All language subtitles for Human Universe 3of5 Are We Alone 720p HDTV

The remote island of Rapa Nui
was once home

to Earth's most isolated
civilisation.

Famed for their stone effigies,
they lived here

for 1,000 years.

They called the island
the End of the Land.

Easter Island is, and feels,
isolated.

It's at least 1,500 miles
to the west,

to the nearest Pacific island -

five hours flying time

from the coast of South America
and Santiago in Chile.

But on Easter Day, 1722,

the civilisation's solitude
was shattered.

The arrival of a Dutch trading fleet

was the moment they came
face to face

with aliens.

It turned out to be
a solitary encounter.

When other ships returned
50 years later,

the civilisation had fallen.

It had been pure chance that
they'd met at all,

such was the island's isolation.

A pinprick in a vast ocean.

What then of us on Earth?

Are we a lone island of life

lost in a vast galaxy?

Think about this.

There are billions of habitable
Earth-like worlds

out there in the galaxy,

and yet we are alone.

Think about this.

There are billions of habitable
Earth-like worlds

out there in the galaxy

and we are not alone.

There are others.

One of these statements is true.

As far as we know,

we humans are unique in
the universe.

One species carried through
space on one tiny planet...

..amongst the Milky Way's
200 billion stars.

The question is,
is anyone else staring back?

NATIVE AMERICAN CHANTING

In New Mexico, the Navajo tribe
believe the stars

are home
to another consciousness.

Once a year they come together
to communicate with this entity,

in a healing ceremony
called Yeibichai.

It's a nine-day long festival
of ritual and chanting.

The most sacred of which
is the night-chant

performed by medicine men
in private,

never filmed,

and recorded just once.

HIGH-PITCHED VOCALISATION

This recording was made by Willard
Rhodes of Colombia University

in the early 1940s.

HIGH-PITCHED CHANT IN UNISON

And he could not have imagined what
was going to happen to it.

In 1977, the recording was
transferred onto four golden records

and encased in these covers.

Today only two of them remain
on Planet Earth.

The other two are bolted to
the sides of two space probes -

Voyager 1 and Voyager 2 -

and are now travelling into
deep space.

Accompanying the night-chant
are an eclectic mix

of images and sounds from Earth.

MOZART'S Magic Flute
GREETINGS IN MANY LANGUAGES

'As The Secretary General
of The United Nations,

'I send greetings on behalf
of the people of our planet.'

The Voyager's primary mission

was to explore the outer planets
of the solar system.

But adding the discs gave
the spacecraft an extra purpose -

they become Earth's emissaries,

carrying postcards to our
alien neighbours.

If they exist.

But there's more than that -

there's hope engraved into
this cover, as well.

Because here is a map,

this identifies the position
of the Earth in the cosmos.

These lines point to stars called
pulsars

which rotate with
a very specific rate.

So any alien civilisation
that captured this

would be able to find the
positions of the pulsars

and all of these lines
point back to Earth.

And there's also a means of decoding
all that information...here.

This is a picture
of a hydrogen atom -

the most common element
in the universe.

Now hydrogen atoms
radiate radio waves

with a very particular wavelength -
21 centimetres.

That gives you the distance scale -
the key, if you like - to this code.

So any civilisation that
knows anything about physics,

which is to say, any
civilisation worth its salt,

would recognise that
and be able to decode this.

They would be able to determine
the location of Earth -

they would be able to find us.

What a wonderful idea.

Voyager 2 is now 19 billion
kilometres into its journey.

But in galactic terms,
it's only just left home...

..because The Milky Way is a
billion-billion kilometres across.

In truth, the scientists
and engineers who made that record

knew that it wouldn't be found,
but that's not the point.

See the very act of launching it
out into space

expresses something deep,
something primal.

It's a feeling we all share -
I certainly feel it -

and that's that the Navajo
weren't alone in wanting

to talk to the people out there
amongst the stars.

'Wouldn't it be lovely to make
contact with another civilisation

'that has arisen and
evolved independently?'

Every hour of every day,

some among us scan the skies

listening for alien civilisations.

'The chance of success in,

'picking up messages directed
our way depends in large part

'on how serious we are

'at investigating such
possible signals.'

They call themselves SETI -

the Search
for Extra-Terrestrial Intelligence.

This is the Allen Array
in Northern California,

and it is SETI's most ambitious
instrument to date.

This collection of 42
radio telescopes

is optimised to detect signals
from extra-terrestrial civilisations

and it's exquisitely sensitive.

If a signal were beamed to us

with the sort of power
that we can generate

using our current technology

from any one of the million
or so sun-like stars

within 1,000 light years of Earth,
then this would hear it.

This experiment has yet to detect
a call from ET.

But they live in hope,

because SETI did once
pick up a tantalising signal.

HIGH-PITCHED TONE

On August the 15th, 1977,

a radio telescope called
the Big Ear detected a radio signal

from somewhere in the vicinity of
the constellation Sagittarius,

and this is the printout
from that night.

You can see that somewhere around
9.45pm Eastern Standard Time

a very bright radio signal
pulsed in.

It looks something like this -

you can sketch it out.

It was a pulse...

..with a width of around
72 seconds,

and a peak intensity -

the peak brightness -

was over 30 times the brightness

of the background radio emission
from the galaxy.

So it's fascinating.

HIGH-PITCHED TONE

And it came in on a wavelength
that SETI believes

an intelligent civilisation
might choose -

the same wavelength etched
into Voyager's golden disc.

The wavelength was 21 centimetres.

That's the wavelength of light,

of radio waves emitted
from hydrogen atoms.

So the whole sky glows
at that wavelength.

And, back in the 1950s,
radio astronomers speculated

that if an alien civilisation
wanted to communicate with us,

then they might well choose that
very special, natural wavelength

to send their message.

So surprising was this,

that when it was spotted
a few days later

by an astronomer called Jerry Ehman

he circled that pulse -

the 72-second flash of radio waves -

and wrote "Wow!" next to it,

so this has become known
as the Wow! signal.

Today, over 35 years after
the Wow! signal was detected,

there's still no satisfactory
explanation.

It doesn't seem to have been local -
a military signal or a satellite,

and, indeed, nobody's
supposed to transmit

at the hydrogen line frequency,

it's reserved for radio astronomy.

But scientific results
have to be repeatable,

and even though we've turned
our telescopes in the direction

of Sagittarius many times since,

nobody has ever heard anything.

So I suppose it has to remain
just an interesting anomaly.

For now, we remain in
magnificent isolation...

..a lone intelligence in a galaxy
that remains quiet.

But one man more than any other

is convinced that the great silence

will not last forever.

This is not some fantastical idea,

but a serious scientific quest.

RECORDING: 'It would be the
greatest bonanza in knowledge

'you could possibly imagine.

'A surprise for the
whole civilisation.'

Dr Frank Drake is SETI's
founding father

and has spent a lifetime listening.

So if I'd have asked you back
in the 1960s -

By the 21st century, do you expect
to have seen a signal? -

what would you have said?

I would have said,
"Yes, I expect to succeed."

Because we were doing a lot
of searching then.

Does that make you disappointed?

I'm disappointed, yes,
but not surprised.

What we've learned over the years

is that the searching has
to be MUCH more comprehensive,

than we realised 50 years ago.
Mm.

But we know it's still doable,

it just takes much more time
than we imagined.

Let me show you something.

This is an orchid.

A very peculiar and beautiful orchid,

very fragrant,

which actually only blooms
two days a year.

If you would've been here
two days ago,

you wouldn't have seen this orchid

and you might've thought
that this plant never blooms.

Well, so it is with SETI.

We've learned we must search over
and over and over through the years,

till we are in the right place
at the right time

to make the discovery.

What are they called?
This is called a Stanhopea.

It has "hope" in its name,

and we have hope that some day

we'll be in the right place
at the right time in SETI.

But Frank wasn't content just
to hang on the line

waiting for ET's call.

The equation starts out with
the rate of star formation...

In 1961, he devised what remains

a useful scientific framework

for considering how likely it is,

that we share the
galaxy with others.

I realised that there
were seven factors -

if you knew these factors,
and multiplied them together

it would give you a prediction

of the number of detectable
civilisations in our galaxy.

The end result is N -
Big N, as we call it -

which is the number of
detectable civilisations,

which in its own right
is a very interesting number.

But, also, it's very important in
guiding us in planning our searches.

The Drake Equation has become famous

because it gives us a tool
to continue the search.

It dares us to answer a series
of provocative scientific questions.

On the left-hand side is the number
we want to measure -

the number of intelligent
civilisations in the galaxy

that we can communicate with.

But then there's this group of terms

that really ask questions
about ourselves -

How likely is it for life
to arise on a planet?

How special is the Earth?

How special is the sun?

How special is the solar system?

How special is intelligence?

And then there's this last term, L,

which perhaps speaks to us most
profoundly of all -

the lifetime of
intelligent civilisations.

Do civilisations live
for a long or short time?

And if it's short,
what is it that destroys them?

Is it a natural disaster?

We do, after all,
live in a violent universe.

Or are they condemned
to destroy themselves?

We came up with the number 10,000
civilisations in the galaxy,

and that's sort of stood as
a reasonable number to this day.

50 years ago, Frank's number
was really an educated guess.

But our generation is replacing
his hunches with hard data,

so we can arrive at our own
solution to his equation.

And we can begin, as Frank did,

by asking just how rare

is our home planet?

And the key to understanding
that can be found

in the relationship we share
with our nearest star.

The Peruvian high Andes,

home to the mighty Incan Empire,

a lost civilisation that called
themselves the Children of the Sun

and worshipped the star
as their god.

Today, this connection remains
just as vital.

Carmen Pachako and her
family's livelihood

depends entirely on the sun.

WOMAN SINGING

Tucked away deep in the Sacred
Valley is an ancient salt mine.

To this day, hundreds of terraces
are tended by the local community.

Carmen's job is to keep
the whole system running.

She floods each terrace in turn with
saline water from the local spring.

The ponds are then left
to evaporate in the sun

leaving behind
a crust of salt crystals.

This place works because,
here on Earth,

the conditions are just right for
water to exist in three states -

there's liquid, that you can
see in the salt pools,

there's a vapour that can evaporate
up into clouds in the sky,

and a solid as ice and snow,

that you can see over there
on the tops of the mountains.

This constant recycling of water

from one state to another

happens here on Earth

because we receive just the right
amount of energy from the sun.

Can you imagine dragging the
Earth closer to the Sun,

heating it up,

then at some point the temperatures

would rise to 100 degrees
or greater,

all the water would evaporate
or boil away into the atmosphere,

and, in fact, if it got too hot,

then the water molecules themselves
would escape off into space.

Earth would be
a dry and barren rock.

Then, if you imagine dragging
the Earth further out,

further out into the solar system,

then temperatures would drop

and, eventually, they'd be so cold

that all the water would freeze out.

There would be no liquid on
the surface

and, indeed, no clouds
in the atmosphere.

So there's a region
in our solar system,

within which the Earth could orbit

and the conditions would still
be right for liquid water

to exist on its surface.

That region is known as
the habitable zone,

because all life on Earth
requires liquid water.

But in the high Andes,
water is relatively scarce.

So, for the Incas,
habitability was important too,

albeit on a more human scale.

This is a natural sink hole cut out
of the limestone of the high Andes.

At some point,
around 500 or 600 years ago,

the Inca modified it by cutting
these circular steps

into the side of the sink hole.

There's no consensus as to what
this structure was for,

but one of the more
widely accepted theories is

that it was an agricultural
research station.

So the Inca had built this

in order to generate a series of
different micro climates,

different temperatures,

different amounts of irrigation

to see which crops grew best

in which different conditions.

And so it is,
in our own solar system.

On one extreme lies Mars...

..80 million kilometres further
out from the sun,

its surface is mile after mile
of parched red rock...

..but the surface features
still recall a time

when they were rivers
and flood plains -

evidence of a long oceanic period
before the Red Planet

lost most of its water to space.

Closer to the sun, and on the
inner limits of habitability,

lies Venus.

About the same size as Earth,
its dense atmosphere

sees temperatures soar to over
400 degrees Celsius -

far too hot for water.

But, in 2008, the Venus Express
spacecraft detected a signature

highly suggestive that water once
flowed on the surface here, too.

For a brief moment, billions of
years ago, not one, but three worlds

with oceans and rivers
orbited the sun.

THUNDER AND HEAVY RAIN

If alien life ever existed
on Mars or Venus,

no trace of it has yet been found.

What is certain, though,

is that our star is central
to the story of life

in our solar system.

But the sun is just one of billions
of stars in the Milky Way...

..so in our hunt for aliens,

we must determine exactly how many
of those stars

are capable of supporting life.

I'm going to start with that one -
that's called a red dwarf.

Come here.

This could be the sun.

Are you OK for a big one?

There you go.

This is a little tiny white dwarf.

There's a little white dwarf.
There you go.

Gracias. Thank you.

HE LAUGHS

Can you hold that?
There you go!

So there are 200 billion stars
in our galaxy

of many different types,

and we've got the whole menagerie
represented here -

enormous red stars,

enormous blue stars,

tiny red stars,

tiny white stars.

Now, science is not just about
looking at the stars

and saying, "There's a nice yellow
one. There's a nice blue one."

In fact, the very famous physicist
Ernest Rutherford

who discovered the atomic nucleus
in Manchester

once said all science is either
physics, or stamp collecting.

Stamp collecting is just looking
at the stars,

physics is arranging them
in a pattern

and trying to understand the
pattern. That's what we'll do now.

We're going to make what's called
the Hertzsprung-Russell diagram.

Wish me luck.

You guys go all the way over there.

You go there. Yeah.

You go right over there.

You come in here.

You come here.
You, as well, come here.

There's an orange one.

You go there.

There.

Exactly there.

The Hertzsprung-Russell diagram
allows us to organise the stars

according to their colour
and brightness.

When laid out on a graph,

nearly all the galaxies'
different stars

fall neatly onto a line.

This line is called
the Main Sequence -

these are most of the stars
in the galaxy,

they're burning hydrogen into
helium in their cores.

So this is our familiar sun,

the sun that we see
in the sky every day,

and it sits on the Main Sequence.

If you come with me.

Somewhere around here

a yellow star, an average star.

The stars either side of the line

are reaching the end of their lives.

The red giant stars have run
out of hydrogen to burn,

and are swelling in size,
engulfing any nearby planets.

So we can discount the red giants.

Over there. Over there.

Others, like the blue giants,

are so massive that they burn
their fuel quickly...

..collapsing and illuminating the
galaxy in a supernova explosion.

So the search for stars with
habitable planets

must focus on the Main Sequence.

Complex life took three billion
years to evolve -

it wouldn't have time to evolve
around these brightest of stars,

the bright, short-lived stars.

So the white stars can go.

Go on.

If you want to look for planets
that could support life

and civilisations, you'd look at
the sun-like stars -

the G and K stars,
the orange and yellow stars.

These make up one fifth
of all the stars in our galaxy.

But new research suggests
that there's another type of star

that might significantly expand
this number.

Could we find planets around
red dwarfs

that can support complex life
or civilisations?

Well, the jury's still out -
there are big problems.

Red dwarfs - certainly early on in
their lives - are very active stars.

So even though they're dim,

they throw out vast amounts of
radiation,

which many scientists think would
sterilise planets

and prevent life from appearing
on those planets.

But others think that with the right
atmosphere,

a planet could cope with
the radiation.

If they're right, then our odds
of being alone shorten immensely...

..because red dwarfs are by far

the most numerous stars in
our galaxy.

So our theoretical search for life
suggests

that there are billions

of potentially habitable star
systems...

..and it's to these,
that we must now look.

Trailing behind Earth,
lies a space telescope

nearing the end of its mission

to seek out new worlds.

Known as Kepler, it has stared
unblinkingly at 150,000 sun-like

and red dwarf stars
for over four years.

Kepler looked in the direction
of the constellation of Lyra.

He studied a small patch -

about 0.3% of the entire sky -

and about 3,000 light years deep.

It was looking for a tell-tale,

minute dip in light
from a distant star

that would reveal a new world.

A planet passing in front
of its star.

It found 603 planets,

of which ten were Earth-like

and received about the same amount
of energy from their parent star,

as we do here on Earth -
in other words -

ten Earths within the
habitable zone.

Kepler has proved that our
life-giving planet is not alone...

..and there may be far more out
there waiting to be discovered.

Kepler can only see planets that
pass across the face of the star

as seen from Earth,

and solar systems have many
different orientations.

Also, because it only observed
for around four years,

it can't see planets that have
orbital periods

that last longer than four years.

But, we can correct for all
those things.

And when we do that, we find that
there are 10,000 Earth-size planets

in orbit around Kepler's stars.

But that's just in that
tiny piece of sky -

10,000 Earths...just there...

in a small volume of space.

To get the full picture,

we have to extend that
over the entire sky

and when we do that, we find

that there are ten billion
habitable worlds out there

in the Milky Way galaxy
around sun-like stars.

And there could be even more.

The red dwarfs,
the so-called M-class stars...

If we admit the possibility
that M stars

might also have habitable
zones around them,

and we can multiply that
by three or four,

that means there might be 30 billion
Earth-like worlds out there

in our galaxy.

We've long suspected that in
a galaxy with so many star systems

Earth couldn't stand alone.

But now we're on the verge of
discovering

not just a handful of other Earths,

but billions of worlds

where water flows freely on
the surface,

and rain falls down from
the skies -

billions of homes for life.

For the first time in human history,

we have the possibility of
glimpsing our planet's twin.

A second Earth.

Perhaps, just perhaps,
we have company.

And it's a lovely thought
that maybe one day

Voyager, our galactic message
in a bottle,

might encounter such a planet.

MUSIC: Johnny B Goode by Chuck Berry

# Deep down in Louisiana,
close to New Orleans... #

SONG FADES INTO DISTANCE

The sounds of Earth may wash up
on a distant shore.

The question is, will there
be anyone around to listen?

The final turns in the Drake
equation shift the focus

from astronomy to biology...

..and they begin by tackling

one of the most
enduring mysteries in science -

..the question of how often life

spontaneously arises on a planet.

The problem is that we only
have one example of life

anywhere in the universe.

Now you may think that's a
strange thing to say -

there's me, there's fish,

there's coral, insects -

all the living things
you could imagine.

But the remarkable thing is
we're all chemically the same,

in a biochemical sense.

We all share the same
basic building blocks.

And that's because
we're all descended

from the same common ancestor
3.8 billion years ago.

This is one of the very few places
where you can see living structures

reminiscent of early life on Earth.

Well, this alien-looking landscape
that you see here

is made of structures
called stromatolites.

What I'm allowed to do
is break a piece off...

It looks like rock.

But, you see...
it's not really rock,

it's covered in a bit of algae
and it's quite soft to the touch.

They're formed by the action
of bacteria,

specifically, in this case,
Cyanobacteria.

In fact inside Cyanobacteria,
it's now known,

is the place where photosynthesis
first evolved.

Structures like these would have
been the only visible signs

of life on Earth

for perhaps two billion years.

This is what the primordial ocean
would have looked like.

Whilst there are no living examples

of what came before
the stromatolites,

we've found clues
that life was around even earlier.

There's very strong evidence

that life on Earth began around
3.8 billion years ago,

which is pretty much as soon as
it could have begun.

So that's taken as one piece
of evidence to suggest that life,

given the right conditions,
is inevitable.

Indeed, there are many scientists

who think that life may be
created in a laboratory

in the not-too-distant future.

So that means that most scientists,
I think,

suspect that we will find simple
life somewhere out there.

It's a dizzying thought.

If they're right, and habitable
planets do generate life,

then with billions of such planets
out there in the galaxy,

surely there must've been
a second genesis?

But we must be careful, because
the story of life on this planet

shows that the transition from
single celled life to complex life,

may not have been inevitable.

Life had to overcome
a significant bottleneck.

Everything we would call a complex
living thing today,

shares the same basic structure -

it's built out of cells
called eukaryotic cells.

They're, roughly speaking,
the same in every tree,

every blade of grass, every fish,
every insect

and even in every piece of skin
in my knee -

every cell in my body -

is a eukaryotic cell.

And they're extremely different
to the simple cells,

the bacteria that we've seen
photosynthesising

in the stromatolites.

So how did those cells
come to exist?

One popular theory is that it was
two simple cells merging together

that formed what
we'd recognise today

as the complex cells in your body.

Somehow the invader managed to
survive the host cell's defences.

And when the cells reproduced,
they reproduced together.

We still struggle to understand
how this happened,

because it's incredibly unusual.

Here's the point - because every
complex animal and plant

shares that same
basic building block

we are very confident that
it only happened once -

somewhere in the oceans of
the primordial Earth.

Biologists call this merger
the Fateful Encounter hypothesis,

for a good reason.

It seems that that kind of merger
between two simpler life forms

may be extremely rare.

The idea that one organism
can get inside another

and doesn't kill it -

that they both survive
to produce something

that's actually capable of
doing magnificent things,

things that are far more
complicated and wonderful

than the two simple building blocks
can manage on their own

seems to be...
It seems to be unlikely,

a "fateful encounter".

If that's the case, then that may
suggest that complex life -

that intelligent life - is extremely
rare INDEED in the universe.

Maybe there is a profound bottleneck

in the evolution of complex
life in the Milky Way.

And perhaps this is why
we continue to bear

the great silence.

But life here, did squeeze through.

And the story of our planet
suggests that after it did

all hell broke loose.

So given the right conditions

then 550 million years ago,

at least, here on Earth,

the Cambrian explosion happened

and all the...what Darwin called

"endless forms most beautiful"

that we see on this planet,

emerged almost in the blink
of an eye

including one civilisation,

an intelligent civilisation,

that's managed, ultimately,

to begin to launch spaceships

beyond our solar system into
the stars,

and, it seems,

has also produced aquatic pigs!

Such is the wonder

of evolution by natural selection

which I would argue

is a law of nature.

Anywhere you get the right
conditions

you will get...

You'll get animals like this.

Just 100 million years after
the start of the Cambrian explosion

our ancestors crawled from
the oceans...

And in the cosmic blink of an eye,

evolution threw up an
inquisitive species

prone to asking questions
about the world around it.

If you're interested in
communicating

with an alien intelligence

then I suppose a natural question
arises, which is -

Are there animals on Earth
other than ourselves,

that are intelligent enough
to communicate with?

Now, early on in the history
of SETI

there was one animal in particular
that got an iconic reputation -

in fact, Frank Drake, Carl Sagan,
John Lilly,

and the other SETI pioneers

called themselves, named themselves,
after this animal -

they called themselves
The Order of The Dolphin.

And, indeed, in the '60s and '70s,
John Lilly, in particular,

carried out a series of
increasingly eccentric experiments

to try and communicate
with dolphins.

Dolphins remain at the forefront

of inter-species
communication efforts.

Where's your giggle?

VOCALISES A RAPID STUTTER

BLOWS WHISTLE

Good!

So far, there have been
no meaningful conversations -

however there is a dolphin

in the Grassy Key Research Centre
in Florida

that demonstrates
a form of intelligence.

He's a 13-year-old bottlenose
male called Talon.

Good boy.

OK, Brian.

We're going to find out whether
Talon can understand

the abstract concept
of more or less.

OK, so what we're going to do,
Brian, is put these boards up

and Talon's going to choose the one
that has the least amount of dots.

The dots are randomised
by the computer,

and the side of the correct answer.

So put this one up first.

Oh, yeah.
So always the left first?

Always the left first.

That way if Talon's watching
from the other side -

even though we have these little
blinders -

we're not giving him any kind
of cues,

on which side the correct
answer might be on.

Left board up first...

TALON SPLASHES
BRIAN CHUCKLES

Yeah. Hi, buddy!

That is Talon. He is looking.

Most humans acquire the ability
to distinguish between

numerical amounts fairly early
in life -

at around the age of six months.

Talon!

The experiment tests if dolphins
share the same cognitive ability.

Less.

Yes!

Yeah, you can move our hands back
out... Excellent!

Less.

Yes!

Yeah, good job, Talon!

This is quite an interesting example

because this one's five and
this one is six

but because of the difference
in area, and placement,

I think it does take a while even
for a human to look at it.

Probably, what we'd do,
is count them.

Less!

Yes.

Straightaway.
You see how quickly he chose that?

That was impressive, actually.

He was actually going for that board
before she even gave him the signal.

It's not known how Talon
processes the information,

but his success rate is interesting.

All right. That was awesome!

Good job, Talon.

What this experiment shows
is that dolphins exhibit

a kind of behaviour that
we associate with intelligence -

scientists call it
relative numerosity -

the ability to say which board
has got more dots on it.

And that's interesting,

because if you ask the question -

Where is the common ancestor between
me and a dolphin?

- you've got to trace the timeline
back about 90 million years.

That's when dinosaurs
were on the Earth.

So our common ancestor certainly
wouldn't be able to do that,

wouldn't exhibit that
kind of intelligence.

So intelligence of a sort has arisen
actually many times on Earth.

But from the point of view of SETI -

the Search for Extra-Terrestrial
Intelligence -

that doesn't matter, at all.

We need a civilisation
that can do astronomy,

that can build radio telescopes
or spacecraft,

that can leave a signature

or send a message
that we can recognise.

This is perhaps the
greatest mystery.

We simply don't know how often
complex, intelligent life...

..develops into a technologically
advanced civilisation.

# I watch the moon
hang in the sky

# I feel the traffic rushing by

# Freight train engine in the night

# I'm still here

# Waiting for you

# I watch the moon hang in the air

# I feel the cool breeze
through my hair

# My eyes' blind by her light's glare

# I'm still here

# Waiting for you... #

Just three decades ago,

we knew of no planets beyond
our solar system.

Today we've found thousands of them

and suspect that there
are billions more.

Our knowledge of the origin of life
has also deepened,

as has our understanding
of our own evolution.

# I'm still here

# Waiting for you... #

All this leads me to think
that there must be

at least simple life

elsewhere in the galaxy.

# For all the pain we saw... #

Yet the silence persists.

We remain alone.

# And I'm still here

# Waiting for you... #

It's a puzzle,

but I think Easter Island

may offer us an answer.

The civilisation here survived
in isolation for over 1,000 years...

..until, 300 years ago,

it vanished,

leaving behind only a handful

of stone statues.

What seems to have happened

is that they used the resources
of this tiny island,

chopped all the trees down,

they killed all the animals,
they overused the land,

the population grew too big,

they started warring,

it's thought that they
toppled their own statues

in battles between rival villages

and so they essentially
destroyed themselves.

This collapse speaks powerfully

to the last factor in
the Drake equation -

the length of time
a civilisation lasts.

Imagine there are,

and have been thousands or
millions of civilisations

in the history of the
Milky Way galaxy -

and imagine their lifetimes
are short.

No matter how they're distributed
in space and time...

..they never overlap.

And I think this is quite
a sobering thought -

the reason we have never

and WILL never hear

from any other civilisation,

is because none of them
ever last long enough

to contact each other.

But I don't think that's necessarily
the answer.

I think the story of the
Rapa Nui people

hints at something else.

Their island was the
final destination

of the human colonisation of Earth.

A journey that took us from
our origins in East Africa

to across the planet

in less than 60,000 years.

And I think a sufficiently
advanced alien civilisation

would mirror this
process of colonisation.

In the 1940s, the mathematician
John von Neumann

thought about the possibility
that we could build

self-replicating machines -

he called them
Universal Constructors.

So these would be space probes that
could fly out to a solar system,

land on an asteroid, or a moon,
or a planet,

and then mine the resources

they needed to copy themselves.

In this way von Neumann's
replicating machines could spread

across the entire galaxy...

..just as humans spread across
the Earth.

Think about how the Polynesians
colonised the Pacific islands -

they sailed across the ocean,

they landed on some uninhabited
rock like Easter Island,

and they used the resources
they found there

to make copies of themselves.

We call it breeding.

Now, modern computer models suggest

that such a strategy would allow an
advanced alien civilisation

to colonise the entire Milky Way
galaxy in only ten million years -

the blink of an eye in cosmic time.

All this sounds
like science fiction,

but if they're possible in principle

then you have to construct
some kind of argument

as to why we don't see them,

and I can't construct one.

It bothers me.

It follows that if such an advanced
civilisation had existed,

we'd know about it.

We'd have encountered one
of von Neumann's machines.

And I think that suggests
that there is only one

technologically advanced
civilisation in the Milky Way,

and there only has ever been one -
and that's us.

We are unique.

Could it be that we alone

have passed through the
evolutionary bottlenecks

that seem to have
prevented civilisations

from arising elsewhere?

If the answer is yes,

we are the only
intelligent civilisation

in the galaxy,

and that makes us indescribably
precious and valuable.

We are the only island of meaning
in an infinite sea of lonely stars.

And without wishing to be overly
romantic or sentimental about it,

that would seem to me to confer
on us a responsibility -

the responsibility to act together
as a civilisation to survive

and, ultimately, to explore
those stars.

"As The Secretary General
of the United Nations

"I send greetings on behalf
of the people of our planet."

SMALL CHILD: 'Hello, from
the children of Planet Earth...'