46:49,010 --> 00:46:52,330
To see it,
conditions have to be just right.

468
00:46:56,650 --> 00:46:58,970
Here we go!

469
00:46:58,970 --> 00:47:03,450
Clear dark skies must combine with
strong solar winds.

470
00:47:12,650 --> 00:47:14,170
You're going to burn it.

471
00:47:25,730 --> 00:47:28,410
All the family can do is wait.

472
00:47:53,250 --> 00:47:56,450
Oh, oh, oh, oh.
I can see some uprights there.

473
00:47:59,050 --> 00:48:01,370
Oh, wow.

474
00:48:10,370 --> 00:48:12,730
- Oh, yeah.
- Here we go. Look at it.

475
00:48:15,010 --> 00:48:16,570
Beautiful!

476
00:48:17,570 --> 00:48:19,370
Look at it!

477
00:48:21,010 --> 00:48:22,730
Look how bright that is up there,
look!

478
00:48:32,010 --> 00:48:33,850
Yay! Yes!

479
00:48:35,210 --> 00:48:37,290
Now that's what I'm talking about.

480
00:48:39,690 --> 00:48:41,890
Look. Ooh!

481
00:48:44,770 --> 00:48:47,250
Look, look, look, look, look!
Corona, corona!

482
00:48:48,530 --> 00:48:51,930
- Look at it.
- Look at the reds at the top!

483
00:48:51,930 --> 00:48:54,130
- Stunning. - Wow.

484
00:48:58,930 --> 00:49:00,050
Oh, oh, oh!

485
00:49:00,050 --> 00:49:02,410
Oh, look at that.

486
00:49:04,690 --> 00:49:07,570
- And on the left, on the left,
look, there's some pink. - Yay!

487
00:49:07,570 --> 00:49:09,290
Look, look, look, look, look!

488
00:49:12,210 --> 00:49:14,970
But the aurora
isn't simply beautiful.

489
00:49:14,970 --> 00:49:16,850
It contains information.

490
00:49:22,970 --> 00:49:26,490
As charged particles strike
gas atoms,

491
00:49:26,490 --> 00:49:28,210
a photon of light is released.

492
00:49:31,850 --> 00:49:36,210
And because the atoms of every
element have a unique structure,

493
00:49:36,210 --> 00:49:39,850
each one gives off
a photon of a unique colour.

494
00:49:43,810 --> 00:49:46,690
The green comes from oxygen atoms,

495
00:49:46,690 --> 00:49:48,530
pink from nitrogen.

496
00:49:57,890 --> 00:49:59,410
Yes!

497
00:50:02,610 --> 00:50:05,050
The colours dancing across the sky

498
00:50:05,050 --> 00:50:08,090
reveal the composition
of our atmosphere.

499
00:50:10,850 --> 00:50:15,090
And it's these fingerprints
encoded in photons of light

500
00:50:15,090 --> 00:50:18,250
that have shown us
the entire universe.

501
00:51:13,490 --> 00:51:15,450
In the last few hundred years,

502
00:51:15,450 --> 00:51:19,890
astronomers have gazed out from
the edges of our home world...

503
00:51:22,210 --> 00:51:24,690
..and dreamt of the universe beyond.

504
00:51:33,290 --> 00:51:37,610
In the shadow of the Eiger,
that is the Sphinx.

505
00:51:37,610 --> 00:51:38,890
And despite appearances,

506
00:51:38,890 --> 00:51:42,690
it isn't the lair of
some Alpine super-villain.

507
00:51:42,690 --> 00:51:44,050
It's an observatory.

508
00:51:44,050 --> 00:51:46,130
It was built in 1937,
and since then,

509
00:51:46,130 --> 00:51:49,650
astronomers have been coming here
from all over the world

510
00:51:49,650 --> 00:51:51,170
to look at the night sky.

511
00:52:00,850 --> 00:52:06,330
And by decoding messages hidden in
the light reaching our telescopes,

512
00:52:06,330 --> 00:52:09,570
we've discovered the true nature
of the cosmos.

513
00:52:15,330 --> 00:52:17,770
That's an aurora in a box.

514
00:52:17,770 --> 00:52:22,130
It's a tube full of hydrogen gas
that's heated up.

515
00:52:22,130 --> 00:52:25,250
And what you're seeing
in that pinky-red colour

516
00:52:25,250 --> 00:52:30,570
are the fundamental laws of nature
themselves in action.

517
00:52:30,570 --> 00:52:36,330
They tell you that a single proton,
which is the nucleus of hydrogen,

518
00:52:36,330 --> 00:52:40,450
and a single electron,
can only behave in certain ways.

519
00:52:40,450 --> 00:52:45,170
The electrons can only go in certain
places around the proton,

520
00:52:45,170 --> 00:52:47,010
so when you heat them up,

521
00:52:47,010 --> 00:52:49,090
they can only jump high up.

522
00:52:50,410 --> 00:52:52,170
And then when they cool down,

523
00:52:52,170 --> 00:52:54,810
they can only fall back
in a specific way,

524
00:52:54,810 --> 00:52:57,570
releasing light
of a particular energy

525
00:52:57,570 --> 00:53:00,050
which equals a particular colour.

526
00:53:00,050 --> 00:53:02,970
And those laws don't just apply
here on Earth.

527
00:53:02,970 --> 00:53:05,610
They apply everywhere
across the universe.

528
00:53:05,610 --> 00:53:10,890
So anywhere that you see glowing
hydrogen gas, let's say in a nebula,

529
00:53:10,890 --> 00:53:15,730
heated up by young stars, then
you'll see that specific colour.

530
00:53:22,690 --> 00:53:25,290
Vast clouds of glowing hydrogen

531
00:53:25,290 --> 00:53:28,650
create some of the most spectacular
sights in the cosmos...

532
00:53:32,770 --> 00:53:36,050
..their pink hue revealing
the identity of the gas

533
00:53:36,050 --> 00:53:37,730
from which they're made.

534
00:53:49,690 --> 00:53:52,250
And what's true for nebulae

535
00:53:52,250 --> 00:53:55,410
is true for everything
in the universe.

536
00:53:58,610 --> 00:54:00,770
By analysing the light
from the stars,

537
00:54:00,770 --> 00:54:04,290
we know that they're not just
twinkling lights in the sky.

538
00:54:04,290 --> 00:54:08,210
They're other suns
made out of the same stuff as ours.

539
00:54:08,210 --> 00:54:11,930
And if there are other suns,
then there must be other planets -

540
00:54:11,930 --> 00:54:14,130
maybe even other Earths.

541
00:54:29,730 --> 00:54:33,490
By understanding the origin
of the colours of our world -

542
00:54:34,890 --> 00:54:36,530
the blue of its oceans...

543
00:54:39,330 --> 00:54:41,210
..the green of its life...

544
00:54:44,370 --> 00:54:47,370
..and the colours
dancing in its atmosphere...

545
00:54:49,370 --> 00:54:52,050
..we've uncovered
the colour signature

546
00:54:52,050 --> 00:54:54,090
of a life-supporting planet.

547
00:54:58,890 --> 00:55:02,850
And because we understand
the signature of our own world,

548
00:55:02,850 --> 00:55:07,330
we can look for worlds like ours
elsewhere in the cosmos.

549
00:55:14,570 --> 00:55:17,690
Here's what I think is
the most wonderful thought of all.

550
00:55:17,690 --> 00:55:22,450
When you look out to those
distant star systems and planets,

551
00:55:22,450 --> 00:55:24,450
you're connected to them.

552
00:55:24,450 --> 00:55:27,850
An electron somewhere out there

553
00:55:27,850 --> 00:55:30,890
radiated a photon

554
00:55:30,890 --> 00:55:33,130
which hit an electron

555
00:55:33,130 --> 00:55:35,570
in your eye.

556
00:55:35,570 --> 00:55:38,930
Alien photons
crossing light years of space,

557
00:55:38,930 --> 00:55:43,170
entering your retina and carrying
stories of distant worlds.

558
00:55:49,530 --> 00:55:53,290
In the constellation of Pegasus,
around a sun-like star,

559
00:55:53,290 --> 00:55:55,690
there's a planet called Osiris.

560
00:55:55,690 --> 00:56:00,370
It's way closer to its parent star
than Mercury is to our sun,

561
00:56:00,370 --> 00:56:04,610
but it's a massive gas giant,
a hot Jupiter.

562
00:56:04,610 --> 00:56:08,210
And by measuring the light
that shines through its atmosphere,

563
00:56:08,210 --> 00:56:14,130
we found that the atmosphere is full
of hydrogen and methane and water.

564
00:56:16,450 --> 00:56:20,370
And just 24 light years away
in the constellation of Scorpius,

565
00:56:20,370 --> 00:56:25,970
there's a small, rocky world, one of
the most earthlike worlds we know.

566
00:56:25,970 --> 00:56:28,810
It orbits around
a red dwarf star,

567
00:56:28,810 --> 00:56:32,530
which is one of three
stars in the system.

568
00:56:32,530 --> 00:56:35,410
A planet with three suns in the sky.

569
00:56:40,130 --> 00:56:41,810
In the last 20 years,

570
00:56:41,810 --> 00:56:45,770
we've found hundreds of planets
around distant suns.

571
00:56:45,770 --> 00:56:48,130
We've began to
analyse their atmospheres,

572
00:56:48,130 --> 00:56:50,890
and search for life-giving water.

573
00:56:59,090 --> 00:57:01,490
Science is about gathering data,

574
00:57:01,490 --> 00:57:03,490
information about the natural world,

575
00:57:03,490 --> 00:57:05,210
and trying to understand it.

576
00:57:05,210 --> 00:57:08,170
And put like that,
it seems a bit dry.

577
00:57:08,170 --> 00:57:12,250
But when that information
carries stories of alien worlds

578
00:57:12,250 --> 00:57:15,610
around distant suns
hundreds of light years away,

579
00:57:15,610 --> 00:57:17,890
that's a whole different thing.

580
00:57:17,890 --> 00:57:21,810
Astronomy turns data into dreams -

581
00:57:21,810 --> 00:57:24,610
dreams of worlds of ice and snow,

582
00:57:24,610 --> 00:57:27,370
dreams of world with hemispheres

583
00:57:27,370 --> 00:57:30,530
in perpetual day
and permanent night.

584
00:57:30,530 --> 00:57:33,890
Dreams of world with moons
and moonbows,

585
00:57:33,890 --> 00:57:36,330
and perhaps, just perhaps,

586
00:57:36,330 --> 00:57:39,370
alien astronomers
observing the light

587
00:57:39,370 --> 00:57:43,170
passing through the atmosphere
of our blue world,

588
00:57:43,170 --> 00:57:44,930
and dreaming of us.