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Under the cover of darkness,

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the world lies hidden from view.

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Without light,
I've no idea what lies beyond
my immediate surroundings.

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I'm closed in,
enveloped on all sides

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by the unknown.

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For much of human history,

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when the sun went down
and the dark set in,

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we were at the mercy of the night.

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But over the centuries,

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we've developed our own sources
of illumination.

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We've lit our homes, our streets,
our cities,

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and doing so, we've banished
the darkness into the shadows.

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And just as we've used light
to illuminate our world,

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the more we've discovered
about light's properties,

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the more of the Universe
it's shown us.

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We've seen into the depths
of space...

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..and back to the beginning of time.

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But as we've looked deeper,

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we've come to realise
how little we've seen

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and that the cosmos's
greatest mysteries

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remain hidden in the dark.

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Light and dark
is essentially the story

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of everything we know

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and everything we don't know
about our Universe.

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And it all begins with light.

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It's such an integral part
of the way we perceive the world,

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it's easy to take it for granted.

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But for centuries,
understanding what light really is

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has been one of science's
most enduring questions.

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The first steps toward understanding
the properties of light

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were made in the third century BC

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by the renowned Greek mathematician
Euclid.

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He did it by thinking about
something so obvious,

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most of us don't give it
any thought at all.

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Placing the tiny chair
very close to the camera

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produces a large image
on the retina,

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and because we're not used to seeing
tiny chairs in everyday life,

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our brains are tricked into thinking

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it's a normal-sized chair
in the middle of the room.

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The reason this illusion works
at all

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is because, to judge distances,

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our brains rely on a simple fact -

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the further away things are,
the smaller they appear to the eye.

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And it was by focusing on
exactly why

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distant objects
could appear the same size

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as much smaller ones closer up...

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..that led Euclid to discover
of one of light's
most fundamental properties.

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Obviously the London Eye is much
bigger than my fingers, I know that,

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and yet to me
they look the same size.

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So, how do we explain this?

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Well, Euclid came up
with an elegant solution.

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For my finger to appear
at the top of the wheel,

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my eye, my finger
and the top of the wheel

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must all lie on the same line.

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But Euclid's insight didn't just
explain the tricks of perspective,

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it revealed a basic truth
about light itself.

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Euclid had discovered that
light travels in straight lines.

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Realising how it travels

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marks the beginning of our
scientific understanding of light.

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And it also meant
that if we could divert it
from its straight-line path,

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we could change the way
we see the world.

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But that leap wouldn't happen
for another 2,000 years.

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It was eventually made
in Renaissance Italy

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by one of the founding fathers
of modern science.

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In the summer of 1609,

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Galileo Galilei made the short
but fateful journey
from his home in Padua

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to Venice,
capital of the Venetian Republic.

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Galileo had flame-red hair,
a full beard,

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and was well-known for his love of
fine wines and generous hospitality,

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and also
for his anti-establishment views.

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By this time, he'd also built up
a reputation as a natural
philosopher and mathematician

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and he was regarded as a valuable
asset to the Venetian Republic.

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But although, as a professor,
he had a regular income,

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Galileo was never far
from financial troubles.

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When his father died in 1591,

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Galileo, the eldest of four
surviving siblings,

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became the head of the household

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and, effectively,
took on responsibility
for supporting his brother,

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a poor itinerant musician,

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and for paying his sisters' dowries.

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By the time he came to Venice,

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he still owed a significant amount
of money to his two brothers in law

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and so was always on the lookout
for a money-making scheme.

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That summer,

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Venice was abuzz
with rumours of a device

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that appeared to do
the impossible...

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..a Dutch spyglass

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that could bring distant objects
closer.

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It was just opportunity
Galileo was looking for.

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Back in the 17th Century,

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the spyglass
was cutting-edge technology

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and the details of how it worked
were a closely-guarded secret.

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All Galileo knew
was that it consisted of two lenses
arranged in a tube,

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and so when he developed his own,
he kept it very secret, as well.

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But we do know from a shopping list

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that he got his glass from the small
island of Murano, out in the lagoon,

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and because no tools existed,
he had to improvise,

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for instance,
buying an artillery ball

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to grind the curved surfaces
of the lenses.

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It had been known since
the first spectacles were produced,

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in the middle of the 13th century,

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that glass had the strange property
of bending light.

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But unlike spectacles,

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the spyglass, an early telescope,

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required a combination of lenses

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in a very specific arrangement.

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This is how
Galileo's telescope works.

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Rays of light come in
from a distant object

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so they're almost parallel
where they meet his first lens.

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This is the objective lens,
and it's plano-convex,

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which means it's flat on one side
and curved on the other.

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It's the sort of lens
used to treat long-sightedness.

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What it does is bend the
rays of light towards each other

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so that they would meet at a point.

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But before this focal point,

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Galileo places his second lens, the
ocular lens, which is plano-concave,

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and this bends the rays of light
back out again

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so they emerge parallel,
where they enter the eye,

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and then the eye's lens
focuses them on the retina.

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Now the magnification of a telescope
depends on the ratio

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of the focal lengths
of the two lenses -

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the distances F1 and F2.

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The difficulty for Galileo

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was grinding down the convex surface
of his objective lens

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to make it as shallow as possible

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in order to maximise the length F1,

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because the longer
he could make that,

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the greater the magnification
of his telescope.

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Produced in just a few weeks,

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Galileo's telescope
had a magnification of eight times

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and was far more powerful
than the original spyglass.

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All he needed to do now

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was cash in on his new invention.

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Ever the showman,
on the 21st August, 1609,

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Galileo climbed one of the city's
bell towers.

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BELLS CHIME

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LIFT MUZAK: "The Girl from Ipanema"

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Obviously,
he would've used the stairs!

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At the top,
in front of an assembled group
of Venetian noblemen and senators,

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Galileo demonstrated his telescope.

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It was a sensation.

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Using it, the Venetians would be
able to see approaching ships

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two hours earlier
than with naked eye.

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The military and economic advantage
of knowing who was sailing over the
horizon

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was lost on no-one
watching that day.

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Three days later,
as a grand gesture,

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Galileo presented his telescope
to the duke as a gift.

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In return,
he was guaranteed his job for life,

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at double his salary.

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With his finances now secure,

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Galileo went on to develop
a more powerful telescope,

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and with it,
use the ability to bend light

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to change our perspective
on the cosmos.

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This is the book Galileo published
in 1610.

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It's called "Sidereus Nuncius",

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which in Latin means
"The Starry Messenger".

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In it, he recorded his first
observations of the night sky

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the first anyone had ever made

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using anything other
than the naked eye.

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Today, it's hard to imagine

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how anything contained in this
little book was controversial,

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but you have to remember
that when it was written,

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the nature of heavens was thought
to be knowable only to God

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and the Earth was considered to be
at the centre of the Universe.

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These are his drawings of the moon.

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Since ancient times,
all heavenly bodies
were thought to be perfect spheres,

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but with his telescope, Galileo saw
texture in the surface of the moon,

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deep craters and mountains

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that, from the shadows they cast
across the lunar surface,

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he estimated to be some
six kilometres tall.

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As well as showing the heavens
to be imperfect...

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..his telescope began to uncover
their true extent,

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revealing ten-times more stars

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than are visible to the naked eye.

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And in the final chapters,

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Galileo reports the discovery
of four stars

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that appeared to form
a straight line

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near the planet Jupiter.

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His drawings show
how their positions change
from night to night.

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Although they moved, they always
did so along the same straight line,

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and from that, Galileo deduced that
they had to be orbiting Jupiter.

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00:14:26,020 --> 00:14:29,460
They weren't stars at all,
they were moons.

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Through his telescope,

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00:14:36,620 --> 00:14:38,340
Galileo had seen evidence

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that overturned the accepted dogma

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that the Earth was the fulcrum

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about which everything
in the Universe revolved.

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Seeing moons in orbit around Jupiter

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meant that not everything
went round the Earth.

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00:14:56,220 --> 00:14:58,940
So, far from being
the centre of the Universe,

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the Earth was just another planet.

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The telescope had allowed Galileo

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to glimpse the true nature
of the cosmos

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and our place within it.

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00:15:28,780 --> 00:15:31,500
But this way of manipulating light

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00:15:31,500 --> 00:15:35,540
had another powerful application,

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00:15:35,540 --> 00:15:40,740
one that would allow us to see
into another world.

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00:15:43,380 --> 00:15:46,220
BELLS CHIME

199
00:15:50,540 --> 00:15:52,780
In 17th-century London,

200
00:15:52,780 --> 00:15:55,500
one of the most prominent scientists
of the age

201
00:15:55,500 --> 00:15:59,420
was using lenses
in a very different way.

202
00:16:04,980 --> 00:16:09,740
Robert Hooke had taken
the basic principle of the telescope

203
00:16:09,740 --> 00:16:14,140
and used it to build a microscope.

204
00:16:23,100 --> 00:16:26,820
Galileo uses the telescope to
discover a new world in the heavens,

205
00:16:26,820 --> 00:16:30,340
and Hooke uses the microscope
to discover a new world

206
00:16:30,340 --> 00:16:33,100
in the very, very small.

207
00:16:33,100 --> 00:16:37,060
But there's a difference,
because what Galileo had presented

208
00:16:37,060 --> 00:16:40,860
was a world that was bigger
and more plentiful,

209
00:16:40,860 --> 00:16:43,660
but it was a world that people
were at least vaguely familiar with

210
00:16:43,660 --> 00:16:46,300
because you can look up in the sky
and see the stars,

211
00:16:46,300 --> 00:16:48,740
whereas the world
that Hooke presented

212
00:16:48,740 --> 00:16:51,380
was really something spectacular
and new.

213
00:16:51,380 --> 00:16:55,260
It was a world inside
the tiniest particles of matter

214
00:16:55,260 --> 00:16:58,900
that no-one had ever imagined
to be there before.

215
00:16:58,900 --> 00:17:00,900
People didn't even realise

216
00:17:00,900 --> 00:17:04,860
that there was a microscopic world
there to reveal.

217
00:17:08,660 --> 00:17:10,620
Hooke trained his microscope

218
00:17:10,620 --> 00:17:15,060
on a huge range of materials
and living things.

219
00:17:19,540 --> 00:17:24,780
But it was his drawings
of the exquisite detail
he saw in the bodies of insects

220
00:17:24,780 --> 00:17:27,820
that would become famous.

221
00:17:29,780 --> 00:17:34,820
Up here, you can see a human flea,
Pulex irritans, a very tiny creature,

222
00:17:34,820 --> 00:17:38,100
and here we've got the plate
from "Micrographia",

223
00:17:38,100 --> 00:17:42,420
which is a huge image of the flea
that Hooke produced,

224
00:17:42,420 --> 00:17:45,020
and it's really something
spectacular.

225
00:17:45,020 --> 00:17:47,900
This would've folded out in the book,
so it was really very large.

226
00:17:47,900 --> 00:17:50,020
Some people said
it was as big as a cat.

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00:17:50,020 --> 00:17:54,700
It's a work of art, really.
I mean, there's so much
intricate detail in there.

228
00:17:54,700 --> 00:17:57,500
It is, and there was nothing like it
before Hooke.

229
00:17:57,500 --> 00:17:59,580
They really were unprecedented

230
00:17:59,580 --> 00:18:05,140
and the shading and the quality
of the images is just superb.

231
00:18:05,140 --> 00:18:08,420
And it's accurate. I mean, it's...
It is, it's absolutely accurate.

232
00:18:08,420 --> 00:18:13,500
I was looking yesterday at images of,
er, photographs of the flea and, er,
there's really -

233
00:18:13,500 --> 00:18:15,580
made with an electron microscope -

234
00:18:15,580 --> 00:18:19,100
and there's really nothing to chose
between Hooke and, er,

235
00:18:19,100 --> 00:18:21,980
the current images.

236
00:18:23,340 --> 00:18:25,660
This is an image
of the compound eye of a fly,

237
00:18:25,660 --> 00:18:28,740
which Hooke shows in amazing detail
for the first time.

238
00:18:28,740 --> 00:18:31,580
This is an image of the foot
of a fly.

239
00:18:31,580 --> 00:18:35,380
Hooke shows you
the foot has little spikes in it

240
00:18:35,380 --> 00:18:39,660
that allow it to clasp
into the pores on a surface.

241
00:18:39,660 --> 00:18:44,100
This image looks less interesting,
less intricate than the others.

242
00:18:44,100 --> 00:18:46,180
It doesn't look terribly interesting

243
00:18:46,180 --> 00:18:49,180
but, actually,
it's really quite a profound picture,

244
00:18:49,180 --> 00:18:53,500
because what Hooke is looking at here
is a very thin slice of cork,

245
00:18:53,500 --> 00:18:55,940
which he cut with a penknife,

246
00:18:55,940 --> 00:19:00,340
and he's looking at
the little individual components
that make it up.

247
00:19:00,340 --> 00:19:03,820
And he calls them pores,
and then he calls them caverns,

248
00:19:03,820 --> 00:19:07,220
he calls them boxes
and then he calls them cells,

249
00:19:07,220 --> 00:19:09,780
and cell, of course,
is the term that stuck.

250
00:19:09,780 --> 00:19:13,980
These are the little
constituent parts, not just of cork,
but of all living things,

251
00:19:13,980 --> 00:19:17,380
and so it's a profoundly important
discovery

252
00:19:17,380 --> 00:19:21,900
and a name that has become standard
in biology.

253
00:19:49,700 --> 00:19:52,580
Using glass to bend light

254
00:19:52,580 --> 00:19:56,420
revealed our true place
in the Universe...

255
00:20:01,860 --> 00:20:06,860
..and the intricate architecture
of the microscopic world.

256
00:20:14,100 --> 00:20:16,100
The more we looked,

257
00:20:16,100 --> 00:20:19,140
the more we saw.

258
00:20:19,140 --> 00:20:22,900
With each new insight
into the nature of light

259
00:20:22,900 --> 00:20:26,500
came a fresh understanding
of the cosmos.

260
00:20:32,300 --> 00:20:34,140
And the next discovery

261
00:20:34,140 --> 00:20:37,060
would take us far further...

262
00:20:38,060 --> 00:20:43,580
..and enable us to read the story
of the stars.

263
00:20:55,900 --> 00:21:01,780
And it began with something Hooke
had glimpsed through his microscope.

264
00:21:03,220 --> 00:21:07,460
This is Robert Hooke's book
The Micrographia,

265
00:21:07,460 --> 00:21:10,060
published in 1664,

266
00:21:10,060 --> 00:21:13,100
350 years ago.

267
00:21:14,420 --> 00:21:16,660
It's full of...

268
00:21:16,660 --> 00:21:18,500
..his famous diagrams.

269
00:21:18,500 --> 00:21:21,260
Here's his picture of the flea.

270
00:21:21,260 --> 00:21:23,780
It's incredible seeing it
in its original form.

271
00:21:23,780 --> 00:21:26,980
It really is the size of a cat!

272
00:21:26,980 --> 00:21:32,180
These images really captured
the public imagination
and they made the book a sensation,

273
00:21:32,180 --> 00:21:35,820
but for me, The Micrographia
is about much more than that.

274
00:21:35,820 --> 00:21:38,220
The chapter that interests me
as a physicist

275
00:21:38,220 --> 00:21:42,460
is one that contains
hardly any images at all.

276
00:21:43,500 --> 00:21:45,580
And it's this one here -

277
00:21:45,580 --> 00:21:47,260
"Of the Colours observable

278
00:21:47,260 --> 00:21:51,020
"in Muscovy Glass,
and other thin Bodies".

279
00:21:51,020 --> 00:21:55,300
Here Hooke describes the iridescent
patterns of rainbow colours

280
00:21:55,300 --> 00:21:57,180
he sees through his microscope

281
00:21:57,180 --> 00:22:00,500
as light passes through
thin materials,

282
00:22:00,500 --> 00:22:03,940
like soap bubbles and Muscovy-glass,

283
00:22:03,940 --> 00:22:07,820
a silicate mineral that's made up
of lots of thin layers.

284
00:22:07,820 --> 00:22:12,020
At the time,
it was thought that white light,
like sunlight, was pure,

285
00:22:12,020 --> 00:22:14,380
that it came directly from God,

286
00:22:14,380 --> 00:22:17,380
and so Hooke concluded
that the colours he was seeing

287
00:22:17,380 --> 00:22:19,500
must have somehow been added to the
light,

288
00:22:19,500 --> 00:22:21,900
that they were effectively created

289
00:22:21,900 --> 00:22:25,540
as the light passed through
the materials.

290
00:22:27,660 --> 00:22:30,580
But Hooke's theory
about coloured light

291
00:22:30,580 --> 00:22:35,100
was about to be challenged
by his greatest rival.

292
00:22:39,660 --> 00:22:45,300
Isaac Newton is one of the world's
most revered scientists...

293
00:22:46,860 --> 00:22:52,020
..best known for his theory
of universal gravitation.

294
00:23:04,300 --> 00:23:06,980
And just like his laws of gravity,

295
00:23:06,980 --> 00:23:10,180
Newton's discoveries
about the nature of light

296
00:23:10,180 --> 00:23:13,620
are among his most
celebrated achievements.

297
00:23:14,700 --> 00:23:20,100
But the story of how that work began
is much less familiar,

298
00:23:20,100 --> 00:23:24,580
And this time,
there was no fruit involved.

299
00:23:24,580 --> 00:23:27,420
This is Stourbridge Common,

300
00:23:27,420 --> 00:23:31,820
a sleepy riverside meadow
on the banks of the River Cam.

301
00:23:31,820 --> 00:23:36,220
But when Newton's visited in 1664,
it would've been very different.

302
00:23:36,220 --> 00:23:40,860
For over 700 years, every September
this place would be transformed

303
00:23:40,860 --> 00:23:44,980
into what was, at its height,
the largest fair in Europe.

304
00:23:44,980 --> 00:23:47,740
For several weeks each year,
people would descend on the common

305
00:23:47,740 --> 00:23:52,620
for an annual festival of commerce
and debauchery.

306
00:23:52,620 --> 00:23:55,500
DOGS BARK and SHOUTING

307
00:23:55,500 --> 00:23:58,860
SWORDS CLASH and APPLAUSE

308
00:24:02,060 --> 00:24:06,740
This whole common would've been
packed with make-shift stalls -

309
00:24:06,740 --> 00:24:11,300
farming produce, brandy houses,
goldsmiths, silk merchants.

310
00:24:11,300 --> 00:24:15,260
There'd have been slack-rope
dancing, puppet shows, music,

311
00:24:15,260 --> 00:24:17,580
temptations of every kind,

312
00:24:17,580 --> 00:24:23,700
packed into row upon row
of wooden booths and tents.

313
00:24:30,700 --> 00:24:35,740
Stourbridge Fair
was a place you could buy anything
you could imagine,

314
00:24:35,740 --> 00:24:39,780
but when Newton came here,
it's said he bought just one thing -

315
00:24:39,780 --> 00:24:41,580
a prism.

316
00:24:45,700 --> 00:24:48,940
He bought it
because it performed the same magic

317
00:24:48,940 --> 00:24:51,620
Hooke had seen with his microscope.

318
00:25:02,660 --> 00:25:05,780
Newton would later write that,
using his new purchase,

319
00:25:05,780 --> 00:25:10,820
he would "try the celebrated
phenomena of colours"...

320
00:25:10,820 --> 00:25:13,500
..a rather understated introduction

321
00:25:13,500 --> 00:25:18,500
to work that would produce
one of the most profound insights
into the nature of light.

322
00:25:27,340 --> 00:25:30,380
Newton devised
an ingenious experiment

323
00:25:30,380 --> 00:25:35,060
to discover precisely how these
rainbow colours were produced

324
00:25:35,060 --> 00:25:37,340
and to put Hooke's theory -

325
00:25:37,340 --> 00:25:42,380
that they were created
by the prism itself - to the test.

326
00:25:45,460 --> 00:25:50,700
This is Newton's own drawing of what
he called his "Crucial Experiment".

327
00:25:50,700 --> 00:25:54,580
In it, he arranged a prism
so that sunlight -

328
00:25:54,580 --> 00:25:58,700
coming in from a small hole
he'd made in the shutters
of his bedroom window -

329
00:25:58,700 --> 00:26:03,300
passed through it and projected
coloured light onto a screen.

330
00:26:03,300 --> 00:26:06,740
Well, here's my light source

331
00:26:06,740 --> 00:26:07,860
and here's my prism

332
00:26:07,860 --> 00:26:12,460
which, if I arrange carefully,

333
00:26:12,460 --> 00:26:16,620
I can get projected
onto the back pillar.

334
00:26:16,620 --> 00:26:18,940
Of course, none of this was new.

335
00:26:18,940 --> 00:26:21,980
People knew that prisms
produced coloured light,

336
00:26:21,980 --> 00:26:25,580
but what Newton did next
had never been done before.

337
00:26:25,580 --> 00:26:31,820
He first isolated one of the colours
using a slit,

338
00:26:31,820 --> 00:26:34,900
so in this case,

339
00:26:34,900 --> 00:26:37,420
the orange light.

340
00:26:37,420 --> 00:26:41,260
He then passed that orange light
through a second prism.

341
00:26:41,260 --> 00:26:43,540
Now, if Hooke was right,

342
00:26:43,540 --> 00:26:47,380
then this prism should add
the other colours to the orange

343
00:26:47,380 --> 00:26:50,820
and reproduce the rainbow.

344
00:26:55,580 --> 00:26:58,780
But all Newton saw was orange
light.

345
00:26:58,780 --> 00:27:01,820
The prism wasn't adding
any extra colour.

346
00:27:01,820 --> 00:27:06,660
He concluded that the colours
must be contained in the white light
in the first place,

347
00:27:06,660 --> 00:27:08,900
that white light wasn't pure

348
00:27:08,900 --> 00:27:11,260
and prisms don't add anything to it.

349
00:27:11,260 --> 00:27:16,620
Instead, they split it up
into its constituent parts.

350
00:27:21,940 --> 00:27:25,660
Newton named the colours
that make up white light

351
00:27:25,660 --> 00:27:27,580
"the spectrum",

352
00:27:27,580 --> 00:27:31,540
and when this discovery
was combined with the telescope

353
00:27:31,540 --> 00:27:33,860
it would show us
something remarkable.

354
00:27:33,860 --> 00:27:35,980
The spectrum would reveal

355
00:27:35,980 --> 00:27:40,660
precisely what it was
we were looking at out in space.

356
00:27:41,700 --> 00:27:43,780
This is a spectroscope.

357
00:27:43,780 --> 00:27:48,180
As sunlight comes in, it's broken up
into its constituent colours

358
00:27:48,180 --> 00:27:51,980
and spread out much more finely
than you'd get with a simple prism.

359
00:27:51,980 --> 00:27:54,300
Now, with this camera,

360
00:27:54,300 --> 00:27:58,100
I should be able to show you
what I can see.

361
00:27:58,100 --> 00:28:01,140
I'll just check that it's working.

362
00:28:01,140 --> 00:28:04,140
Yes. OK.

363
00:28:05,580 --> 00:28:09,660
When scientists first did this
in the middle of the 19th century...

364
00:28:09,660 --> 00:28:13,300
I'm placing the spectroscope on top.

365
00:28:13,300 --> 00:28:17,340
..they saw something
completely unexpected.

366
00:28:18,500 --> 00:28:22,420
You can see
the colours of the spectrum
as Newton would've seen them,

367
00:28:22,420 --> 00:28:25,220
but if you look more closely,
you can see something else.

368
00:28:25,220 --> 00:28:27,100
It's not continuous,

369
00:28:27,100 --> 00:28:31,100
it's broken up
by lots of thin black lines.

370
00:28:31,100 --> 00:28:33,140
These are gaps in the spectrum.

371
00:28:33,140 --> 00:28:35,940
It was soon realised
that these gaps

372
00:28:35,940 --> 00:28:39,740
were due to atoms
in the outer atmosphere of the sun

373
00:28:39,740 --> 00:28:44,180
absorbing certain wavelengths
of light coming from its interior,

374
00:28:44,180 --> 00:28:45,860
and that they could be used

375
00:28:45,860 --> 00:28:49,860
to work out the chemical composition
of the sun.

376
00:28:59,980 --> 00:29:05,300
Every element absorbs
a unique pattern of wavelengths -

377
00:29:05,300 --> 00:29:07,580
an optical fingerprint

378
00:29:07,580 --> 00:29:10,260
that can be used to determine
the chemicals

379
00:29:10,260 --> 00:29:15,140
that make up any bright object
you can see in the sky.

380
00:29:17,300 --> 00:29:19,500
And in Rome,

381
00:29:19,500 --> 00:29:22,580
one man was using this technique
to study light

382
00:29:22,580 --> 00:29:26,740
whose origins lay
far beyond the sun.

383
00:29:26,740 --> 00:29:30,340
Father Angelo Secchi
was no ordinary priest.

384
00:29:30,340 --> 00:29:35,340
He was charismatic
and viewed as something of a heretic
by his fellow Jesuits.

385
00:29:35,340 --> 00:29:38,900
That's because
he was also a professor of physics,

386
00:29:38,900 --> 00:29:42,620
with a evangelical passion
for astronomy.

387
00:29:50,580 --> 00:29:52,420
In 1852,

388
00:29:52,420 --> 00:29:57,260
Secchi was appointed
Director of the Vatican Observatory.

389
00:29:58,500 --> 00:30:01,940
Within a year,
he'd built a new observatory

390
00:30:01,940 --> 00:30:06,940
on the roof of St Ignatius Church,
in the heart of the city.

391
00:30:09,700 --> 00:30:14,180
At the time, most astronomers
were interested in mapping
the positions of the stars

392
00:30:14,180 --> 00:30:17,340
and charting their motions
across the heavens.

393
00:30:17,340 --> 00:30:18,820
But Secchi was different.

394
00:30:18,820 --> 00:30:21,740
He wanted to know
what they actually were.

395
00:30:21,740 --> 00:30:25,940
So from his vantage point,
high above the streets
of the Eternal City,

396
00:30:25,940 --> 00:30:30,220
he began to meticulously analyse
their light.

397
00:30:41,580 --> 00:30:45,460
Fitting a spectroscope
to the observatory's telescope,

398
00:30:45,460 --> 00:30:48,660
Father Secchi laboriously recorded
the spectra

399
00:30:48,660 --> 00:30:52,340
of more than 4,000 stars.

400
00:30:58,820 --> 00:31:02,020
This is Secchi's book "Le Stelle",
The Stars,

401
00:31:02,020 --> 00:31:04,580
which he published in 1877.

402
00:31:04,580 --> 00:31:06,580
And flicking through it,

403
00:31:06,580 --> 00:31:11,180
you can see many of the observations
that he made.

404
00:31:11,180 --> 00:31:12,940
This one in particular
is interesting.

405
00:31:12,940 --> 00:31:15,300
It shows some of the spectra
he recorded.

406
00:31:15,300 --> 00:31:18,340
The top one here is from the sun,

407
00:31:18,340 --> 00:31:21,180
but the second one is starlight.

408
00:31:21,180 --> 00:31:24,180
It's from Sirius A, the Dog Star,

409
00:31:24,180 --> 00:31:28,100
which is the brightest star
in the night sky.

410
00:31:34,700 --> 00:31:37,180
It's 8.6 light years from Earth

411
00:31:37,180 --> 00:31:41,100
and over 20 times as luminous
as the sun.

412
00:31:43,260 --> 00:31:46,500
You can see from its spectrum
this clear sequence of bands,

413
00:31:46,500 --> 00:31:48,820
which is the signature of hydrogen,

414
00:31:48,820 --> 00:31:51,260
because it's a relatively
young star.

415
00:31:58,140 --> 00:32:01,820
The Universe's hottest,
brightest stars

416
00:32:01,820 --> 00:32:04,780
have spectra rich
in the two lightest elements -

417
00:32:04,780 --> 00:32:07,740
hydrogen and helium.

418
00:32:10,100 --> 00:32:12,460
But as they age, they cool,

419
00:32:12,460 --> 00:32:16,820
and their spectra reveal the
presence of many heavier elements.

420
00:32:19,860 --> 00:32:22,980
This third one
is from the star Betelgeuse,

421
00:32:22,980 --> 00:32:25,780
which is a red supergiant.

422
00:32:25,780 --> 00:32:27,580
It's near the end of its life

423
00:32:27,580 --> 00:32:30,460
and so you can see
from the many bands here

424
00:32:30,460 --> 00:32:34,260
that it's composed
of lots of different elements.

425
00:32:39,620 --> 00:32:41,900
What's remarkable about this image
is that,

426
00:32:41,900 --> 00:32:45,980
I mean, it really is one of the key
moments in the history of astronomy,

427
00:32:45,980 --> 00:32:50,100
that we can learn so much about
what distant stars are made of

428
00:32:50,100 --> 00:32:53,420
just by examining their light.

429
00:33:05,260 --> 00:33:11,700
But because Secchi
had catalogued the spectra
of so many stars of different ages,

430
00:33:11,700 --> 00:33:17,180
his observations led to something
even more profound -

431
00:33:17,180 --> 00:33:20,060
that by analysing starlight,

432
00:33:20,060 --> 00:33:24,100
we can determine
the stars' life cycles...

433
00:33:25,980 --> 00:33:29,060
..when they were born...

434
00:33:30,780 --> 00:33:33,660
..and when they'll die.

435
00:33:39,780 --> 00:33:42,580
Understanding the spectrum

436
00:33:42,580 --> 00:33:47,140
had allowed us to read the story
of the stars.

437
00:33:49,220 --> 00:33:51,780
It's quite incredible to think

438
00:33:51,780 --> 00:33:56,380
that what began as a simple
experiment in a darkened room

439
00:33:56,380 --> 00:34:00,860
could reveal so much
about the Universe,

440
00:34:00,860 --> 00:34:05,380
that the scant light from those
tiny points in the night sky

441
00:34:05,380 --> 00:34:10,580
could contain within it
the epic drama of the heavens.

442
00:34:19,540 --> 00:34:23,580
But that wasn't all the spectrum
could tell us.

443
00:34:24,620 --> 00:34:30,580
We know that it's made up of light
of many different wavelengths,

444
00:34:30,580 --> 00:34:36,900
and that those wavelengths extend
way beyond the range we can see.

445
00:34:36,900 --> 00:34:41,500
The spectrum,
from the longest wavelengths
used in radio communications,

446
00:34:41,500 --> 00:34:44,140
to the very shortest wavelength,
gamma rays,

447
00:34:44,140 --> 00:34:47,580
covers a range
of 30 orders of magnitude.

448
00:34:47,580 --> 00:34:50,700
The longest are
1-followed-by-30-zeros

449
00:34:50,700 --> 00:34:52,940
bigger than the shortest.

450
00:34:52,940 --> 00:34:56,100
That's the same as
a spread in range of weights

451
00:34:56,100 --> 00:34:59,140
from that of a single grain of sand

452
00:34:59,140 --> 00:35:03,020
to the weight of all the water
in all the oceans on the planet.

453
00:35:03,020 --> 00:35:05,540
And within that vast spread,

454
00:35:05,540 --> 00:35:08,220
visible light -
the frequencies we can see -

455
00:35:08,220 --> 00:35:11,060
covers a factor of just two.

456
00:35:11,060 --> 00:35:13,540
That's the same as the difference
in weight

457
00:35:13,540 --> 00:35:17,660
between this pebble
and one twice its size.

458
00:35:22,020 --> 00:35:23,940
Are we all set, Doctor?
Yes, I think so.

459
00:35:27,060 --> 00:35:30,100
And throughout the 20th century,

460
00:35:30,100 --> 00:35:32,540
opening our eyes
to the full spectrum

461
00:35:32,540 --> 00:35:36,380
revealed even more of the Universe.

462
00:35:36,380 --> 00:35:41,420
If you had infrared eyes,
here's how the sky would look.

463
00:35:41,420 --> 00:35:46,660
Infrared allowed us to see
the Universe's coolest stars,

464
00:35:46,660 --> 00:35:48,580
while radio telescopes,

465
00:35:48,580 --> 00:35:51,420
sensitive to the longest
wavelengths,

466
00:35:51,420 --> 00:35:55,060
revealed a cosmos in turmoil...

467
00:35:55,060 --> 00:35:57,620
It's the violent events
that are picked up,

468
00:35:57,620 --> 00:36:00,700
exploded stars and galaxies.

469
00:36:00,700 --> 00:36:03,540
..and satellites scoured the heavens

470
00:36:03,540 --> 00:36:06,580
for short-wavelength ultra violet.

471
00:36:06,580 --> 00:36:10,660
The OAO picks the ultra-violet light
from hot stars,

472
00:36:10,660 --> 00:36:14,020
which the atmosphere cuts off
from ground telescopes.

473
00:36:14,020 --> 00:36:17,980
And here's the very latest window -
gamma rays -

474
00:36:17,980 --> 00:36:22,020
which are like very energetic x-rays.

475
00:36:22,020 --> 00:36:24,100
Seeing beyond the visible

476
00:36:24,100 --> 00:36:28,020
has allowed us to peer deep
into the cosmos.

477
00:36:28,020 --> 00:36:29,740
I was cock-a-hoop about this.

478
00:36:29,740 --> 00:36:34,020
I, too, was wildly excited
when I heard of this discovery.

479
00:36:40,940 --> 00:36:44,820
But the very fact that light
had proved such a useful tool

480
00:36:44,820 --> 00:36:47,020
for exploring the Universe

481
00:36:47,020 --> 00:36:51,660
depended on one of its most
mysterious properties.

482
00:36:51,660 --> 00:36:53,860
Light behaves like a wave,

483
00:36:53,860 --> 00:36:57,380
but if it is a wave,
what is it a wave in?

484
00:36:57,380 --> 00:37:00,780
Waves are carried across the ocean
by the water.

485
00:37:00,780 --> 00:37:04,580
The sound you can hear now
is due to waves in the air.

486
00:37:04,580 --> 00:37:06,300
In the vacuum of space,
there is no air

487
00:37:06,300 --> 00:37:08,340
so there is no sound.

488
00:37:08,340 --> 00:37:12,380
But the reason you can see me
is because I'm lit by sunlight

489
00:37:12,380 --> 00:37:15,580
that has travelled 150 million
kilometres

490
00:37:15,580 --> 00:37:17,460
through empty space.

491
00:37:17,460 --> 00:37:19,140
So, what is light,

492
00:37:19,140 --> 00:37:22,540
and how can you have a wave
in nothing?

493
00:37:23,460 --> 00:37:27,740
Answering that question
would not only reveal what light is,

494
00:37:27,740 --> 00:37:30,220
it would ultimately
allow us to glimpse

495
00:37:30,220 --> 00:37:32,700
the beginning of the Universe.

496
00:37:45,540 --> 00:37:47,700
And the first part of the solution

497
00:37:47,700 --> 00:37:51,580
was a discovery that challenged
our most basic assumptions

498
00:37:51,580 --> 00:37:54,620
about how we see the world.

499
00:37:56,660 --> 00:38:00,140
To our eyes,
light appears to be everywhere,

500
00:38:00,140 --> 00:38:02,660
instantaneously.

501
00:38:02,660 --> 00:38:06,460
When I look out at the view,
there seems to be no time lag,

502
00:38:06,460 --> 00:38:09,780
no delay, while I wait for the light
to reach me.

503
00:38:09,780 --> 00:38:12,380
But towards the end
of the 17th century,

504
00:38:12,380 --> 00:38:15,740
it was discovered
that our senses are mistaken.

505
00:38:21,580 --> 00:38:27,180
In 1672, the Danish astronomer
Ole Romer arrived in Paris

506
00:38:27,180 --> 00:38:31,140
to begin work
at the city's observatory

507
00:38:31,140 --> 00:38:36,300
and to continue his observations
of the moons of Jupiter.

508
00:38:45,580 --> 00:38:47,260
For more than a decade

509
00:38:47,260 --> 00:38:50,500
Giovanni Cassini,
the observatory's director,

510
00:38:50,500 --> 00:38:54,660
had been documenting their orbits
in minute detail.

511
00:39:16,100 --> 00:39:18,580
Jupiter's innermost moon Io

512
00:39:18,580 --> 00:39:22,300
is known to make a complete circuit
around the gas giant

513
00:39:22,300 --> 00:39:25,740
once every 1.77 Earth days -

514
00:39:25,740 --> 00:39:28,580
that's every 42.5 hours.

515
00:39:28,580 --> 00:39:32,980
Now, from Earth, we can see
it disappear behind Jupiter

516
00:39:32,980 --> 00:39:35,700
and then re-emerge
round the other side

517
00:39:35,700 --> 00:39:38,260
as it travels around in its orbit.

518
00:39:38,260 --> 00:39:41,020
But here in Paris in the 1660s,

519
00:39:41,020 --> 00:39:43,340
Giovanni Cassini had noticed

520
00:39:43,340 --> 00:39:47,860
that the timing of these eclipses
seemed to vary,

521
00:39:47,860 --> 00:39:52,460
sometimes sooner,
sometimes later than expected.

522
00:39:57,940 --> 00:40:00,180
Soon after he arrived in Paris,

523
00:40:00,180 --> 00:40:05,300
Romer noticed
that these fluctuations
weren't happening at random.

524
00:40:05,300 --> 00:40:08,740
When the Earth was closer
to Jupiter,

525
00:40:08,740 --> 00:40:13,180
Io would be seen to disappear
and re-emerge earlier.

526
00:40:13,180 --> 00:40:15,100
But as the year went by

527
00:40:15,100 --> 00:40:17,620
and the Earth moved in its orbit
around the sun

528
00:40:17,620 --> 00:40:20,380
so that it was further away
from Jupiter,

529
00:40:20,380 --> 00:40:25,460
then the eclipses appeared to happen
later than expected.

530
00:40:28,820 --> 00:40:34,540
Romer knew the moon always took the
same time to travel around Jupiter.

531
00:40:34,540 --> 00:40:39,220
His great insight was to realise
that the variations
were due to the fact

532
00:40:39,220 --> 00:40:43,860
that light itself
takes time to travel through space.

533
00:40:47,580 --> 00:40:49,420
Here's how it works...

534
00:40:49,420 --> 00:40:52,740
The eclipses of Io
appear later than expected

535
00:40:52,740 --> 00:40:55,380
when the Earth is further
from Jupiter,

536
00:40:55,380 --> 00:41:00,060
because light takes a longer time
to cover the greater distance,

537
00:41:00,060 --> 00:41:04,540
but they appear earlier
when the Earth is closer

538
00:41:04,540 --> 00:41:09,380
because light needs less time
to reach the Earth.

539
00:41:10,420 --> 00:41:13,500
Light isn't instantaneous.

540
00:41:13,500 --> 00:41:17,100
It travels at a finite speed.

541
00:41:24,180 --> 00:41:29,060
Today, we've not only measured
light's speed
with incredible accuracy,

542
00:41:29,060 --> 00:41:32,100
we've seen it in motion.

543
00:41:32,100 --> 00:41:35,460
This is a video
made by scientists at MIT,

544
00:41:35,460 --> 00:41:40,380
using a camera designed to monitor
extremely fast, chemical reactions.

545
00:41:40,380 --> 00:41:44,180
It has a shutter speed
of around a picosecond.

546
00:41:44,180 --> 00:41:47,220
That's a millionth of a millionth
of a second -

547
00:41:47,220 --> 00:41:52,500
the time it takes light to travel
just a third of a millimetre.

548
00:41:52,500 --> 00:41:55,660
Now, look what happens
when I press play.

549
00:42:07,740 --> 00:42:10,580
What you can see here
is a pulse of laser light

550
00:42:10,580 --> 00:42:13,260
moving through
a water-filled bottle.

551
00:42:13,260 --> 00:42:18,140
To us, this would appear
as the briefest of flashes,

552
00:42:18,140 --> 00:42:22,180
but the camera reveals how the pulse
travels through the bottle,

553
00:42:22,180 --> 00:42:26,940
scattering and bouncing around
as it hits the water molecules.

554
00:42:34,180 --> 00:42:36,300
Light travels so fast -

555
00:42:36,300 --> 00:42:39,020
300,000 kilometres per second -

556
00:42:39,020 --> 00:42:41,460
that slowed down by the same amount,

557
00:42:41,460 --> 00:42:44,500
a bullet would take an entire year

558
00:42:44,500 --> 00:42:47,660
to travel the length of the bottle.

559
00:42:49,820 --> 00:42:53,500
It's one thing to know that light
travels at a finite speed,

560
00:42:53,500 --> 00:42:57,500
quite another
to actually see it move.

561
00:43:00,580 --> 00:43:05,700
The discovery of the speed of light
was hugely significant.

562
00:43:06,900 --> 00:43:09,580
Not least because it proved crucial

563
00:43:09,580 --> 00:43:13,580
to uncovering what light
actually is.

564
00:43:20,500 --> 00:43:23,060
Born in the summer of 1831,

565
00:43:23,060 --> 00:43:27,100
James Clerk Maxwell would become
one of the leading lights

566
00:43:27,100 --> 00:43:29,420
of 19th-century physics.

567
00:43:29,420 --> 00:43:32,180
GASPS and APPLAUSE

568
00:43:32,180 --> 00:43:35,300
His work on electricity
and magnetism

569
00:43:35,300 --> 00:43:38,940
was one of the greatest achievements
of the age.

570
00:43:42,020 --> 00:43:45,060
This is Glenlair
in south-west Scotland,

571
00:43:45,060 --> 00:43:47,980
Maxwell's family home.

572
00:43:47,980 --> 00:43:52,900
While he was growing up here,
he developed an insatiable curiosity
about the world around him,

573
00:43:52,900 --> 00:43:55,380
a desire to understand nature

574
00:43:55,380 --> 00:43:58,260
that he would never lose.

575
00:44:09,460 --> 00:44:13,140
The young Maxwell
seems to have taken great delight

576
00:44:13,140 --> 00:44:15,620
in tormenting his parents
and his nanny

577
00:44:15,620 --> 00:44:19,780
by constantly asking them
how things worked.

578
00:44:19,780 --> 00:44:21,740
"What's the go o'that?" he'd say.

579
00:44:21,740 --> 00:44:23,580
If anyone ventured an answer,

580
00:44:23,580 --> 00:44:26,500
the young Maxwell would only
be satisfied for a moment

581
00:44:26,500 --> 00:44:28,900
before asking them how they knew.

582
00:44:35,020 --> 00:44:38,460
Of course, none of this
is particularly unusual for a child,

583
00:44:38,460 --> 00:44:40,540
but what sets Maxwell apart

584
00:44:40,540 --> 00:44:43,220
is that he was just 14 years old

585
00:44:43,220 --> 00:44:45,980
when he wrote his first
scientific paper.

586
00:44:45,980 --> 00:44:48,140
So young,
that a friend of the family

587
00:44:48,140 --> 00:44:52,620
had to present it to the Royal
Society of Edinburgh on his behalf.

588
00:45:02,180 --> 00:45:04,820
Maxwell was one of the
greatest scientists who ever lived

589
00:45:04,820 --> 00:45:09,740
and it was here that he carried out
his most important work.

590
00:45:10,940 --> 00:45:13,260
During the 1860s,

591
00:45:13,260 --> 00:45:16,780
Maxwell produced a virtuoso piece
of mathematics

592
00:45:16,780 --> 00:45:19,580
that showed electricity
and magnetism

593
00:45:19,580 --> 00:45:23,500
were different aspects
of the same thing.

594
00:45:24,580 --> 00:45:28,260
But his calculations
would show something else.

595
00:45:28,260 --> 00:45:33,700
Quite by accident, they would reveal
the true nature of light.

596
00:45:33,700 --> 00:45:36,420
These are Maxwell's
four famous equations

597
00:45:36,420 --> 00:45:41,860
that describe the relationship
between electric
and magnetic fields.

598
00:45:41,860 --> 00:45:44,020
Curl of E

599
00:45:44,020 --> 00:45:47,820
is minus DB by DT.

600
00:45:47,820 --> 00:45:51,020
E is the electric field,
B is the magnetic field.

601
00:45:51,020 --> 00:45:54,420
Curl of B over mu nought,

602
00:45:54,420 --> 00:45:57,340
div of E equals zero,

603
00:45:57,340 --> 00:46:00,740
equals epsilon nought equals nought.

604
00:46:00,740 --> 00:46:02,780
With a bit of algebra
and manipulation,

605
00:46:02,780 --> 00:46:07,060
these four equations can be combined
to give one single equation.

606
00:46:07,060 --> 00:46:09,740
So the way it's done is like this...

607
00:46:09,740 --> 00:46:12,060
We take the curl of curl of E...

608
00:46:12,060 --> 00:46:15,020
Hidden deep within his mathematics

609
00:46:15,020 --> 00:46:19,340
was something that even Maxwell
didn't expect.

610
00:46:19,340 --> 00:46:21,900
..epsilon nought... Grad E 2 div...

611
00:46:21,900 --> 00:46:24,700
This second term is zero

612
00:46:24,700 --> 00:46:28,700
and I'm left with Del squared
of E...

613
00:46:28,700 --> 00:46:32,700
..minus mu nought, epsilon nought

614
00:46:32,700 --> 00:46:35,060
D 2 E...

615
00:46:35,060 --> 00:46:37,740
..by DT squared.

616
00:46:39,620 --> 00:46:42,380
This is the wave equation.

617
00:46:42,380 --> 00:46:45,100
It tells us how
an electromagnetic field

618
00:46:45,100 --> 00:46:47,220
travels through space.

619
00:46:47,220 --> 00:46:50,020
Now, the important bit
is this here -

620
00:46:50,020 --> 00:46:52,140
mu nought, epsilon nought -

621
00:46:52,140 --> 00:46:55,580
because it's related to the speed
that the wave is travelling.

622
00:46:55,580 --> 00:46:59,060
In fact, the speed is given...

623
00:46:59,060 --> 00:47:04,540
..by one over the square root
of mu nought epsilon nought.

624
00:47:04,540 --> 00:47:07,060
And if you work that out,
you arrive at...

625
00:47:07,060 --> 00:47:11,780
..3 times 10 to the power 8
metres per second,

626
00:47:11,780 --> 00:47:15,220
or 300,000 kilometres per second -

627
00:47:15,220 --> 00:47:18,140
the speed of light.

628
00:47:19,180 --> 00:47:22,900
If electromagnetic waves moved
at the speed of light,

629
00:47:22,900 --> 00:47:26,220
it could only mean one thing.

630
00:47:26,220 --> 00:47:30,540
Maxwell knew this had to be more
than just a coincidence.

631
00:47:30,540 --> 00:47:37,020
It meant that light itself
had to be an electromagnetic wave.

632
00:47:47,980 --> 00:47:52,420
The discovery that light
is an electromagnetic wave

633
00:47:52,420 --> 00:47:56,780
explains one of its
most puzzling properties.

634
00:47:56,780 --> 00:47:59,180
What Maxwell's equations show

635
00:47:59,180 --> 00:48:04,500
is that light consists
of electric and magnetic waves
travelling through space.

636
00:48:04,500 --> 00:48:08,540
So light is simply
electric and magnetic vibrations

637
00:48:08,540 --> 00:48:12,620
feeding off one another
as they move.

638
00:48:15,100 --> 00:48:20,900
And we now know
that these electromagnetic waves
have a remarkable property -

639
00:48:20,900 --> 00:48:23,540
they don't need to be waves
in anything,

640
00:48:23,540 --> 00:48:26,780
they can travel through empty space.

641
00:48:43,060 --> 00:48:47,580
I remember first learning about this
when I was in my second year
at university.

642
00:48:47,580 --> 00:48:53,180
I was in lecture hall 33AC21
of the physics department
at the University of Surrey,

643
00:48:53,180 --> 00:48:55,380
the lecturer was Dr Chivers,

644
00:48:55,380 --> 00:48:57,580
and I remember turning to my friend
next to me

645
00:48:57,580 --> 00:49:01,580
and remarking on how incredible
I thought this was.

646
00:49:01,580 --> 00:49:05,580
I could tell by his reaction that
he thought I was a bit of a geek.

647
00:49:05,580 --> 00:49:10,860
But, actually, it is incredible
that in just a few lines of algebra,

648
00:49:10,860 --> 00:49:14,940
you can tell what light really is.

649
00:49:17,380 --> 00:49:21,180
And the fact that light travels
at a finite speed

650
00:49:21,180 --> 00:49:24,620
has enabled us to do something else.

651
00:49:24,620 --> 00:49:29,340
It allows us to look into the past.

652
00:49:30,580 --> 00:49:33,420
Looking at a mirror one metre away,

653
00:49:33,420 --> 00:49:37,300
you see yourself as you were
six nanoseconds ago.

654
00:49:37,300 --> 00:49:41,580
From Earth, the moon appears
as it was one second ago

655
00:49:41,580 --> 00:49:44,620
and the sun
eight minutes in the past.

656
00:49:44,620 --> 00:49:50,260
The further you look out in space,
the further you look back in time.

657
00:49:51,340 --> 00:49:54,780
Light from the cosmos's
most distant objects

658
00:49:54,780 --> 00:49:58,980
has taken billions of years
to reach the Earth.

659
00:49:58,980 --> 00:50:03,140
But there's one source that
has taken us so far back in time,

660
00:50:03,140 --> 00:50:07,940
we've reached the very limit
of what can be seen with light.

661
00:50:13,020 --> 00:50:18,020
In 1964, while converting
a strange-looking horn antenna

662
00:50:18,020 --> 00:50:21,140
designed for early
satellite communications

663
00:50:21,140 --> 00:50:24,460
to make astronomical observations...

664
00:50:25,820 --> 00:50:28,740
..Arno Penzias and Robert Wilson

665
00:50:28,740 --> 00:50:33,580
began to pick up a mysterious signal
they couldn't explain.

666
00:50:43,260 --> 00:50:46,540
Here, we had purposely picked
a portion of the spectrum,

667
00:50:46,540 --> 00:50:48,260
a wavelength of seven centimetres,

668
00:50:48,260 --> 00:50:51,460
where we expected nothing
or almost nothing,

669
00:50:51,460 --> 00:50:54,100
no radiation at all from the sky.

670
00:50:58,100 --> 00:51:00,980
Instead, what happened is that
we found radiation

671
00:51:00,980 --> 00:51:04,180
coming into our antenna
from all directions.

672
00:51:04,180 --> 00:51:07,780
It's just flooding in at us and, um,

673
00:51:07,780 --> 00:51:12,700
clearly was orders of magnitude more
than we expected from the galaxy.

674
00:51:17,620 --> 00:51:20,700
At first,
they dismissed it as noise,

675
00:51:20,700 --> 00:51:25,380
something unwanted,
generated by the antenna itself.

676
00:51:25,380 --> 00:51:27,540
Now, we had some suspicion

677
00:51:27,540 --> 00:51:32,020
because the throat of the antenna
came into the cab
and was a little bit warmer,

678
00:51:32,020 --> 00:51:34,900
and that was an attractive place
for pigeons,

679
00:51:34,900 --> 00:51:40,020
at least a pair of pigeons
who liked to stay there,
especially in the cold winter.

680
00:51:40,020 --> 00:51:43,260
We didn't mind that
because they flew away when we came,

681
00:51:43,260 --> 00:51:46,940
except that they had coated the
surface with a white sticky material

682
00:51:46,940 --> 00:51:51,140
which might not only absorb
radio waves but emit radio waves,

683
00:51:51,140 --> 00:51:55,060
which could be part
or maybe all of our result.

684
00:52:00,780 --> 00:52:03,500
With the antenna cleaned,
and the pigeons -

685
00:52:03,500 --> 00:52:05,700
well, it didn't end well
for the pigeons -

686
00:52:05,700 --> 00:52:10,300
Penzias and Wilson began searching
for an astronomical explanation.

687
00:52:10,300 --> 00:52:14,380
But the signal wasn't coming
from anything in our own galaxy.

688
00:52:14,380 --> 00:52:17,540
Nor did it appear to be coming
from any other galaxy either.

689
00:52:17,540 --> 00:52:21,780
It seemed to be coming
from everywhere.

690
00:52:23,700 --> 00:52:27,260
No matter when we looked,
day or night, winter or summer,

691
00:52:27,260 --> 00:52:31,260
this background of radiation
appeared everywhere in the sky.

692
00:52:33,100 --> 00:52:37,740
It was not tied to our galaxy
or any other known source
of radio waves.

693
00:52:37,740 --> 00:52:40,780
It was rather as if
the whole Universe had been warmed up

694
00:52:40,780 --> 00:52:44,060
to a temperature about three degrees
above absolute zero.

695
00:52:48,620 --> 00:52:52,940
And so we were left with the
astonishing result

696
00:52:52,940 --> 00:52:55,620
that this radiation
was coming from somewhere

697
00:52:55,620 --> 00:52:58,420
in really deep cosmic space...

698
00:53:00,620 --> 00:53:03,180
..beyond any radio sources

699
00:53:03,180 --> 00:53:07,700
that any of us knew about
or even dreamed existed.

700
00:53:20,260 --> 00:53:23,780
What they'd discovered
was light so ancient,

701
00:53:23,780 --> 00:53:26,380
it had been stretched out
into microwaves

702
00:53:26,380 --> 00:53:31,580
and cooled to just a few
scant degrees above absolute zero,

703
00:53:31,580 --> 00:53:33,340
light that had been travelling
to Earth

704
00:53:33,340 --> 00:53:36,540
for almost the entire age
of the Universe.

705
00:53:36,540 --> 00:53:38,980
It hadn't come from a distant galaxy

706
00:53:38,980 --> 00:53:42,220
and it was far older than any star.

707
00:53:42,220 --> 00:53:47,260
Penzias and Wilson had discovered
that the entire Universe
was awash with light

708
00:53:47,260 --> 00:53:50,820
from the embers of the Big Bang
itself.

709
00:54:04,180 --> 00:54:07,580
Called the Cosmic Microwave
Background,

710
00:54:07,580 --> 00:54:13,780
it was released when the Universe
was just 370,000 years old

711
00:54:13,780 --> 00:54:19,140
and it gives us a snapshot
of the cosmos in its infancy.

712
00:54:23,060 --> 00:54:24,940
And here it is,

713
00:54:24,940 --> 00:54:28,340
the latest image
of the Cosmic Microwave Background,

714
00:54:28,340 --> 00:54:32,340
taken by the Planck satellite
and published in early 2013.

715
00:54:32,340 --> 00:54:37,300
The different colours
are fluctuations in temperature
in the early Universe

716
00:54:37,300 --> 00:54:39,820
and the information they contain

717
00:54:39,820 --> 00:54:43,460
has proved priceless
to cosmologists.

718
00:54:44,500 --> 00:54:49,900
The tiny variations in temperature
are caused by matter
clumping together

719
00:54:49,900 --> 00:54:55,260
into what will eventually become
stars and galaxies.

720
00:54:56,380 --> 00:54:59,260
But what's truly remarkable
about this image

721
00:54:59,260 --> 00:55:02,380
is that it's not just light
from the early Universe,

722
00:55:02,380 --> 00:55:06,860
it's the very first light
there ever was.

723
00:55:29,980 --> 00:55:32,980
During the first era of its life,

724
00:55:32,980 --> 00:55:38,180
the Universe was a fireball
of hot dense plasma

725
00:55:38,180 --> 00:55:42,220
that trapped light,
preventing it from moving.

726
00:55:44,260 --> 00:55:48,380
Then, as the cosmos cooled,
the plasma condensed,

727
00:55:48,380 --> 00:55:51,620
forming the first atoms...

728
00:55:56,940 --> 00:55:59,140
..and the first light,

729
00:55:59,140 --> 00:56:02,740
light that would become
the Cosmic Microwave Background,

730
00:56:02,740 --> 00:56:06,660
was released into the Universe.

731
00:56:20,500 --> 00:56:25,580
It's sort of hard to express what
an astonishing achievement this is,

732
00:56:25,580 --> 00:56:29,940
that from our small planet,
orbiting an unremarkable star,

733
00:56:29,940 --> 00:56:33,260
we've reached out into the Universe

734
00:56:33,260 --> 00:56:38,860
and seen as far
as it's possible to see with light.

735
00:56:54,060 --> 00:56:57,220
The discovery
of the Cosmic Microwave Background

736
00:56:57,220 --> 00:57:02,100
appeared to complete our picture
of the Universe,

737
00:57:02,100 --> 00:57:05,020
the final chapter
in our use of light

738
00:57:05,020 --> 00:57:08,100
to explore the cosmos.

739
00:57:09,260 --> 00:57:14,700
Understanding the nature of light
has allowed us to illuminate
our world.

740
00:57:14,700 --> 00:57:18,860
We've captured it from the depths
of space and the beginning of time.

741
00:57:18,860 --> 00:57:21,460
At the smallest scales,
light has uncovered

742
00:57:21,460 --> 00:57:24,420
the microscopic structure
of living things,

743
00:57:24,420 --> 00:57:28,140
and at the largest, it's shown us
our place in the cosmos

744
00:57:28,140 --> 00:57:30,980
and told us the story of the stars.

745
00:57:30,980 --> 00:57:33,820
Virtually everything we know
about the Universe,

746
00:57:33,820 --> 00:57:37,460
we know because it's been revealed
by light.

747
00:57:44,500 --> 00:57:46,940
But just as it seemed
light would lead us

748
00:57:46,940 --> 00:57:51,100
to a complete understanding
of everything...

749
00:57:52,500 --> 00:57:54,620
..in the last 30 years,

750
00:57:54,620 --> 00:57:57,700
it's shown us something disturbing.

751
00:57:59,300 --> 00:58:04,940
The vast majority of the cosmos
can't be seen at all.

752
00:58:09,860 --> 00:58:12,500
Far from being a Universe of light,

753
00:58:12,500 --> 00:58:15,980
much of it is hidden in the dark.

754
00:58:20,340 --> 00:58:21,860
Next time,

755
00:58:21,860 --> 00:58:24,700
how scientists came
to the realisation

756
00:58:24,700 --> 00:58:28,060
that more than 99 percent
of the Universe

757
00:58:28,060 --> 00:58:32,100
lies concealed in the shadows,

758
00:58:32,100 --> 00:58:34,580
and the extraordinary quest

759
00:58:34,580 --> 00:58:39,020
to uncover what's out there
in the dark.

760
00:58:41,660 --> 00:58:44,100
Whether you want to
step into the light

761
00:58:44,100 --> 00:58:46,740
or explore the mysteries
of the dark,

762
00:58:46,740 --> 00:58:50,340
let the Open University inspire you.
Go to...

763
00:58:53,460 --> 00:58:56,260
and follow links
to The Open University.

764
00:59:42,000 --> 00:59:48,120
As the sun dips below the horizon,
its light begins to fade.

765
00:59:48,120 --> 00:59:52,680
Night falls and our world
descends into darkness.

766
01:00:08,400 --> 01:00:11,800
Today, in our street-lit towns
and cities,

767
01:00:11,800 --> 01:00:15,300
we rarely experience true darkness.

768
01:00:15,300 --> 01:00:17,720
But without our eyes to guide us,

769
01:00:17,720 --> 01:00:21,280
the world becomes
a much more mysterious place.

770
01:00:37,720 --> 01:00:39,640
I can't see anything now,

771
01:00:39,640 --> 01:00:43,040
but strangely I can still sense
the presence of the trees

772
01:00:43,040 --> 01:00:44,680
enveloping me in the gloom.

773
01:00:46,400 --> 01:00:49,760
I can't see them, but I know
there's something out there.

774
01:00:57,080 --> 01:01:01,360
And in the same way,
as we've explored the cosmos,

775
01:01:01,360 --> 01:01:03,600
we've come to realise
we can only see

776
01:01:03,600 --> 01:01:06,040
the merest hint of what's out there.

777
01:01:08,680 --> 01:01:13,480
Our best estimate is that more
than 99% of the universe

778
01:01:13,480 --> 01:01:18,120
lies hidden in the dark,
invisible to our telescopes

779
01:01:18,120 --> 01:01:20,280
and beyond our comprehension.

780
01:01:22,720 --> 01:01:26,320
This film is the story
of how we went from thinking

781
01:01:26,320 --> 01:01:29,840
we were close to a complete
understanding of the universe,

782
01:01:29,840 --> 01:01:34,280
to realising we'd seen
almost none of it,

783
01:01:34,280 --> 01:01:38,360
and the extraordinary quest to
uncover what's really out there

784
01:01:38,360 --> 01:01:39,720
in the dark.

785
01:01:43,400 --> 01:01:47,280
It's perhaps the most important
undertaking in science,

786
01:01:47,280 --> 01:01:51,360
because our universe
was forged in darkness.

787
01:01:51,360 --> 01:01:54,960
And darkness will one day
tear it apart.

788
01:02:14,880 --> 01:02:19,320
For centuries, scientists have used
light to build up a seemingly

789
01:02:19,320 --> 01:02:22,000
comprehensive picture
of the universe.

790
01:02:23,640 --> 01:02:26,400
We'd discovered that the Earth
was just one planet

791
01:02:26,400 --> 01:02:28,000
in orbit around the sun.

792
01:02:29,320 --> 01:02:31,920
And that the sun was itself a star,

793
01:02:31,920 --> 01:02:35,000
made of the same stuff
as the billions upon billions

794
01:02:35,000 --> 01:02:39,520
of stars that light up
a vast - perhaps endless - cosmos.

795
01:02:51,560 --> 01:02:55,000
But there was one niggling problem
that had remained unsolved

796
01:02:55,000 --> 01:02:58,160
for over 400 years,
and it was this -

797
01:02:58,160 --> 01:03:01,840
with so many stars out there,
why was there any darkness at all?

798
01:03:04,280 --> 01:03:07,880
The story of the dark begins
with this simple question.

799
01:03:09,480 --> 01:03:13,000
And at its heart
lies a deep paradox.

800
01:03:15,120 --> 01:03:18,760
In the forest, no matter what
direction I point my torch,

801
01:03:18,760 --> 01:03:21,760
the beam will always hit
the trunk of a tree.

802
01:03:23,240 --> 01:03:26,400
And just as everywhere I look I see
a tree,

803
01:03:26,400 --> 01:03:28,920
if the universe
is sufficiently large,

804
01:03:28,920 --> 01:03:33,800
then every line of sight from
Earth should end in a star.

805
01:03:33,800 --> 01:03:36,360
The night sky shouldn't be
black at all,

806
01:03:36,360 --> 01:03:38,520
it should be ablaze with starlight.

807
01:03:41,840 --> 01:03:44,680
First posed in the 1570s,

808
01:03:44,680 --> 01:03:48,800
this question would become
known as Olbers' Paradox.

809
01:03:50,360 --> 01:03:55,280
One possible solution was that
the Earth was surrounded

810
01:03:55,280 --> 01:04:00,280
by dark stuff that obscured our view
of the stars behind.

811
01:04:00,280 --> 01:04:04,600
But it was soon realised that
these dark clouds would absorb

812
01:04:04,600 --> 01:04:08,760
the light from the stars,
heat up and eventually glow

813
01:04:08,760 --> 01:04:11,440
with the same brightness
as the stars they obscured.

814
01:04:18,040 --> 01:04:22,480
The paradox was only satisfactorily
explained in the 20th century.

815
01:04:25,480 --> 01:04:28,600
The answer -
the reason it gets dark at night

816
01:04:28,600 --> 01:04:32,160
is because the universe
had a beginning.

817
01:04:32,160 --> 01:04:36,760
It began with the big bang
13.8 billion years ago, and so

818
01:04:36,760 --> 01:04:41,760
we only see those stars whose light
has had time to reach us since then.

819
01:04:41,760 --> 01:04:45,240
The sky is dark because light
from the most distant stars

820
01:04:45,240 --> 01:04:47,080
hasn't got here yet.

821
01:04:55,440 --> 01:04:59,800
No mysterious stuff was needed
to block out the light.

822
01:04:59,800 --> 01:05:03,720
The dark spaces that starlight
had yet to reach were empty,

823
01:05:03,720 --> 01:05:07,120
and cosmologists could sleep easy
at night.

824
01:05:09,000 --> 01:05:12,040
But before long, we began to see
hints that there might be

825
01:05:12,040 --> 01:05:14,680
more out there than meets the eye,

826
01:05:14,680 --> 01:05:17,960
that the shadowy recesses
of empty space

827
01:05:17,960 --> 01:05:20,480
might not be so empty after all.

828
01:05:22,760 --> 01:05:26,240
The first clues had in fact begun
to emerge from the gloom

829
01:05:26,240 --> 01:05:29,600
some 200 years ago,

830
01:05:29,600 --> 01:05:33,320
not in the depths of the universe,
but in our own back yard.

831
01:05:44,520 --> 01:05:48,800
The invention of the telescope
in the 17th century had allowed us

832
01:05:48,800 --> 01:05:52,960
to see the dimmest light from the
deepest reaches of the solar system.

833
01:05:54,400 --> 01:05:59,880
And in 1781, it had revealed
a seventh planet, Uranus,

834
01:05:59,880 --> 01:06:02,680
the first to be found
since ancient times.

835
01:06:04,440 --> 01:06:07,680
But there was something odd
about this new planet.

836
01:06:07,680 --> 01:06:12,160
Astronomers found that as time
passed, Uranus's actual position

837
01:06:12,160 --> 01:06:15,560
was drifting further and further
away from the position

838
01:06:15,560 --> 01:06:18,680
the laws of gravity predicted
it should be at.

839
01:06:18,680 --> 01:06:22,560
One explanation was that the laws
themselves were wrong,

840
01:06:22,560 --> 01:06:24,680
but working
at the Paris Observatory,

841
01:06:24,680 --> 01:06:27,480
one man came up with
a different solution.

842
01:06:27,480 --> 01:06:29,600
There was something else out there,

843
01:06:29,600 --> 01:06:33,520
something we couldn't see that was
interfering with Uranus's orbit.

844
01:06:44,760 --> 01:06:49,000
In 1846, the mathematician
Urbain Le Verrier

845
01:06:49,000 --> 01:06:53,840
was employed at the observatory
to calculate the orbits of comets

846
01:06:53,840 --> 01:06:56,040
as they wandered through
the solar system...

847
01:06:58,840 --> 01:07:01,920
..and predict when
they would light up the night sky.

848
01:07:06,000 --> 01:07:09,440
Le Verrier has been described
as having an almost pathological

849
01:07:09,440 --> 01:07:13,760
need to impose order on everything
and everyone around him,

850
01:07:13,760 --> 01:07:17,160
and to have made no allowances
for human error or frailty.

851
01:07:19,000 --> 01:07:21,560
When asked what he was like,
a colleague remarked,

852
01:07:21,560 --> 01:07:24,120
"I do not know whether
Monsieur Le Verrier is actually

853
01:07:24,120 --> 01:07:26,520
"the most detestable man in France,

854
01:07:26,520 --> 01:07:29,880
"but I am quite certain
that he is the most detested."

855
01:07:31,800 --> 01:07:34,520
But he was undoubtedly
a mathematical genius,

856
01:07:34,520 --> 01:07:37,240
and he was as harsh on himself
as he was on others.

857
01:07:39,520 --> 01:07:43,360
And because he was a mathematician,
he set about finding the object

858
01:07:43,360 --> 01:07:47,880
he thought was influencing Uranus
not by scouring the skies

859
01:07:47,880 --> 01:07:52,600
with a telescope, but by determining
its position through calculation.

860
01:08:01,760 --> 01:08:06,600
These are Le Verrier's original
hand-written notes from 1846.

861
01:08:06,600 --> 01:08:07,840
This one is called

862
01:08:07,840 --> 01:08:11,360
"Searches of the disturbing body.
Second approximation."

863
01:08:15,480 --> 01:08:17,920
It contains page after page

864
01:08:17,920 --> 01:08:20,560
of complicated mathematical
calculations.

865
01:08:31,320 --> 01:08:33,920
What Le Verrier was attempting
was quite different

866
01:08:33,920 --> 01:08:36,040
to what was normally done
in astronomy,

867
01:08:36,040 --> 01:08:38,320
where you know where an object is -

868
01:08:38,320 --> 01:08:43,320
say a star or planet or comet -
you then use the laws of gravity

869
01:08:43,320 --> 01:08:47,520
to explain its effects on
nearby objects.

870
01:08:47,520 --> 01:08:50,760
Here, he didn't know
where his disturbing body was.

871
01:08:50,760 --> 01:08:55,480
All he had to go by was the effect
it had on the orbit of Uranus.

872
01:08:55,480 --> 01:08:59,640
So he made some starting assumptions
about its position, and then

873
01:08:59,640 --> 01:09:04,520
carried out a calculation to predict
the effect it would have on Uranus.

874
01:09:04,520 --> 01:09:07,600
He then compared that with
what had been observed.

875
01:09:07,600 --> 01:09:11,120
When the two didn't match,
he went back and adjusted

876
01:09:11,120 --> 01:09:14,360
his starting assumptions
and repeated the calculation.

877
01:09:16,200 --> 01:09:18,480
He did this again and again

878
01:09:18,480 --> 01:09:21,400
until his prediction matched
the observation.

879
01:09:26,560 --> 01:09:30,000
On the 31st of August, 1846,

880
01:09:30,000 --> 01:09:33,080
after three months of
painstaking work,

881
01:09:33,080 --> 01:09:36,520
Le Verrier presented his results
to the French Academy.

882
01:09:36,520 --> 01:09:39,200
He announced that his calculations
had revealed

883
01:09:39,200 --> 01:09:42,200
what he believed was a new planet,

884
01:09:42,200 --> 01:09:45,720
and, crucially, that he had
the co-ordinates in the night sky

885
01:09:45,720 --> 01:09:48,080
that showed where it could be found.

886
01:09:52,600 --> 01:09:55,800
And yet, despite this,
he was unable to persuade

887
01:09:55,800 --> 01:09:58,920
any French astronomers
to search for his planet.

888
01:10:03,560 --> 01:10:06,640
Eventually, Le Verrier
sent his calculations

889
01:10:06,640 --> 01:10:09,960
to Johann Galle
at the Berlin Observatory.

890
01:10:09,960 --> 01:10:13,280
His letter arrived
on the 23rd of September,

891
01:10:13,280 --> 01:10:16,280
and the new planet was found
the same evening

892
01:10:16,280 --> 01:10:20,200
within one degree of
Le Verrier's predicted location.

893
01:10:20,200 --> 01:10:23,040
His calculations were so precise,

894
01:10:23,040 --> 01:10:26,320
it took Galle less than an hour
to find it.

895
01:10:33,520 --> 01:10:37,240
Le Verrier and Galle had discovered
the planet Neptune.

896
01:10:38,400 --> 01:10:42,840
A vast ice giant,
17 times heavier than the Earth

897
01:10:42,840 --> 01:10:45,440
and nearly 60 times its volume,

898
01:10:45,440 --> 01:10:49,840
lurking in the shadows some
4 billion kilometres from the sun.

899
01:10:56,320 --> 01:10:58,520
Neptune had been hard to find

900
01:10:58,520 --> 01:11:01,720
not because there was anything
inherently mysterious about it.

901
01:11:01,720 --> 01:11:05,520
It's dark simply because
it's so far from the sun,

902
01:11:05,520 --> 01:11:08,360
there's precious little light
to illuminate it.

903
01:11:09,960 --> 01:11:12,240
And outside our solar system,

904
01:11:12,240 --> 01:11:15,840
this lack of illumination
is an even bigger problem.

905
01:11:17,640 --> 01:11:21,520
And it means even more stuff
is hidden in the dark.

906
01:11:23,760 --> 01:11:29,560
Stars are thought to contain just
11% of the atoms in the universe.

907
01:11:29,560 --> 01:11:34,200
The rest - clouds of gas
and dust, planets, dead stars -

908
01:11:34,200 --> 01:11:37,600
we can't see, because they
give off hardly any light.

909
01:11:38,760 --> 01:11:42,880
The dark spaces between
the stars aren't empty at all.

910
01:11:42,880 --> 01:11:45,560
In fact, they contain the vast
majority

911
01:11:45,560 --> 01:11:47,440
of the stuff that's out there.

912
01:11:52,360 --> 01:11:56,400
Up until the middle of the
20th century, most astronomers

913
01:11:56,400 --> 01:12:01,480
believed that, although they
couldn't see nearly 90% of it,

914
01:12:01,480 --> 01:12:04,840
the universe was still,
theoretically at least,

915
01:12:04,840 --> 01:12:06,240
entirely visible.

916
01:12:09,040 --> 01:12:11,160
But that was about to change.

917
01:12:14,560 --> 01:12:16,760
Welcome to White Sands Missile Range.

918
01:12:24,400 --> 01:12:29,160
In 1964, NASA scientists fitted
an Aerobee rocket

919
01:12:29,160 --> 01:12:31,400
with an X-ray detector...

920
01:12:31,400 --> 01:12:33,120
'..two, one...'

921
01:12:35,400 --> 01:12:38,320
..and blasted it to the edge
of space.

922
01:12:40,760 --> 01:12:44,240
High above the X-ray-absorbing
layers of the atmosphere,

923
01:12:44,240 --> 01:12:47,120
the detector spotted something
extremely bright

924
01:12:47,120 --> 01:12:49,640
in the constellation of Cygnus.

925
01:12:54,240 --> 01:12:58,000
The young British astronomer
Paul Murdin was fascinated by this

926
01:12:58,000 --> 01:13:01,520
mysterious X-ray source,
known as Cygnus X-1.

927
01:13:01,520 --> 01:13:03,920
And when he joined
the Royal Greenwich Observatory

928
01:13:03,920 --> 01:13:05,880
in the summer of 1971,

929
01:13:05,880 --> 01:13:09,000
he was given with the perfect
opportunity to discover what it was.

930
01:13:11,040 --> 01:13:13,640
It was known that X-rays
were produced

931
01:13:13,640 --> 01:13:18,440
when gas was heated to temperatures
upwards of a million degrees.

932
01:13:18,440 --> 01:13:20,200
DOORBELL CHIMES

933
01:13:20,200 --> 01:13:22,080
Hello, Paul!

934
01:13:22,080 --> 01:13:25,600
'But no-one knew for sure
what could produce such extreme

935
01:13:25,600 --> 01:13:27,680
'conditions out in space.'

936
01:13:29,200 --> 01:13:32,240
What was it about X-ray sources
that interested you?

937
01:13:32,240 --> 01:13:35,560
Celestial X-ray sources
had just been discovered.

938
01:13:35,560 --> 01:13:38,560
They were places in the sky
where X-rays came from.

939
01:13:38,560 --> 01:13:40,200
It's a very energetic radiation,

940
01:13:40,200 --> 01:13:43,120
it means something really powerful
is happening there.

941
01:13:43,120 --> 01:13:46,560
I mean, the X-rays are a flag
which the star is waving at you,

942
01:13:46,560 --> 01:13:50,280
saying, "Look at me, look at me,
look at me - I'm really interesting."

943
01:13:52,520 --> 01:13:56,120
But when Paul trained his optical
telescope on the source,

944
01:13:56,120 --> 01:13:59,720
all he saw was an ordinary,
everyday star,

945
01:13:59,720 --> 01:14:02,680
nowhere near hot enough
to produce X-rays.

946
01:14:04,480 --> 01:14:08,000
Most stars are in systems
where there's two stars,

947
01:14:08,000 --> 01:14:10,560
three stars, even five stars
or many more.

948
01:14:10,560 --> 01:14:14,680
It's really unusual to have a
star like our sun that's on its own.

949
01:14:14,680 --> 01:14:16,800
I decided therefore that I'd try

950
01:14:16,800 --> 01:14:20,240
and look for evidence on the star
that I could see, that there

951
01:14:20,240 --> 01:14:23,920
was another star nearby and
that they were circling one another.

952
01:14:23,920 --> 01:14:27,800
By recording its motion
night after night, Paul discovered

953
01:14:27,800 --> 01:14:33,840
the star was orbiting an invisible
partner, once every 5.6 days.

954
01:14:33,840 --> 01:14:38,360
What you can calculate, once
you know the period of a binary star,

955
01:14:38,360 --> 01:14:43,920
is the mass of the system and the
mass of the component parts of it.

956
01:14:43,920 --> 01:14:45,840
And so,
that was the thing to do next.

957
01:14:45,840 --> 01:14:48,560
And then, maybe within an hour,

958
01:14:48,560 --> 01:14:53,160
I knew that the star which
I couldn't see

959
01:14:53,160 --> 01:14:56,440
was four solar masses or more.

960
01:14:59,280 --> 01:15:03,120
Something that heavy
so close to the star he could see

961
01:15:03,120 --> 01:15:06,400
would strip material
from its outer layers, the immense

962
01:15:06,400 --> 01:15:12,240
frictional forces heating the gas to
such an extent it produced X-rays.

963
01:15:15,040 --> 01:15:19,520
But physicists only knew of one
object that could be that massive

964
01:15:19,520 --> 01:15:22,760
and yet remain completely invisible.

965
01:15:22,760 --> 01:15:25,960
It was something that had
only ever existed in theory.

966
01:15:28,280 --> 01:15:31,120
Paul Murdin had discovered
the first black hole.

967
01:15:35,560 --> 01:15:38,400
I was just... I was just elated.

968
01:15:38,400 --> 01:15:42,600
And I had to get up from my desk
and walk about a bit to calm down.

969
01:15:42,600 --> 01:15:44,480
My pulse raced.

970
01:15:44,480 --> 01:15:48,600
I knew it was big, but I was also
a little bit frightened of it,

971
01:15:48,600 --> 01:15:50,280
so I knew I had to check it
very carefully

972
01:15:50,280 --> 01:15:55,520
and go through it all again
and check what I was doing.

973
01:15:55,520 --> 01:15:57,240
But it was... It was a great hour

974
01:15:57,240 --> 01:16:00,440
and I couldn't really do any serious
work for the rest of the day.

975
01:16:01,600 --> 01:16:04,680
And I felt... I felt really happy
with myself, actually.

976
01:16:09,160 --> 01:16:10,840
Thanks to Paul Murdin,

977
01:16:10,840 --> 01:16:15,760
the universe now had a new
and profoundly dark inhabitant.

978
01:16:18,680 --> 01:16:21,360
Black holes are so incredibly dense,

979
01:16:21,360 --> 01:16:25,640
their gravity warps the fabric
of space and time around them

980
01:16:25,640 --> 01:16:30,560
to such an extent that nothing,
not even light, can escape.

981
01:16:38,480 --> 01:16:40,440
As you approach a black hole,

982
01:16:40,440 --> 01:16:42,680
an observer watching you
from a distance

983
01:16:42,680 --> 01:16:47,000
will see the light coming from you
getting redder and redder.

984
01:16:47,000 --> 01:16:50,040
And you will appear to be moving
in slow motion

985
01:16:50,040 --> 01:16:53,840
as the immense gravitational field
of the black hole

986
01:16:53,840 --> 01:16:57,200
stretches both space and time.

987
01:16:57,200 --> 01:17:01,000
And then, as you pass through
the event horizon,

988
01:17:01,000 --> 01:17:04,920
the point of no return that marks
the edge of a black hole,

989
01:17:04,920 --> 01:17:09,880
you simply disappear,
lost from the universe for ever.

990
01:17:20,280 --> 01:17:23,680
Black holes are objects
that would remain dark

991
01:17:23,680 --> 01:17:26,440
no matter how much light
you shone on them.

992
01:17:30,560 --> 01:17:32,600
Through their effects
on other things,

993
01:17:32,600 --> 01:17:36,440
we've now discovered dozens
of black holes in our own galaxy,

994
01:17:36,440 --> 01:17:39,440
and estimate there must be billions
upon billions

995
01:17:39,440 --> 01:17:41,680
of them throughout the universe.

996
01:17:41,680 --> 01:17:44,920
Including huge, supermassive
black holes

997
01:17:44,920 --> 01:17:47,640
millions of times
the mass of the sun

998
01:17:47,640 --> 01:17:50,400
at the heart of nearly every galaxy.

999
01:17:59,800 --> 01:18:01,960
As strange as black holes are,

1000
01:18:01,960 --> 01:18:05,680
they were at least something
we'd expected to find.

1001
01:18:07,600 --> 01:18:10,280
We had theories
that predicted their existence

1002
01:18:10,280 --> 01:18:12,320
and described their properties.

1003
01:18:15,320 --> 01:18:17,400
But since the 1930s,

1004
01:18:17,400 --> 01:18:22,160
astronomers had seen disturbing
hints of something much stranger.

1005
01:18:24,280 --> 01:18:29,640
Stuff that was both completely
invisible and completely unexpected.

1006
01:18:34,800 --> 01:18:36,400
FAINT WHISPERING

1007
01:18:39,600 --> 01:18:44,000
As a child, Vera Rubin
spent hours awake at night

1008
01:18:44,000 --> 01:18:46,800
staring out of the window
above her bed,

1009
01:18:46,800 --> 01:18:50,040
gazing at the stars as they moved
across the sky.

1010
01:18:54,160 --> 01:18:57,040
Then, in her 30s
and a mother herself,

1011
01:18:57,040 --> 01:18:59,920
she decided to realise
her childhood dream

1012
01:18:59,920 --> 01:19:02,960
and embark on a career
as an astronomer.

1013
01:19:02,960 --> 01:19:04,720
FAINT WHISPERING

1014
01:19:13,040 --> 01:19:17,440
In the mid 1960s, the hottest topic
in astronomy was quasars.

1015
01:19:20,280 --> 01:19:23,000
But the field was extremely crowded

1016
01:19:23,000 --> 01:19:26,680
and because the biggest telescopes
that were needed to study them

1017
01:19:26,680 --> 01:19:29,240
were often in the remotest parts
of the world,

1018
01:19:29,240 --> 01:19:33,840
working on quasars meant
a lot of time spent away from home.

1019
01:19:33,840 --> 01:19:36,240
So Vera needed to find
a research topic

1020
01:19:36,240 --> 01:19:39,600
that was more compatible
with being a working mum,

1021
01:19:39,600 --> 01:19:43,040
and a smaller field where
she could really make her mark.

1022
01:19:49,480 --> 01:19:54,440
So she began a project measuring
the way stars move within galaxies

1023
01:19:54,440 --> 01:19:56,440
like our own Milky Way.

1024
01:20:22,280 --> 01:20:24,560
Whoa!
HE LAUGHS

1025
01:20:31,400 --> 01:20:35,680
Everything in a galaxy
is on the move and rotating.

1026
01:20:35,680 --> 01:20:38,520
In one minute, the Earth travels

1027
01:20:38,520 --> 01:20:41,760
nearly 2,000 kilometres
around the sun.

1028
01:20:44,600 --> 01:20:48,520
But in that same time, the sun
and the entire solar system

1029
01:20:48,520 --> 01:20:54,200
travel 12,000 kilometres around
the centre of the Milky Way galaxy.

1030
01:20:57,000 --> 01:20:59,320
Ah!

1031
01:20:59,320 --> 01:21:01,240
I'm not liking this!

1032
01:21:07,560 --> 01:21:11,040
If you think this is spinning fast,
think about this.

1033
01:21:11,040 --> 01:21:15,480
The Earth is travelling around the
sun at 108,000 kilometres an hour.

1034
01:21:17,520 --> 01:21:20,720
Ha! And the sun and
the entire solar system

1035
01:21:20,720 --> 01:21:24,480
are travelling
at 720,000 kilometres an hour

1036
01:21:24,480 --> 01:21:26,880
around the centre of the galaxy.

1037
01:21:26,880 --> 01:21:28,600
HE LAUGHS

1038
01:21:32,880 --> 01:21:34,880
Can we stop it now?

1039
01:21:45,520 --> 01:21:47,920
That's done me in, that really has.

1040
01:21:47,920 --> 01:21:49,640
Thanks very much.

1041
01:21:55,480 --> 01:21:58,600
But when Vera Rubin
measured the speed of stars

1042
01:21:58,600 --> 01:22:01,080
orbiting the centre
of the Andromeda Galaxy,

1043
01:22:01,080 --> 01:22:04,000
she found something deeply puzzling.

1044
01:22:08,040 --> 01:22:10,720
If I plot a graph of the speed

1045
01:22:10,720 --> 01:22:13,840
at which planets in our solar system
orbit the sun

1046
01:22:13,840 --> 01:22:16,840
against their distance from the sun,

1047
01:22:16,840 --> 01:22:22,680
I find that the closest planet,
Mercury, orbits the fastest.

1048
01:22:22,680 --> 01:22:28,760
It's then followed by Venus,
Earth, Mars and so on.

1049
01:22:28,760 --> 01:22:32,960
The further out you go...

1050
01:22:32,960 --> 01:22:35,000
the slower the orbit.

1051
01:22:35,000 --> 01:22:39,040
In fact, Neptune moves so slowly
relative to the other planets

1052
01:22:39,040 --> 01:22:41,800
and has so far to go in orbit
around the sun,

1053
01:22:41,800 --> 01:22:44,560
that it's only
completed one full circuit

1054
01:22:44,560 --> 01:22:47,080
since it was discovered
167 years ago.

1055
01:22:48,640 --> 01:22:53,960
Now, if I plot the same graph again
of speed against distance,

1056
01:22:53,960 --> 01:22:56,640
but this time, the speed

1057
01:22:56,640 --> 01:23:00,080
at which the stars orbit
the centre of a galaxy

1058
01:23:00,080 --> 01:23:02,040
against their distance
from the centre,

1059
01:23:02,040 --> 01:23:04,760
I'd expect to see
for the outer stars,

1060
01:23:04,760 --> 01:23:08,640
that the speed drops off with
distance, as it did for the planets.

1061
01:23:08,640 --> 01:23:12,120
But when Vera Rubin
plotted her data,

1062
01:23:12,120 --> 01:23:14,600
she found that the further out
you went,

1063
01:23:14,600 --> 01:23:17,680
the speed of the stars
didn't drop off,

1064
01:23:17,680 --> 01:23:20,400
it remained roughly the same.

1065
01:23:22,320 --> 01:23:25,280
The planets move more slowly
the further out they are

1066
01:23:25,280 --> 01:23:27,160
because the further you go,

1067
01:23:27,160 --> 01:23:30,400
the weaker the sun's
gravitational field becomes.

1068
01:23:30,400 --> 01:23:35,720
So anything moving too fast would
simply fly off into outer space.

1069
01:23:35,720 --> 01:23:38,520
But Vera Rubin's result for galaxies

1070
01:23:38,520 --> 01:23:41,440
suggested there must be
an extra source of gravity

1071
01:23:41,440 --> 01:23:45,000
holding all those fast-moving stars
in their orbits.

1072
01:23:48,840 --> 01:23:50,880
This extra gravity was needed

1073
01:23:50,880 --> 01:23:54,560
because when astronomers added up
the gravitational pull

1074
01:23:54,560 --> 01:23:58,680
of all the dark things they thought
might be lurking in the galaxy,

1075
01:23:58,680 --> 01:24:02,480
planets, clouds of dust,
even black holes,

1076
01:24:02,480 --> 01:24:05,680
it always came out
about ten times less

1077
01:24:05,680 --> 01:24:07,640
than that needed to account for

1078
01:24:07,640 --> 01:24:10,360
the stellar speeds
Vera Rubin had measured.

1079
01:24:12,200 --> 01:24:14,440
There were two possible
explanations.

1080
01:24:14,440 --> 01:24:17,840
Either Einstein's
theory of gravity was wrong,

1081
01:24:17,840 --> 01:24:21,920
or galaxies were full of
a completely new kind of stuff.

1082
01:24:21,920 --> 01:24:23,760
Something that wasn't made of atoms,

1083
01:24:23,760 --> 01:24:27,080
was completely invisible
and very heavy.

1084
01:24:27,080 --> 01:24:29,320
A new form of dark matter.

1085
01:24:29,320 --> 01:24:32,080
Something astronomers named...
dark matter.

1086
01:24:46,640 --> 01:24:49,280
Unsurprisingly, rather than accept

1087
01:24:49,280 --> 01:24:53,520
that galaxies were full
of some mysterious unseen stuff,

1088
01:24:53,520 --> 01:24:57,280
some physicists once again
thought tweaking the laws of gravity

1089
01:24:57,280 --> 01:24:59,160
might be the simplest solution.

1090
01:25:02,160 --> 01:25:05,960
That was until astronomers
captured an astonishing image.

1091
01:25:07,040 --> 01:25:08,760
For me, this is one of the most

1092
01:25:08,760 --> 01:25:11,120
amazing pictures
in modern astronomy.

1093
01:25:11,120 --> 01:25:16,440
It's an image of a cluster of
galaxies called the Bullet Cluster.

1094
01:25:16,440 --> 01:25:19,640
It gets its name from this
bullet-shaped cloud of gas,

1095
01:25:19,640 --> 01:25:24,440
which is actually a shockwave
caused by the collision

1096
01:25:24,440 --> 01:25:29,000
not of just clouds of gas
or stars or even whole galaxies,

1097
01:25:29,000 --> 01:25:32,280
but clusters of galaxies
coming together

1098
01:25:32,280 --> 01:25:36,480
and passing through each other
at 10-million kilometres an hour.

1099
01:25:39,960 --> 01:25:44,040
It almost gives me vertigo trying to
imagine the immensity of the scale.

1100
01:25:46,560 --> 01:25:48,960
But it's not the magnitude
of the collision

1101
01:25:48,960 --> 01:25:51,480
that makes this image so important.

1102
01:25:51,480 --> 01:25:54,880
It's what it did to the clusters'
constituent parts.

1103
01:25:56,200 --> 01:25:58,600
As the clusters came together,

1104
01:25:58,600 --> 01:26:00,600
the stars and planets
in the galaxies

1105
01:26:00,600 --> 01:26:02,480
pretty much passed
through each other

1106
01:26:02,480 --> 01:26:06,360
because although they're big, the
distances between them are so vast

1107
01:26:06,360 --> 01:26:10,840
that the chances of any two stars
colliding is actually very small.

1108
01:26:10,840 --> 01:26:13,600
But that doesn't apply to
the dust and gas

1109
01:26:13,600 --> 01:26:18,880
that makes up 90% by mass of all
the stuff we can see in a galaxy.

1110
01:26:18,880 --> 01:26:22,680
When these collide,
they create a huge, hot cloud -

1111
01:26:22,680 --> 01:26:25,640
these two pink regions
in the centre of the image.

1112
01:26:27,080 --> 01:26:30,640
But if most of the mass
is trapped here in the clouds,

1113
01:26:30,640 --> 01:26:34,520
then you'd expect most of the
gravity to be centred there, too.

1114
01:26:34,520 --> 01:26:36,480
But that's not what you see.

1115
01:26:36,480 --> 01:26:41,200
These outer blue regions
show where light has been bent round

1116
01:26:41,200 --> 01:26:45,280
as gravity warps
the fabric of space itself.

1117
01:26:45,280 --> 01:26:48,840
That means most of the gravity
is centred out here,

1118
01:26:48,840 --> 01:26:51,040
rather than in the middle.

1119
01:26:51,040 --> 01:26:53,760
The simplest way to explain this

1120
01:26:53,760 --> 01:26:56,240
is that it wasn't just stars
and planets

1121
01:26:56,240 --> 01:26:59,000
that passed
through as the clusters collided,

1122
01:26:59,000 --> 01:27:00,800
something else did, too.

1123
01:27:00,800 --> 01:27:03,800
Something massive, yet invisible.

1124
01:27:03,800 --> 01:27:07,280
This image is the best evidence
we have yet

1125
01:27:07,280 --> 01:27:09,160
for the existence of dark matter.

1126
01:27:16,960 --> 01:27:20,520
It's now generally accepted
that dark matter is real,

1127
01:27:20,520 --> 01:27:24,640
which means there's far more stuff
in the universe than we'd thought.

1128
01:27:26,400 --> 01:27:28,440
In fact, there's four times

1129
01:27:28,440 --> 01:27:31,680
as much dark matter
as there is normal matter.

1130
01:27:34,320 --> 01:27:39,040
And so vast swathes of the universe
are not just unseen,

1131
01:27:39,040 --> 01:27:41,840
they're fundamentally unseeable.

1132
01:27:43,520 --> 01:27:45,600
The reason dark matter
is so elusive

1133
01:27:45,600 --> 01:27:50,000
is because it doesn't reflect light
and it doesn't emit light.

1134
01:27:50,000 --> 01:27:52,720
So we can't see it.

1135
01:27:52,720 --> 01:27:56,440
And worse than that, what gives
normal matter its solidity

1136
01:27:56,440 --> 01:27:59,040
is the electromagnetic force.

1137
01:27:59,040 --> 01:28:02,160
And dark matter particles
don't feel that force,

1138
01:28:02,160 --> 01:28:04,760
so they just pass
straight through matter.

1139
01:28:04,760 --> 01:28:09,040
The only hope we have is if they
hit an atomic nucleus head-on.

1140
01:28:09,040 --> 01:28:12,840
And even if they do,
that's really hard to detect.

1141
01:28:17,120 --> 01:28:19,640
And so the hunt for dark matter

1142
01:28:19,640 --> 01:28:24,960
has turned from the incredibly large
to the unimaginably small.

1143
01:28:30,240 --> 01:28:32,800
From scouring the skies
with telescopes

1144
01:28:32,800 --> 01:28:35,640
to detectors buried deep
underground.

1145
01:28:37,080 --> 01:28:39,280
When it comes to the search
for dark matter,

1146
01:28:39,280 --> 01:28:42,800
the place I'm going to is pretty
much the centre of the universe.

1147
01:28:49,960 --> 01:28:52,480
The Gran Sasso National Laboratory

1148
01:28:52,480 --> 01:28:56,560
lies beneath almost a kilometre
and a half of solid rock.

1149
01:28:59,640 --> 01:29:01,880
And can only be reached
through a tunnel

1150
01:29:01,880 --> 01:29:04,440
cut deep into the Italian Apennines.

1151
01:29:09,760 --> 01:29:13,080
The reason you'd build a laboratory
underneath a mountain

1152
01:29:13,080 --> 01:29:18,000
is because our planet is constantly
being bombarded by cosmic rays.

1153
01:29:18,000 --> 01:29:20,400
These collide with the upper
atmosphere,

1154
01:29:20,400 --> 01:29:22,720
creating a cascade of particles

1155
01:29:22,720 --> 01:29:25,640
that shower down onto the surface
of the Earth.

1156
01:29:25,640 --> 01:29:31,160
The rock above me effectively forms
a 1400-metre-thick roof

1157
01:29:31,160 --> 01:29:34,200
that absorbs most of these
particles,

1158
01:29:34,200 --> 01:29:37,640
shielding and protecting
the equipment below.

1159
01:29:37,640 --> 01:29:40,400
But crucially
for dark-matter hunters,

1160
01:29:40,400 --> 01:29:42,520
it passes straight through
normal matter,

1161
01:29:42,520 --> 01:29:43,920
straight through the rock,

1162
01:29:43,920 --> 01:29:46,360
and the hope is,
into their detectors.

1163
01:29:51,600 --> 01:29:53,840
Oh!

1164
01:29:53,840 --> 01:29:56,160
It looks like a Bond villain's
evil lair.

1165
01:30:25,600 --> 01:30:29,200
Gran Sasso is the world's largest
underground laboratory.

1166
01:30:35,000 --> 01:30:36,800
And for the last ten years,

1167
01:30:36,800 --> 01:30:40,800
it's been home to dark matter
scientists like Dr Chamkaur Ghag,

1168
01:30:40,800 --> 01:30:43,640
who works on DarkSide-50,

1169
01:30:43,640 --> 01:30:46,760
one of five dark matter experiments
based here.

1170
01:30:57,320 --> 01:30:59,640
So hairnet. Hairnet.

1171
01:30:59,640 --> 01:31:02,040
Or head net, in my case.

1172
01:31:04,080 --> 01:31:09,040
Milligram levels of dust
can destroy the experiment. Right.

1173
01:31:25,360 --> 01:31:27,560
That looks very impressive.

1174
01:31:27,560 --> 01:31:29,200
Yep. Very sci-fi.

1175
01:31:34,160 --> 01:31:36,440
So tell me,
how does the experiment work?

1176
01:31:36,440 --> 01:31:39,160
Well, the entire experiment
is configured like a Russian doll,

1177
01:31:39,160 --> 01:31:41,800
where the first outer layer
is the mountain itself,

1178
01:31:41,800 --> 01:31:45,400
protecting the experiment
from radiation from space.

1179
01:31:45,400 --> 01:31:48,040
Then we have this tank
that we're standing in.

1180
01:31:48,040 --> 01:31:50,560
And this tank is going to be
flooded full of water.

1181
01:31:50,560 --> 01:31:52,560
What, the whole cylinder?

1182
01:31:52,560 --> 01:31:55,280
Absolutely. This is all
completely filled to the brim.

1183
01:31:55,280 --> 01:31:58,600
About 750 cubic metres of water
will fill this thing

1184
01:31:58,600 --> 01:32:02,520
to stop radiation coming from the
laboratory and the rock around us.

1185
01:32:02,520 --> 01:32:05,840
That's protecting this huge metal
sphere right here,

1186
01:32:05,840 --> 01:32:07,600
which is the final layer
of protection

1187
01:32:07,600 --> 01:32:10,800
before we get to DarkSide itself,
which is inside there right now.

1188
01:32:10,800 --> 01:32:13,200
That's the detector,
that's the heart of the experiment.

1189
01:32:13,200 --> 01:32:15,440
That's the thing that will be
detecting dark matter.

1190
01:32:17,680 --> 01:32:20,680
You haven't got a light switch
up there. No.

1191
01:32:20,680 --> 01:32:22,600
I'm going to get up there
and have a look.

1192
01:32:29,760 --> 01:32:32,280
DarkSide-50 is designed to detect

1193
01:32:32,280 --> 01:32:34,920
a new class of fundamental particles

1194
01:32:34,920 --> 01:32:38,120
called weakly interacting
massive particles.

1195
01:32:39,480 --> 01:32:43,160
Predicted by theory, it's
thought that these WIMPs

1196
01:32:43,160 --> 01:32:46,400
might be the stuff
of which dark matter is made.

1197
01:32:49,760 --> 01:32:53,120
So that metal sphere in the centre,
that's DarkSide?

1198
01:32:53,120 --> 01:32:56,240
That's right. That's a detector
full of 150kg of liquid argon.

1199
01:32:56,240 --> 01:32:57,880
Dark matter particles should be

1200
01:32:57,880 --> 01:33:00,240
streaming through the detector
all the time,

1201
01:33:00,240 --> 01:33:02,360
but most of them just go
straight through

1202
01:33:02,360 --> 01:33:04,920
because they're very
weakly interacting particles.

1203
01:33:04,920 --> 01:33:08,280
If we're lucky, one will collide
with the nucleus of an argon atom,

1204
01:33:08,280 --> 01:33:10,960
producing flashes of light
that the detector will pick up.

1205
01:33:16,760 --> 01:33:19,720
DarkSide is yet to begin its search,

1206
01:33:19,720 --> 01:33:22,640
but elsewhere in the laboratory's
labyrinth of tunnels,

1207
01:33:22,640 --> 01:33:26,120
they're already seeing
tantalising hints.

1208
01:33:28,480 --> 01:33:31,080
This is the XENON100 experiment
that's already running

1209
01:33:31,080 --> 01:33:32,880
and taking data
and has been for a while.

1210
01:33:32,880 --> 01:33:36,520
It's the most sensitive dark matter
detector in the world right now.

1211
01:33:36,520 --> 01:33:40,480
And this is a live feed of dark
matter data coming in right now.

1212
01:33:40,480 --> 01:33:43,920
So, what exactly... What sort of
signal or shape are you looking for?

1213
01:33:43,920 --> 01:33:46,920
Well, what we're looking for
is an initial flash of light

1214
01:33:46,920 --> 01:33:48,800
which will be a very sharp
peak like this,

1215
01:33:48,800 --> 01:33:52,360
followed by a much larger
peak like that one,

1216
01:33:52,360 --> 01:33:55,560
which is light being generated
in a gas layer

1217
01:33:55,560 --> 01:33:57,560
on top of the liquid xenon.

1218
01:33:57,560 --> 01:34:00,000
Oh. That could be a good one
as well, actually.

1219
01:34:00,000 --> 01:34:01,280
There you go.

1220
01:34:01,280 --> 01:34:03,520
So any one of those events,
those spikes,

1221
01:34:03,520 --> 01:34:05,440
could be a dark matter particle?

1222
01:34:05,440 --> 01:34:07,960
That's right.
Any one of these events

1223
01:34:07,960 --> 01:34:10,800
could be the signature
of dark matter

1224
01:34:10,800 --> 01:34:12,280
interacting in XENON100.

1225
01:34:12,280 --> 01:34:15,120
It's just we won't know for sure
until the data's been analysed.

1226
01:34:17,120 --> 01:34:19,160
Because it's so sensitive,

1227
01:34:19,160 --> 01:34:22,000
the overwhelming majority
of the spikes

1228
01:34:22,000 --> 01:34:24,840
are due to radiation
emitted by the metal

1229
01:34:24,840 --> 01:34:26,960
that makes up the detector itself.

1230
01:34:32,200 --> 01:34:34,800
But the hope is experiments
like this

1231
01:34:34,800 --> 01:34:38,040
will definitively detect
dark matter particles

1232
01:34:38,040 --> 01:34:39,760
within the next ten years.

1233
01:34:47,160 --> 01:34:50,600
Today, we think that dark matter
not only exists,

1234
01:34:50,600 --> 01:34:53,880
but that it is a vital part
of our universe,

1235
01:34:53,880 --> 01:34:58,280
because without it, the world that
we can see wouldn't exist

1236
01:34:58,280 --> 01:35:02,240
and that's because dark matter
not only holds galaxies together,

1237
01:35:02,240 --> 01:35:06,680
it's dark matter that brought
the clouds of gas together

1238
01:35:06,680 --> 01:35:11,520
to form the galaxies in which stars
could ignite in the first place.

1239
01:35:19,960 --> 01:35:22,960
Dark matter has gone from being
a curious quirk

1240
01:35:22,960 --> 01:35:26,520
of the way stars move around
the fringes of galaxies

1241
01:35:26,520 --> 01:35:30,000
to the reason there are stars
and galaxies at all.

1242
01:35:36,000 --> 01:35:39,800
But in the late 1990s,
scientists attempting

1243
01:35:39,800 --> 01:35:43,000
to measure exactly how much
dark matter there was

1244
01:35:43,000 --> 01:35:45,640
made an astonishing discovery.

1245
01:35:45,640 --> 01:35:48,680
There was something
even more mysterious

1246
01:35:48,680 --> 01:35:51,160
and even more elusive out there.

1247
01:35:52,720 --> 01:35:54,880
And to understand what that is,

1248
01:35:54,880 --> 01:35:59,440
you have to go back to the very
beginning of everything.

1249
01:35:59,440 --> 01:36:02,280
The universe began
with a gigantic fireball.

1250
01:36:09,880 --> 01:36:14,440
13.8 billion years ago,
the universe was born.

1251
01:36:15,400 --> 01:36:17,000
In the so-called big bang,

1252
01:36:17,000 --> 01:36:19,400
everything was created
simultaneously.

1253
01:36:22,640 --> 01:36:24,400
See that great flash of light?

1254
01:36:24,400 --> 01:36:28,200
That's all the pieces of the atoms
joining together to make a gas.

1255
01:36:28,200 --> 01:36:30,760
And now the gas is getting lumpy.

1256
01:36:30,760 --> 01:36:33,200
It's making the giant galaxies
of stars.

1257
01:36:36,640 --> 01:36:39,480
The expansion of the universe
that we now see

1258
01:36:39,480 --> 01:36:42,960
is just a remnant of the initial
violent explosion.

1259
01:36:49,720 --> 01:36:52,080
The big bang means that in the past,

1260
01:36:52,080 --> 01:36:54,960
the universe was much smaller
than it is today.

1261
01:36:57,600 --> 01:36:59,960
And it's been getting bigger
ever since.

1262
01:37:11,000 --> 01:37:12,480
According to the big bang theory,

1263
01:37:12,480 --> 01:37:17,680
the universe has been expanding
for the past 13.8 billion years.

1264
01:37:17,680 --> 01:37:19,560
And for most of that time,

1265
01:37:19,560 --> 01:37:22,560
you'd expect the expansion
to be slowing down

1266
01:37:22,560 --> 01:37:24,960
due to the combined
gravitational attraction

1267
01:37:24,960 --> 01:37:27,480
of all the mass in the universe

1268
01:37:27,480 --> 01:37:30,160
trying to pull it back together
again.

1269
01:37:30,160 --> 01:37:31,520
Now, here's the clever bit,

1270
01:37:31,520 --> 01:37:34,080
Cosmologists realised
that by measuring

1271
01:37:34,080 --> 01:37:36,400
how much the expansion was slowing,

1272
01:37:36,400 --> 01:37:40,280
they could calculate
how much stuff was out there.

1273
01:37:40,280 --> 01:37:44,120
In a sense, it would allow them
to weigh the entire universe.

1274
01:37:46,880 --> 01:37:50,480
But in order to measure
how the universe is expanding,

1275
01:37:50,480 --> 01:37:54,560
you need a reliable way
to measure distances in space.

1276
01:38:01,720 --> 01:38:06,960
Something of known brightness,
astronomers call a standard candle.

1277
01:38:08,880 --> 01:38:12,560
The flame in this lantern
produces a fixed amount of light.

1278
01:38:12,560 --> 01:38:17,240
It has a specific brightness that
I can measure here on the ground.

1279
01:38:17,240 --> 01:38:20,480
But if I let the lantern go,
it'll drift away

1280
01:38:20,480 --> 01:38:23,240
and the light will appear to get
dimmer and dimmer

1281
01:38:23,240 --> 01:38:24,720
the further away it gets.

1282
01:38:28,040 --> 01:38:30,400
Because I know how bright
it really is,

1283
01:38:30,400 --> 01:38:33,560
by comparing that
with how bright it appears,

1284
01:38:33,560 --> 01:38:36,400
I can calculate how far away it is.

1285
01:38:59,480 --> 01:39:02,320
And because every lantern's
the same,

1286
01:39:02,320 --> 01:39:05,120
I can use the brightness
to calculate the distance

1287
01:39:05,120 --> 01:39:07,560
to any lantern I see in the sky.

1288
01:39:13,280 --> 01:39:16,360
The astronomical equivalent
of a Chinese lantern

1289
01:39:16,360 --> 01:39:22,840
is a particular species of exploding
star called a Type 1a supernova.

1290
01:39:36,440 --> 01:39:41,840
These stars always explode when
they reach the same critical mass

1291
01:39:41,840 --> 01:39:45,160
and so always explode with
the same brightness.

1292
01:39:48,360 --> 01:39:51,000
So by measuring how bright
they appear,

1293
01:39:51,000 --> 01:39:53,480
we can tell how far
they are from the Earth.

1294
01:39:57,960 --> 01:40:01,040
As well as telling us
how far away they are,

1295
01:40:01,040 --> 01:40:06,000
the light reaching us from distant
supernovae tells us something else.

1296
01:40:06,000 --> 01:40:10,040
As it travels across the cosmos,
light gets stretched

1297
01:40:10,040 --> 01:40:13,840
because the space it's travelling
through is expanding.

1298
01:40:13,840 --> 01:40:19,400
And as its wavelength increases,
the light gets redder and redder.

1299
01:40:19,400 --> 01:40:24,880
And this red shift tells us
how fast the universe was expanding

1300
01:40:24,880 --> 01:40:28,880
when the light left its source,
when the star exploded.

1301
01:40:33,120 --> 01:40:37,760
But when scientists analysed light
from the more distant supernovae

1302
01:40:37,760 --> 01:40:40,440
they found something strange.

1303
01:40:40,440 --> 01:40:42,800
It was less stretched than expected.

1304
01:40:45,080 --> 01:40:47,280
It meant that, in the past,

1305
01:40:47,280 --> 01:40:51,400
the universe was expanding more
slowly than it is today.

1306
01:40:51,400 --> 01:40:56,400
In other words, the expansion of the
universe wasn't slowing down at all,

1307
01:40:56,400 --> 01:40:57,760
it was speeding up.

1308
01:41:02,560 --> 01:41:06,720
The only way the universe's
expansion could be accelerating...

1309
01:41:09,440 --> 01:41:13,920
..was if there was a mysterious new
force pushing it apart.

1310
01:41:17,600 --> 01:41:21,280
And just as with dark matter,
physicists thought the key

1311
01:41:21,280 --> 01:41:23,600
to understanding this new force

1312
01:41:23,600 --> 01:41:26,760
might lie at the smallest possible
scales...

1313
01:41:29,040 --> 01:41:33,880
..because quantum physics appeared
to provide a ready-made explanation.

1314
01:41:36,520 --> 01:41:41,720
According to quantum field theory,
empty space is anything but empty.

1315
01:41:41,720 --> 01:41:45,240
Particles are constantly appearing
and disappearing,

1316
01:41:45,240 --> 01:41:49,000
created out of energy borrowed
from the vacuum itself.

1317
01:41:51,080 --> 01:41:54,800
The hope was that this theoretical
vacuum energy

1318
01:41:54,800 --> 01:41:58,600
might be the very thing that was
pushing the universe apart.

1319
01:41:59,640 --> 01:42:02,880
And the theory allows me to
calculate the energy density

1320
01:42:02,880 --> 01:42:06,480
of the vacuum, that's the amount of
energy you'd expect to find

1321
01:42:06,480 --> 01:42:08,440
in a given volume.

1322
01:42:08,440 --> 01:42:12,320
And so if I take
the energy of the vacuum

1323
01:42:12,320 --> 01:42:17,040
to be a sum over J
of half h-bar omega J,

1324
01:42:17,040 --> 01:42:19,280
and if I take the cut-off energy

1325
01:42:19,280 --> 01:42:22,200
to be of the order
of 10 tera electronvolts

1326
01:42:22,200 --> 01:42:24,040
which is just above
the known physics

1327
01:42:24,040 --> 01:42:27,360
at the Large Hadron Collider,
then the formula for the vacuum...

1328
01:42:27,360 --> 01:42:31,920
'All they needed to do was check the
energy density the theory predicted

1329
01:42:31,920 --> 01:42:36,600
'matched that needed to drive
the universe's acceleration

1330
01:42:36,600 --> 01:42:39,600
'and the mysterious
force would be explained.'

1331
01:42:39,600 --> 01:42:46,360
HE MUTTERS EQUATIONS

1332
01:43:03,080 --> 01:43:08,000
So that would give me a value
for the energy density

1333
01:43:08,000 --> 01:43:13,360
of the vacuum of 10 to the 35
kilograms per cubic metre.

1334
01:43:15,320 --> 01:43:18,520
The trouble is, the value observed
by astronomers

1335
01:43:18,520 --> 01:43:22,960
is 10 to the minus 27 kilograms
per cubic metre.

1336
01:43:22,960 --> 01:43:26,120
That means the theoretical number
and the experimental number

1337
01:43:26,120 --> 01:43:30,120
are out by a factor of
10 to the power 62.

1338
01:43:30,120 --> 01:43:33,080
That's one followed by 62 zeros.

1339
01:43:34,280 --> 01:43:36,880
To give you a sense
of the scale of the error,

1340
01:43:36,880 --> 01:43:40,040
there've been only
10 to the 17 seconds

1341
01:43:40,040 --> 01:43:44,840
since the big bang and the diameter
of the entire visible universe

1342
01:43:44,840 --> 01:43:47,440
is 10 to the 27 metres...

1343
01:43:49,560 --> 01:43:51,040
So it's a pretty big error.

1344
01:43:52,960 --> 01:43:58,160
And that meant that whatever was
actually pushing the universe apart,

1345
01:43:58,160 --> 01:44:00,960
it was something completely new.

1346
01:44:05,480 --> 01:44:08,480
The truth is, we know very little
about what's causing

1347
01:44:08,480 --> 01:44:11,240
the expansion of the universe
to accelerate,

1348
01:44:11,240 --> 01:44:14,960
but we do have a name for it -
dark energy.

1349
01:44:14,960 --> 01:44:17,600
And we know that for it to have
the effect that it does,

1350
01:44:17,600 --> 01:44:19,600
there must be
an awful lot of it about.

1351
01:44:22,720 --> 01:44:26,520
Einstein's famous equation E=mc2

1352
01:44:26,520 --> 01:44:31,440
says that energy and matter are
different forms of the same thing.

1353
01:44:31,440 --> 01:44:34,840
And the equivalent mass of
dark energy dwarfs that

1354
01:44:34,840 --> 01:44:37,040
of everything else in the universe.

1355
01:44:39,120 --> 01:44:41,440
And it means that, today,

1356
01:44:41,440 --> 01:44:45,320
normal matter makes up
just 4% of the cosmos.

1357
01:44:45,320 --> 01:44:48,680
23% of it is elusive dark matter.

1358
01:44:50,080 --> 01:44:54,120
And a colossal 73% of the universe

1359
01:44:54,120 --> 01:44:57,160
consists of mysterious dark energy.

1360
01:45:01,680 --> 01:45:04,040
Just think about it for a moment.

1361
01:45:04,040 --> 01:45:06,200
100 billion galaxies,

1362
01:45:06,200 --> 01:45:10,080
each one containing more than
100 billion stars,

1363
01:45:10,080 --> 01:45:14,080
home in turn to billions upon
billions of planets and moons.

1364
01:45:15,200 --> 01:45:21,360
All of that is mere flotsam adrift
on a vast and unfathomable ocean.

1365
01:45:21,360 --> 01:45:25,680
Dark matter we can't see and dark
energy we can barely comprehend.

1366
01:45:30,760 --> 01:45:35,360
And the very nature of dark energy
means the universe is getting

1367
01:45:35,360 --> 01:45:38,200
more unknowable all the time.

1368
01:45:41,400 --> 01:45:45,120
As space expands and distances
become bigger,

1369
01:45:45,120 --> 01:45:49,560
most forces get weaker, because you
have the same amount of mass

1370
01:45:49,560 --> 01:45:53,240
or electric charge,
only now everything's further apart.

1371
01:45:54,880 --> 01:45:58,360
But dark energy behaves
completely differently.

1372
01:45:58,360 --> 01:46:02,920
As the universe has expanded,
the stronger it's become.

1373
01:46:02,920 --> 01:46:06,040
The more space there is,
the more dark energy there is

1374
01:46:06,040 --> 01:46:09,360
and so the faster
the universe expands,

1375
01:46:09,360 --> 01:46:13,400
creating ever more space
and ever more dark energy.

1376
01:46:18,640 --> 01:46:21,960
And that has a profound
consequence.

1377
01:46:21,960 --> 01:46:25,360
Just as dark matter pulled
the galaxies together

1378
01:46:25,360 --> 01:46:27,520
to create the cosmos
as we know it...

1379
01:46:29,000 --> 01:46:33,040
..so dark energy will tear
the universe apart.

1380
01:46:35,080 --> 01:46:37,520
In the future, as space gets bigger,

1381
01:46:37,520 --> 01:46:40,800
dark energy will become
ever more dominant.

1382
01:46:40,800 --> 01:46:44,560
And so it will ultimately shape
the universe's destiny.

1383
01:46:44,560 --> 01:46:48,080
And if it continues to increase as
it appears to be doing today,

1384
01:46:48,080 --> 01:46:51,640
then it will push the galaxies
further and further apart

1385
01:46:51,640 --> 01:46:54,840
until, eventually,
they slip out of view,

1386
01:46:54,840 --> 01:46:58,840
creating a cosmos that will become
ever more dark

1387
01:46:58,840 --> 01:47:00,240
and ever more desolate.

1388
01:47:10,520 --> 01:47:14,680
The ultimate goal of modern
cosmology is to understand

1389
01:47:14,680 --> 01:47:18,160
dark energy
and the fate of the universe,

1390
01:47:18,160 --> 01:47:22,160
and to witness how dark matter
brought everything together

1391
01:47:22,160 --> 01:47:23,560
in the first place.

1392
01:47:29,080 --> 01:47:33,880
And so to shed light on both the
beginning and end of the universe,

1393
01:47:33,880 --> 01:47:38,560
cosmologists have embarked on
a quest of epic proportions -

1394
01:47:38,560 --> 01:47:43,640
to map everywhere in space over
the entire lifespan of the cosmos...

1395
01:47:45,200 --> 01:47:50,160
..starting with the darkest
period in its past,

1396
01:47:50,160 --> 01:47:54,760
an era that began as the afterglow
of the big bang faded away.

1397
01:47:56,600 --> 01:47:59,800
We talk about the ages of
the universe in the same way

1398
01:47:59,800 --> 01:48:03,680
that we talk about the stages in our
own lives, from its birth,

1399
01:48:03,680 --> 01:48:08,000
through childhood, adolescence,
adulthood and even death.

1400
01:48:08,000 --> 01:48:10,840
So mapping the universe
is really about

1401
01:48:10,840 --> 01:48:13,600
filling in the photo album
of its life.

1402
01:48:14,800 --> 01:48:20,440
Here's a picture of me
from 20 years ago with my children.

1403
01:48:20,440 --> 01:48:23,000
I know it because I have a lot
more hair there.

1404
01:48:23,000 --> 01:48:27,200
And here's a picture of me
in my early 20s on graduation.

1405
01:48:27,200 --> 01:48:29,400
And here's one of me as a teenager.

1406
01:48:31,280 --> 01:48:34,320
In the same way,
by looking out into space,

1407
01:48:34,320 --> 01:48:36,400
we have good images of the universe

1408
01:48:36,400 --> 01:48:38,560
all the way back
to its teenage years,

1409
01:48:38,560 --> 01:48:42,200
when large galaxies first formed.

1410
01:48:42,200 --> 01:48:46,520
But before that,
we have nothing but a single image -

1411
01:48:46,520 --> 01:48:50,440
a picture of the universe
when it was just 400,000 years old,

1412
01:48:50,440 --> 01:48:55,040
the cosmic microwave background -
the afterglow of the big bang.

1413
01:48:55,040 --> 01:48:58,040
It's as though,
in the photo album of my life,

1414
01:48:58,040 --> 01:49:01,680
I have nothing before this picture
of me aged 16,

1415
01:49:01,680 --> 01:49:04,960
apart from this one of me and my
parents in Iraq

1416
01:49:04,960 --> 01:49:06,840
when I was just a few months old.

1417
01:49:08,800 --> 01:49:11,720
This gap in the childhood
of the universe,

1418
01:49:11,720 --> 01:49:15,560
the period between its earliest
moments, through the birth

1419
01:49:15,560 --> 01:49:19,600
of the first stars to the formation
of large galaxies

1420
01:49:19,600 --> 01:49:22,920
is a time known
as the dark ages of the universe.

1421
01:49:28,760 --> 01:49:34,120
The universe's dark ages
lasted for around a billion years

1422
01:49:34,120 --> 01:49:37,560
and they get their name because
there were precious few stars

1423
01:49:37,560 --> 01:49:39,120
to illuminate them.

1424
01:49:44,000 --> 01:49:48,760
So to fill in those pages in the
cosmic photo album, we'd need

1425
01:49:48,760 --> 01:49:52,920
something capable of seeing where
there was next to no light.

1426
01:49:59,800 --> 01:50:04,160
During the Second World War, Bernard
Lovell had developed a machine

1427
01:50:04,160 --> 01:50:06,520
that could see in the dark.

1428
01:50:06,520 --> 01:50:08,440
He'd worked on airborne radar

1429
01:50:08,440 --> 01:50:11,240
that mapped bombers' targets
on the ground.

1430
01:50:14,640 --> 01:50:16,720
But his real ambition was to build

1431
01:50:16,720 --> 01:50:19,440
something capable of
mapping the heavens.

1432
01:50:52,240 --> 01:50:56,000
The giant dish at Jodrell Bank
was Bernard Lovell's baby.

1433
01:50:56,000 --> 01:51:00,360
It was designed to be the world's
largest fully manoeuvrable

1434
01:51:00,360 --> 01:51:03,640
radio telescope,
capable of scouring the entire sky

1435
01:51:03,640 --> 01:51:07,000
and picking up the
longest-wavelength radio signals

1436
01:51:07,000 --> 01:51:09,920
coming from the deepest
recesses of space.

1437
01:51:32,600 --> 01:51:35,000
The Lovell Telescope
has a collecting area

1438
01:51:35,000 --> 01:51:39,000
of 4,560 square metres,

1439
01:51:39,000 --> 01:51:44,560
made up of more than
2,400 galvanised steel plates.

1440
01:51:49,720 --> 01:51:53,600
In the original designs,
this bowl of the telescope

1441
01:51:53,600 --> 01:51:56,200
wasn't meant to be
solid like this.

1442
01:51:56,200 --> 01:51:59,840
The plan was for it to be built of
much lighter wire mesh.

1443
01:52:03,200 --> 01:52:05,240
The dish was redesigned

1444
01:52:05,240 --> 01:52:09,840
because astronomers had discovered
a new way of seeing in the dark,

1445
01:52:09,840 --> 01:52:12,720
something that might ultimately
allow them

1446
01:52:12,720 --> 01:52:15,480
to map the universe's dark ages.

1447
01:52:17,840 --> 01:52:21,200
Hydrogen permeates every galaxy.

1448
01:52:21,200 --> 01:52:22,720
It was produced in the big bang

1449
01:52:22,720 --> 01:52:26,480
and is the basic constituent of all
normal matter, including us.

1450
01:52:26,480 --> 01:52:28,840
And like most normal matter,

1451
01:52:28,840 --> 01:52:31,920
it wasn't thought
to give off any light.

1452
01:52:31,920 --> 01:52:35,240
But then astronomers discovered
something remarkable.

1453
01:52:35,240 --> 01:52:37,320
As it floats around in space,

1454
01:52:37,320 --> 01:52:41,840
neutral hydrogen gas is constantly
producing radio waves

1455
01:52:41,840 --> 01:52:48,320
and, crucially, those waves are
always the same wavelength - 21cm.

1456
01:52:48,320 --> 01:52:51,880
And this meant that hydrogen
could be used to map

1457
01:52:51,880 --> 01:52:53,400
the galaxies that it fills.

1458
01:52:56,360 --> 01:53:01,560
By detecting the 21cm signal,
the Lovell Telescope helped reveal

1459
01:53:01,560 --> 01:53:05,280
the spiral structure
of the Milky Way

1460
01:53:05,280 --> 01:53:08,960
and produced detailed maps
of distant galaxies.

1461
01:53:13,480 --> 01:53:17,680
But galaxies aren't the only place
in the cosmos you find hydrogen gas.

1462
01:53:17,680 --> 01:53:19,600
During the dark ages
of the universe,

1463
01:53:19,600 --> 01:53:23,600
there were no galaxies,
but there was plenty of hydrogen.

1464
01:53:23,600 --> 01:53:28,560
So by detecting the 21cm signal
from these primordial gas clouds,

1465
01:53:28,560 --> 01:53:31,120
you could see the universe
in its infancy

1466
01:53:31,120 --> 01:53:33,880
and peer into the dark ages
themselves.

1467
01:53:40,600 --> 01:53:44,360
And by doing so, we'll be able
to watch dark matter

1468
01:53:44,360 --> 01:53:46,320
pull the cosmos together...

1469
01:53:48,240 --> 01:53:50,120
..and light up the heavens.

1470
01:53:53,600 --> 01:53:56,800
It was during the dark ages
that the hydrogen gas created

1471
01:53:56,800 --> 01:54:01,840
in the big bang was compressed into
stars and moulded into galaxies.

1472
01:54:01,840 --> 01:54:06,640
It was in this era that
the cosmos as we know it was born,

1473
01:54:06,640 --> 01:54:10,160
sculpted by the gravitational
pull of dark matter.

1474
01:54:16,640 --> 01:54:20,440
But the machine scientists
are building to map the dark ages

1475
01:54:20,440 --> 01:54:22,640
will see far more.

1476
01:54:24,120 --> 01:54:28,720
With an effective collecting area
of more than 200 times that

1477
01:54:28,720 --> 01:54:32,640
of the Lovell Telescope,
the square kilometre array

1478
01:54:32,640 --> 01:54:36,240
will be capable of mapping
a billion galaxies,

1479
01:54:36,240 --> 01:54:40,520
tracking the expansion and evolution
of the entire universe

1480
01:54:40,520 --> 01:54:42,600
more accurately than ever before.

1481
01:54:48,400 --> 01:54:50,800
And the hope is, that by doing so,

1482
01:54:50,800 --> 01:54:54,440
it will provide clues
to the nature of dark energy

1483
01:54:54,440 --> 01:54:57,040
and the universe's
ultimate fate.

1484
01:55:16,920 --> 01:55:21,960
Using hydrogen to map the cosmos
might just represent the final

1485
01:55:21,960 --> 01:55:27,520
chapter of humankind's exploration
of the universe using light,

1486
01:55:27,520 --> 01:55:32,760
a journey that began in earnest
some 400 years ago.

1487
01:55:32,760 --> 01:55:37,640
In December 1609, Galileo Galilei
began making observations

1488
01:55:37,640 --> 01:55:39,720
of the night sky.

1489
01:55:39,720 --> 01:55:42,800
Before then, what was thought to be
out there was essentially

1490
01:55:42,800 --> 01:55:44,680
a matter of faith.

1491
01:55:44,680 --> 01:55:48,400
The universe at large
lay unseen and unseeable.

1492
01:55:48,400 --> 01:55:50,240
But now, for the first time,

1493
01:55:50,240 --> 01:55:53,320
the nature of the heavens
was something knowable -

1494
01:55:53,320 --> 01:55:56,560
you simply had to look up
and see it.

1495
01:55:56,560 --> 01:55:59,720
The light captured in Galileo's
simple telescope

1496
01:55:59,720 --> 01:56:02,960
began a chain of discoveries
that would reveal

1497
01:56:02,960 --> 01:56:05,200
the true nature of the cosmos.

1498
01:56:13,640 --> 01:56:14,960
We've seen galaxies

1499
01:56:14,960 --> 01:56:17,920
billions of light years'
distance from the Earth.

1500
01:56:20,520 --> 01:56:23,800
And as we've come to understand
light's properties,

1501
01:56:23,800 --> 01:56:26,920
we've discovered the stuff
of which stars are made...

1502
01:56:30,760 --> 01:56:34,040
..and glimpsed the beginning
of the universe itself.

1503
01:56:41,280 --> 01:56:47,120
But the realisation that most
normal matter can't be seen

1504
01:56:47,120 --> 01:56:52,160
and the discovery of dark matter
and dark energy

1505
01:56:52,160 --> 01:56:57,840
mean that more than 99% of the
universe lies hidden in the shadows.

1506
01:57:00,880 --> 01:57:05,800
And as dark energy pushes
the galaxies ever further apart,

1507
01:57:05,800 --> 01:57:09,120
what few lights there are
will begin to go out.

1508
01:57:10,800 --> 01:57:13,720
As the universe expands ever faster,

1509
01:57:13,720 --> 01:57:18,200
one by one the galaxies
will disappear from view.

1510
01:57:18,200 --> 01:57:22,600
All that will remain visible will be
the stars in our own galaxy.

1511
01:57:22,600 --> 01:57:25,840
It would be almost as if we'd never
invented the telescope at all.

1512
01:57:27,320 --> 01:57:30,160
For the vast majority
of the universe's life,

1513
01:57:30,160 --> 01:57:33,760
there'll be no way of discovering
all the things we have about it.

1514
01:57:35,600 --> 01:57:39,600
So I don't feel disheartened
that so much of the cosmos

1515
01:57:39,600 --> 01:57:41,840
is hidden in the shadows.

1516
01:57:41,840 --> 01:57:43,000
The real miracle is

1517
01:57:43,000 --> 01:57:46,200
that when we first looked out into
the depths of space

1518
01:57:46,200 --> 01:57:48,880
there was any light to see at all.

1519
01:57:59,360 --> 01:58:01,880
Whether you want to
step into the light

1520
01:58:01,880 --> 01:58:04,440
or explore
the mysteries of the dark,

1521
01:58:04,440 --> 01:58:06,920
let the Open University
inspire you.

1522
01:58:06,920 --> 01:58:08,240
Go to...

1523
01:58:10,880 --> 01:58:13,640
..and follow links to
The Open University.