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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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...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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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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00:17:38,662 --> 00:17:42,021
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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00:18:04,702 --> 00:18:07,621
The sun's rays are much weaker here.

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00:18:07,622 --> 00:18:10,261
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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00:18:41,702 --> 00:18:43,422
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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00:19:04,782 --> 00:19:07,661
I've never seen anything like it.

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00:19:07,662 --> 00:19:13,821
The reason for that purity
is that the water's come 50km,

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00:19:13,822 --> 00:19:17,061
sometimes going deep underground,

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00:19:17,062 --> 00:19:20,021
and being filtered by these rocks.

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

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00:19:25,622 --> 00:19:30,421
Really, really spectacular -
look at that.

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You can see forever.

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00:19:39,502 --> 00:19:41,901
Descending deeper into the fissure,

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the effect on the colour of
my drysuit

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00:19:44,662 --> 00:19:46,781
hints at the process by which

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the oceans acquire
their distinctive blue.

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00:19:56,542 --> 00:19:58,662
Well, now we're down at about,
erm...

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...well, 12 metres.

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00:20:02,702 --> 00:20:07,781
And you see that down here,
my beautiful red diving suit

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00:20:07,782 --> 00:20:10,382
is no longer quite as red as it was.

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00:20:11,542 --> 00:20:13,182
In fact, it's looking quite black.

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00:20:14,382 --> 00:20:15,822
And the reason for that...

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...is the structure of
water molecules themselves.

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00:20:22,102 --> 00:20:27,542
See, a water molecule is an oxygen
with two hydrogens attached.

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00:20:28,822 --> 00:20:33,701
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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00:20:55,182 --> 00:20:57,221
The blue light doesn't do that.

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00:20:57,222 --> 00:21:00,661
So the blue light
can scatter around in the water

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00:21:00,662 --> 00:21:02,262
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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00:21:20,222 --> 00:21:23,701
Vibrations inside
the water molecules themselves

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give the oceans their colour.

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00:21:26,542 --> 00:21:30,902
The weaker bonds between water
molecules also absorb sunlight.

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Together, these processes absorb
so much energy

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00:21:35,822 --> 00:21:39,701
that there isn't a lot left
to increase the sea temperature.

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00:21:41,142 --> 00:21:44,021
Which is why humpbacks
must travel so far

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to find water warm enough
to give birth.

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00:22:06,022 --> 00:22:08,501
Because water is hard to heat up,

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00:22:08,502 --> 00:22:12,701
the temperature of the oceans
remains very stable,

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00:22:12,702 --> 00:22:16,822
creating the perfect conditions
for life to thrive.

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00:22:27,142 --> 00:22:30,701
Liquid water
is the essential ingredient

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00:22:30,702 --> 00:22:32,942
for any life-supporting world.

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00:22:36,822 --> 00:22:41,061
Oceans cover 70% of
the Earth's surface,

250
00:22:41,062 --> 00:22:45,221
their distinctive blue
broadcasting to the universe

251
00:22:45,222 --> 00:22:49,342
the message that Earth
is a possible home for life.

252
00:22:54,062 --> 00:22:57,061
And beyond the waves, on the land,

253
00:22:57,062 --> 00:23:00,141
we see life's other
signature colour -

254
00:23:00,142 --> 00:23:02,502
a blanket of green plants.

255
00:23:04,182 --> 00:23:08,701
As with all living things,
they depend on water to survive.

256
00:23:08,702 --> 00:23:11,501
And when the seasonal rains recede,

257
00:23:11,502 --> 00:23:15,582
the land reverts to the arid colours
of the naked earth.

258
00:23:27,862 --> 00:23:29,981
The intense African sun

259
00:23:29,982 --> 00:23:34,501
delivers 1,000 watts of solar power
per square metre,

260
00:23:34,502 --> 00:23:38,022
baking the plains of
the Serengeti to dust.

261
00:23:42,422 --> 00:23:44,381
Scorched brown -

262
00:23:44,382 --> 00:23:47,702
barely a single blade
of green grass survives.

263
00:23:51,902 --> 00:23:56,822
For the Maasai, the dry season
means months of hunger and hardship.

264
00:24:01,142 --> 00:24:04,021
Parakapuni and his son
and brother

265
00:24:04,022 --> 00:24:06,502
must say goodbye
to their home valley...

266
00:24:08,462 --> 00:24:11,901
...and drive their cows
across the plains

267
00:24:11,902 --> 00:24:13,941
in search of enough grass and water

268
00:24:13,942 --> 00:24:16,702
to keep them alive
until the rains return.

269
00:24:56,022 --> 00:24:58,181
At the end of their long journeys,

270
00:24:58,182 --> 00:25:00,981
Maasai warriors
from across the Serengeti

271
00:25:00,982 --> 00:25:03,942
gather on the banks of Lake Masek.

272
00:25:37,542 --> 00:25:40,661
But survival on the plains of
the Serengeti

273
00:25:40,662 --> 00:25:45,022
forces Parakapuni and his family
to spend the dry season apart.

274
00:25:48,582 --> 00:25:51,821
Because while he tends the cattle,

275
00:25:51,822 --> 00:25:53,781
his wife Nomipuni must stay behind

276
00:25:53,782 --> 00:25:58,262
to look after those too young
or too old to make the trip...

277
00:25:59,662 --> 00:26:03,822
...and eke out an existence
on the brown, lifeless plains.

278
00:26:29,942 --> 00:26:31,661
With the cows away,

279
00:26:31,662 --> 00:26:34,821
the family must survive
on their meagre stores

280
00:26:34,822 --> 00:26:36,582
of maize flour and water.

281
00:27:32,662 --> 00:27:35,741
Now, after four long months apart,

282
00:27:35,742 --> 00:27:38,182
things are about to change.

283
00:28:08,502 --> 00:28:11,902
The coming of the rains
means the grass can grow again.

284
00:28:20,742 --> 00:28:24,101
Revived, new shoots emerge,

285
00:28:24,102 --> 00:28:27,662
packed with the green pigment
chlorophyll...

286
00:28:31,302 --> 00:28:35,222
...which allows plants
to harvest the sun's energy.

287
00:28:40,662 --> 00:28:46,262
And the plains are transformed from
dusty brown to vibrant green.

288
00:28:51,382 --> 00:28:55,902
In just a few days,
the landscape is reborn.

289
00:29:15,982 --> 00:29:18,781
I suppose we think of green
as the colour of life,

290
00:29:18,782 --> 00:29:22,301
but actually, it's the colour
that life throws away,

291
00:29:22,302 --> 00:29:26,381
waste green photons
reflected back into our eyes.

292
00:29:26,382 --> 00:29:28,981
Plants absorb
most of the rest of the rainbow,

293
00:29:28,982 --> 00:29:31,661
the blue and red photons,

294
00:29:31,662 --> 00:29:34,342
and use their energy
to power photosynthesis.

295
00:29:37,862 --> 00:29:43,181
Photosynthesis is the process
by which plants harness light.

296
00:29:43,182 --> 00:29:48,381
They are the bridge between nuclear
reactions 93 million miles away

297
00:29:48,382 --> 00:29:49,982
and life on Earth.

298
00:29:53,102 --> 00:29:57,421
Energy released from nuclear fusion
reactions in the sun's core

299
00:29:57,422 --> 00:30:00,741
heats everything up
and shakes electrons around,

300
00:30:00,742 --> 00:30:04,381
and those electrons
will emit photons,

301
00:30:04,382 --> 00:30:07,461
which travel across space
for eight minutes,

302
00:30:07,462 --> 00:30:12,541
and then hit an electron
in a chlorophyll molecule.

303
00:30:12,542 --> 00:30:16,541
But instead of that energy
being dissipated away as heat,

304
00:30:16,542 --> 00:30:18,661
chlorophyll is clever,

305
00:30:18,662 --> 00:30:22,541
and ultimately the energy
imparted to that electron

306
00:30:22,542 --> 00:30:24,781
is used to do
all sorts of clever things

307
00:30:24,782 --> 00:30:27,261
through an intricate piece
of machinery.

308
00:30:27,262 --> 00:30:30,821
Split water up, force electrons
onto carbon dioxide,

309
00:30:30,822 --> 00:30:34,982
and ultimately, build sugars,
which allows the plant to grow.

310
00:30:39,702 --> 00:30:44,741
On the plains of the Serengeti, the
intricate process of photosynthesis

311
00:30:44,742 --> 00:30:49,662
means Parakapuni can begin the
long journey to rejoin his family.

312
00:31:01,222 --> 00:31:04,941
After months apart,
the greening of the Serengeti

313
00:31:04,942 --> 00:31:07,661
transforms the harshest
of environments

314
00:31:07,662 --> 00:31:11,782
into a place
the Maasai can call home.

315
00:32:13,302 --> 00:32:16,022
Animals eat plants
that feed on sunlight.

316
00:32:22,542 --> 00:32:26,342
In this way, the sun's energy
powers the entire food chain.

317
00:32:32,262 --> 00:32:36,022
But photosynthesis does more than
provide life with energy.

318
00:32:40,022 --> 00:32:43,901
Around two and a half billion
years ago, it began to transform

319
00:32:43,902 --> 00:32:47,141
the composition of
the Earth's atmosphere itself,

320
00:32:47,142 --> 00:32:51,261
by filling it with a waste product -
oxygen.

321
00:32:51,262 --> 00:32:55,542
The gas upon which
all complex life depends.

322
00:33:05,262 --> 00:33:09,502
Without air to breathe, there would
be no intelligent life on Earth.

323
00:33:16,062 --> 00:33:19,621
We rely absolutely
on the oxygen that forms

324
00:33:19,622 --> 00:33:23,702
a fifth of the thin blue line
that envelops our planet.

325
00:33:30,742 --> 00:33:33,621
And, perhaps paradoxically,

326
00:33:33,622 --> 00:33:36,381
why the sky is blue
is best explained

327
00:33:36,382 --> 00:33:38,542
in the dead of night.

328
00:33:51,022 --> 00:33:54,541
It's a beautiful, crisp autumn night
here in the south of England.

329
00:33:54,542 --> 00:33:57,662
Rolling hills
illuminated by moonlight.

330
00:33:59,342 --> 00:34:01,901
Which is the light from the sun

331
00:34:01,902 --> 00:34:03,702
reflected off
the surface of the moon.

332
00:34:06,382 --> 00:34:09,301
It's quite silent, actually,
almost eerie.

333
00:34:09,302 --> 00:34:13,581
But out there, the Earth's shadow
is racing through space,

334
00:34:13,582 --> 00:34:16,701
and actually
it's just beginning to cut off

335
00:34:16,702 --> 00:34:19,422
the top left-hand corner
of the moon.

336
00:34:23,542 --> 00:34:25,821
On a few nights every decade,

337
00:34:25,822 --> 00:34:28,981
the sun, moon and Earth line up

338
00:34:28,982 --> 00:34:32,741
to create one of the wonders
of the night sky.

339
00:34:32,742 --> 00:34:34,542
A total lunar eclipse.

340
00:34:37,662 --> 00:34:41,661
You get a real sense of
the celestial mechanics in action

341
00:34:41,662 --> 00:34:43,661
during an eclipse.

342
00:34:43,662 --> 00:34:46,501
The sun is somewhere over there
behind me,

343
00:34:46,502 --> 00:34:48,621
shining on our planet,

344
00:34:48,622 --> 00:34:52,381
and our planet is casting a shadow
through space,

345
00:34:52,382 --> 00:34:56,582
which falls on the surface
of our satellite, the moon.

346
00:35:01,542 --> 00:35:04,821
As the Earth moves between
the moon and the sun,

347
00:35:04,822 --> 00:35:08,582
it blocks light from falling
directly onto the lunar surface.

348
00:35:16,182 --> 00:35:19,222
Now the moon is completely
covered by the Earth's shadow.

349
00:35:20,382 --> 00:35:22,341
But it's not completely blank.

350
00:35:22,342 --> 00:35:25,462
It's glowing a deep blood red.

351
00:35:29,662 --> 00:35:33,421
At the height of the eclipse,
the only photons reaching the moon

352
00:35:33,422 --> 00:35:36,862
have passed through
the atmosphere of our planet.

353
00:35:40,062 --> 00:35:43,141
And it's red
because of the way that light

354
00:35:43,142 --> 00:35:46,061
interacts with
the Earth's atmosphere.

355
00:35:46,062 --> 00:35:50,661
Imagine the rain photons
passing through the atmosphere

356
00:35:50,662 --> 00:35:53,781
from the sun,
all the colours of the rainbow.

357
00:35:53,782 --> 00:35:57,221
Well, the blue photons,
the higher-energy photons,

358
00:35:57,222 --> 00:36:00,941
have a higher probability of
bouncing off the molecules

359
00:36:00,942 --> 00:36:04,262
in the Earth's atmosphere
so they scatter around.

360
00:36:06,662 --> 00:36:09,541
That just leaves the lower-energy
red photons,

361
00:36:09,542 --> 00:36:13,101
which have a much lower probability
of bouncing around,

362
00:36:13,102 --> 00:36:16,621
and therefore can pass through
the Earth's atmosphere

363
00:36:16,622 --> 00:36:18,181
relatively unscathed.

364
00:36:18,182 --> 00:36:23,141
So they are the ones that make it
onto the moon's surface,

365
00:36:23,142 --> 00:36:26,581
and reflect back into my eye,
and that's why,

366
00:36:26,582 --> 00:36:29,862
in a lunar eclipse,
you get a blood-red moon.

367
00:36:41,262 --> 00:36:45,461
And by day, the blue photons
the atmosphere scatters

368
00:36:45,462 --> 00:36:47,742
are what paint the skies blue.

369
00:36:51,142 --> 00:36:53,861
So when you look up into the sky
on a summer's day

370
00:36:53,862 --> 00:36:55,741
and see that deep blue,

371
00:36:55,742 --> 00:36:59,501
what you're actually seeing
is blue photons

372
00:36:59,502 --> 00:37:02,782
scattering around off the molecules
in the atmosphere.

373
00:37:09,102 --> 00:37:13,061
Red, green, blue -

374
00:37:13,062 --> 00:37:16,141
the primary colours of our planet.

375
00:37:16,142 --> 00:37:19,461
The Earth is painted by the photons
that rain down

376
00:37:19,462 --> 00:37:21,222
from the surface of the sun.

377
00:37:23,262 --> 00:37:28,982
But for us to see them, those
photons must take one final journey.

378
00:37:54,062 --> 00:37:57,621
Salma lives with
her seven-year-old son Majidil

379
00:37:57,622 --> 00:38:00,542
on the banks of the Gorai River
in Bangladesh.

380
00:38:19,862 --> 00:38:21,982
This is a vibrant place to grow up.

381
00:38:23,822 --> 00:38:27,942
But unlike the other children,
Majidil can't experience it.

382
00:38:50,862 --> 00:38:55,021
For us to see,
photons must travel into our eyes,

383
00:38:55,022 --> 00:38:58,502
but Majidil was born
with severe cataracts.

384
00:39:00,222 --> 00:39:02,781
The lenses of his eyes
are so clouded,

385
00:39:02,782 --> 00:39:05,982
he's been effectively blind
since birth.

386
00:39:27,902 --> 00:39:31,342
But today,
Majidil's life is about to change.

387
00:39:39,502 --> 00:39:43,342
His family are making the eight-hour
journey to the capital, Dhaka.

388
00:39:51,022 --> 00:39:52,821
Because this afternoon,

389
00:39:52,822 --> 00:39:56,862
Majidil will undergo surgery
designed to restore his sight.

390
00:40:15,862 --> 00:40:17,781
If the surgery is successful,

391
00:40:17,782 --> 00:40:21,101
replacing his faulty lenses
with artificial ones

392
00:40:21,102 --> 00:40:24,902
will allow light to travel
directly into Majidil's eyes.

393
00:40:37,222 --> 00:40:40,821
It's at the back of the eye
that photons collide with cells

394
00:40:40,822 --> 00:40:44,702
in the retina, sensitive to red,
green, and blue light...

395
00:40:47,302 --> 00:40:51,222
...and trigger the nerve impulses
that allow us to see in colour.

396
00:41:41,142 --> 00:41:43,741
For the first time in his life,

397
00:41:43,742 --> 00:41:46,782
photons are able to
enter Majidil's eyes unimpeded.

398
00:41:54,182 --> 00:41:58,182
For the first time in his life,
he can see.

399
00:42:47,662 --> 00:42:50,421
Our eyes reveal to us
the colours of the Earth.

400
00:42:55,422 --> 00:42:59,182
The first step towards
understanding their meaning.

401
00:43:05,222 --> 00:43:08,782
The blue that signals the presence
of liquid water.

402
00:43:11,422 --> 00:43:16,342
The green that marks out our
planet as a home for complex life.

403
00:43:19,182 --> 00:43:22,981
And, in this most distant image of
the Earth ever taken,

404
00:43:22,982 --> 00:43:26,141
from six billion kilometres away,

405
00:43:26,142 --> 00:43:27,981
by the Voyager spacecraft,

406
00:43:27,982 --> 00:43:31,061
the sunlight
bouncing off our atmosphere

407
00:43:31,062 --> 00:43:34,741
reveals our planet
as a pale blue dot

408
00:43:34,742 --> 00:43:37,502
hanging in the void of space.

409
00:43:39,782 --> 00:43:44,261
And even from this distance,
anyone who happened to be looking

410
00:43:44,262 --> 00:43:49,142
could tell our planet contained
the ingredients for life.

411
00:43:50,342 --> 00:43:53,461
Because in the most delicate
colour signals,

412
00:43:53,462 --> 00:43:57,702
we have measured with precision
what the universe is made of.

413
00:44:03,302 --> 00:44:08,582
And those signals can be glimpsed at
the very edge of our own atmosphere.

414
00:44:23,782 --> 00:44:25,501
What are we drawing?

415
00:44:25,502 --> 00:44:26,981
A sun dragon.

416
00:44:26,982 --> 00:44:28,701
What's a sun dragon?

417
00:44:28,702 --> 00:44:30,422
It's just like a sun.

418
00:44:31,742 --> 00:44:35,342
And it's got smoke
coming out of its mouth.

419
00:44:37,182 --> 00:44:40,981
Cos it's starting to make fire
on the world.

420
00:44:43,862 --> 00:44:47,221
Now, let's just do them there.

421
00:44:47,222 --> 00:44:49,822
We need to get ready to go,
don't we?

422
00:44:51,502 --> 00:44:52,661
Push.

423
00:44:52,662 --> 00:44:56,742
For seven years, the Nation family
have been consumed by an obsession.

424
00:44:59,902 --> 00:45:02,661
Ready to drop everything
at a moment's notice,

425
00:45:02,662 --> 00:45:05,262
in pursuit of a light show
like no other.

426
00:45:09,102 --> 00:45:11,542
I'm Aurora Chaser!

427
00:45:16,422 --> 00:45:19,101
They've left their home
on the Sussex coast

428
00:45:19,102 --> 00:45:22,381
for the cold climes
of northern Norway...

429
00:45:22,382 --> 00:45:24,782
73, 98, 97.

430
00:45:25,982 --> 00:45:28,541
69, 98, 100.

431
00:45:28,542 --> 00:45:30,301
...spending their weekends

432
00:45:30,302 --> 00:45:34,181
chasing one of the planet's
most extraordinary phenomena -

433
00:45:34,182 --> 00:45:35,941
the Northern lights.

434
00:45:35,942 --> 00:45:37,822
Aurora borealis.

435
00:45:40,822 --> 00:45:44,221
It's an addiction. It's an absolute
full-blown addiction.

436
00:45:44,222 --> 00:45:47,021
It's a buzz
and it's a rush of adrenaline.

437
00:45:47,022 --> 00:45:51,102
And it is a holding
of your breath and... wow.

438
00:45:53,222 --> 00:45:55,381
The aurora was once thought to
be caused

439
00:45:55,382 --> 00:45:57,542
by dragons fighting in the sky.

440
00:46:00,222 --> 00:46:02,741
But today, we know
its ethereal light

441
00:46:02,742 --> 00:46:06,821
is created by charged particles
known as the solar wind

442
00:46:06,822 --> 00:46:10,142
interacting with the gases of
the upper atmosphere.

443
00:46:15,022 --> 00:46:16,981
Our eldest, Aurora, she's 11.

444
00:46:16,982 --> 00:46:18,141
She's fascinated by,

445
00:46:18,142 --> 00:46:19,821
why do those different gases

446
00:46:19,822 --> 00:46:21,661
make those different colours?

447
00:46:21,662 --> 00:46:24,501
She's quite fascinated by it. Oce...

448
00:46:24,502 --> 00:46:26,461
she likes the pink,
because she likes pink.

449
00:46:26,462 --> 00:46:28,701
It's her favourite colour,
so pink's cool.

450
00:46:28,702 --> 00:46:31,301
Anything else,
just wake me up if it goes pink.

451
00:46:31,302 --> 00:46:34,581
And Lyrica is
quite a fact box on everything.

452
00:46:34,582 --> 00:46:37,141
She can explain to you
what causes an aurora,

453
00:46:37,142 --> 00:46:39,301
where it comes from,
the technical names,

454
00:46:39,302 --> 00:46:42,021
and, you know,
what the magnetic field does

455
00:46:42,022 --> 00:46:44,982
and how it stops the solar
wind from harming us.

456
00:46:50,062 --> 00:46:52,701
But the aurora is elusive.

457
00:46:52,702 --> 00:46:56,022
To see it,
conditions have to be just right.

458
00:47:00,342 --> 00:47:02,661
Here we go!

459
00:47:02,662 --> 00:47:07,142
Clear dark skies must combine with
strong solar winds.

460
00:47:16,342 --> 00:47:17,862
You're going to burn it.

461
00:47:29,422 --> 00:47:32,102
All the family can do is wait.

462
00:47:56,942 --> 00:48:00,142
Oh, oh, oh, oh.
I can see some uprights there.

463
00:48:02,742 --> 00:48:05,062
Oh, wow.

464
00:48:14,062 --> 00:48:16,422
- Oh, yeah.
- Here we go. Look at it.

465
00:48:18,702 --> 00:48:20,262
Beautiful!

466
00:48:21,262 --> 00:48:23,062
Look at it!

467
00:48:24,702 --> 00:48:26,422
Look how bright that is up there,
look!

468
00:48:35,702 --> 00:48:37,542
Yay! Yes!

469
00:48:38,902 --> 00:48:40,982
Now that's what I'm talking about.

470
00:48:43,382 --> 00:48:45,582
Look. Ooh!

471
00:48:48,462 --> 00:48:50,942
Look, look, look, look, look!
Corona, corona!

472
00:48:52,222 --> 00:48:55,621
- Look at it.
- Look at the reds at the top!

473
00:48:55,622 --> 00:48:57,822
- Stunning.
- Wow.

474
00:49:02,622 --> 00:49:03,741
Oh, oh, oh!

475
00:49:03,742 --> 00:49:06,102
Oh, look at that.

476
00:49:08,382 --> 00:49:11,261
- And on the left, on the left, look, there's some pink.
- Yay!

477
00:49:11,262 --> 00:49:12,982
Look, look, look, look, look!

478
00:49:15,902 --> 00:49:18,661
But the aurora
isn't simply beautiful.

479
00:49:18,662 --> 00:49:20,542
It contains information.

480
00:49:26,662 --> 00:49:30,181
As charged particles strike
gas atoms,

481
00:49:30,182 --> 00:49:31,902
a photon of light is released.

482
00:49:35,542 --> 00:49:39,901
And because the atoms of every
element have a unique structure,

483
00:49:39,902 --> 00:49:43,542
each one gives off
a photon of a unique colour.

484
00:49:47,502 --> 00:49:50,381
The green comes from oxygen atoms,

485
00:49:50,382 --> 00:49:52,222
pink from nitrogen.

486
00:50:01,582 --> 00:50:03,102
Yes!

487
00:50:06,302 --> 00:50:08,741
The colours dancing across the sky

488
00:50:08,742 --> 00:50:11,782
reveal the composition
of our atmosphere.

489
00:50:14,542 --> 00:50:18,781
And it's these fingerprints
encoded in photons of light

490
00:50:18,782 --> 00:50:21,942
that have shown us
the entire universe.

491
00:51:17,182 --> 00:51:19,141
In the last few hundred years,

492
00:51:19,142 --> 00:51:23,582
astronomers have gazed out from
the edges of our home world...

493
00:51:25,902 --> 00:51:28,382
...and dreamt of the universe beyond.

494
00:51:36,982 --> 00:51:41,301
In the shadow of the Eiger,
that is the Sphinx.

495
00:51:41,302 --> 00:51:42,581
And despite appearances,

496
00:51:42,582 --> 00:51:46,381
it isn't the lair of
some Alpine super-villain.

497
00:51:46,382 --> 00:51:47,741
It's an observatory.

498
00:51:47,742 --> 00:51:49,821
It was built in 1937,
and since then,

499
00:51:49,822 --> 00:51:53,341
astronomers have been coming here
from all over the world

500
00:51:53,342 --> 00:51:54,862
to look at the night sky.

501
00:52:04,542 --> 00:52:10,021
And by decoding messages hidden in
the light reaching our telescopes,

502
00:52:10,022 --> 00:52:13,262
we've discovered the true nature
of the cosmos.

503
00:52:19,022 --> 00:52:21,461
That's an aurora in a box.

504
00:52:21,462 --> 00:52:25,821
It's a tube full of hydrogen gas
that's heated up.

505
00:52:25,822 --> 00:52:28,941
And what you're seeing
in that pinky-red colour

506
00:52:28,942 --> 00:52:34,261
are the fundamental laws of nature
themselves in action.

507
00:52:34,262 --> 00:52:40,021
They tell you that a single proton,
which is the nucleus of hydrogen,

508
00:52:40,022 --> 00:52:44,141
and a single electron,
can only behave in certain ways.

509
00:52:44,142 --> 00:52:48,861
The electrons can only go in certain
places around the proton,

510
00:52:48,862 --> 00:52:50,701
so when you heat them up,

511
00:52:50,702 --> 00:52:52,782
they can only jump high up.

512
00:52:54,102 --> 00:52:55,861
And then when they cool down,

513
00:52:55,862 --> 00:52:58,501
they can only fall back
in a specific way,

514
00:52:58,502 --> 00:53:01,261
releasing light
of a particular energy

515
00:53:01,262 --> 00:53:03,741
which equals a particular colour.

516
00:53:03,742 --> 00:53:06,661
And those laws don't just apply
here on Earth.

517
00:53:06,662 --> 00:53:09,301
They apply everywhere
across the universe.

518
00:53:09,302 --> 00:53:14,581
So anywhere that you see glowing
hydrogen gas, let's say in a nebula,

519
00:53:14,582 --> 00:53:19,422
heated up by young stars, then
you'll see that specific colour.

520
00:53:26,382 --> 00:53:28,981
Vast clouds of glowing hydrogen

521
00:53:28,982 --> 00:53:32,342
create some of the most spectacular
sights in the cosmos...

522
00:53:36,462 --> 00:53:39,741
...their pink hue revealing
the identity of the gas

523
00:53:39,742 --> 00:53:41,422
from which they're made.

524
00:53:53,382 --> 00:53:55,941
And what's true for nebulae

525
00:53:55,942 --> 00:53:59,102
is true for everything
in the universe.

526
00:54:02,302 --> 00:54:04,461
By analysing the light
from the stars,

527
00:54:04,462 --> 00:54:07,981
we know that they're not just
twinkling lights in the sky.

528
00:54:07,982 --> 00:54:11,901
They're other suns
made out of the same stuff as ours.

529
00:54:11,902 --> 00:54:15,621
And if there are other suns,
then there must be other planets -

530
00:54:15,622 --> 00:54:17,822
maybe even other Earths.

531
00:54:33,422 --> 00:54:37,182
By understanding the origin
of the colours of our world -

532
00:54:38,582 --> 00:54:40,222
the blue of its oceans...

533
00:54:43,022 --> 00:54:44,902
...the green of its life...

534
00:54:48,062 --> 00:54:51,062
...and the colours
dancing in its atmosphere...

535
00:54:53,062 --> 00:54:55,741
...we've uncovered
the colour signature

536
00:54:55,742 --> 00:54:57,782
of a life-supporting planet.

537
00:55:02,582 --> 00:55:06,541
And because we understand
the signature of our own world,

538
00:55:06,542 --> 00:55:11,022
we can look for worlds like ours
elsewhere in the cosmos.

539
00:55:18,262 --> 00:55:21,381
Here's what I think is
the most wonderful thought of all.

540
00:55:21,382 --> 00:55:26,141
When you look out to those
distant star systems and planets,

541
00:55:26,142 --> 00:55:28,141
you're connected to them.

542
00:55:28,142 --> 00:55:31,541
An electron somewhere out there

543
00:55:31,542 --> 00:55:34,581
radiated a photon

544
00:55:34,582 --> 00:55:36,821
which hit an electron

545
00:55:36,822 --> 00:55:39,261
in your eye.

546
00:55:39,262 --> 00:55:42,621
Alien photons
crossing light years of space,

547
00:55:42,622 --> 00:55:46,862
entering your retina and carrying
stories of distant worlds.

548
00:55:53,222 --> 00:55:56,981
In the constellation of Pegasus,
around a sun-like star,

549
00:55:56,982 --> 00:55:59,381
there's a planet called Osiris.

550
00:55:59,382 --> 00:56:04,061
It's way closer to its parent star
than Mercury is to our sun,

551
00:56:04,062 --> 00:56:08,301
but it's a massive gas giant,
a hot Jupiter.

552
00:56:08,302 --> 00:56:11,901
And by measuring the light
that shines through its atmosphere,

553
00:56:11,902 --> 00:56:17,822
we found that the atmosphere is full
of hydrogen and methane and water.

554
00:56:20,142 --> 00:56:24,061
And just 24 light years away
in the constellation of Scorpius,

555
00:56:24,062 --> 00:56:29,661
there's a small, rocky world, one of
the most earthlike worlds we know.

556
00:56:29,662 --> 00:56:32,501
It orbits around
a red dwarf star,

557
00:56:32,502 --> 00:56:36,221
which is one of three
stars in the system.

558
00:56:36,222 --> 00:56:39,102
A planet with three suns in the sky.

559
00:56:43,822 --> 00:56:45,501
In the last 20 years,

560
00:56:45,502 --> 00:56:49,461
we've found hundreds of planets
around distant suns.

561
00:56:49,462 --> 00:56:51,821
We've began to
analyse their atmospheres,

562
00:56:51,822 --> 00:56:54,582
and search for life-giving water.

563
00:57:02,782 --> 00:57:05,181
Science is about gathering data,

564
00:57:05,182 --> 00:57:07,181
information about the natural world,

565
00:57:07,182 --> 00:57:08,901
and trying to understand it.

566
00:57:08,902 --> 00:57:11,861
And put like that,
it seems a bit dry.

567
00:57:11,862 --> 00:57:15,941
But when that information
carries stories of alien worlds

568
00:57:15,942 --> 00:57:19,301
around distant suns
hundreds of light years away,

569
00:57:19,302 --> 00:57:21,581
that's a whole different thing.

570
00:57:21,582 --> 00:57:25,501
Astronomy turns data into dreams -

571
00:57:25,502 --> 00:57:28,301
dreams of worlds of ice and snow,

572
00:57:28,302 --> 00:57:31,061
dreams of world with hemispheres

573
00:57:31,062 --> 00:57:34,221
in perpetual day
and permanent night.

574
00:57:34,222 --> 00:57:37,581
Dreams of world with moons
and moonbows,

575
00:57:37,582 --> 00:57:40,021
and perhaps, just perhaps,

576
00:57:40,022 --> 00:57:43,061
alien astronomers
observing the light

577
00:57:43,062 --> 00:57:46,861
passing through the atmosphere
of our blue world,

578
00:57:46,862 --> 00:57:48,622
and dreaming of us.