Original subtitles

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...'