All language subtitles for C2C918CAC123DB0BDB884978A21CB500_eng

The dream of becoming a citizen
of the cosmos was born here...

more than two millennia ago,
in the city of Alexandria...

named after and conceived
by its dead conqueror, Alexander the Great.

The Ptolemys...

the Greek kings who inherited the
Egyptian portion of Alexander's empire...

built this library
and its associated research institution.

Rarely, if ever, before or since,
has there been a government...

that was willing to spend so much
of its gross national product...

on the acquisition of knowledge.

And it paid off, big time.

Every ship entering Alexandria's harbor
was searched...

not for contraband, but for books
that might be copied and stored here...

in what was then
the greatest library on Earth.

Here, Eratosthenes,
one of the chief librarians...

accurately calculated the size
of the Earth and invented geography.

Pythagoras.

Hypatia.

Euclid.

Euclid set forth the
precepts of geometry...

in a textbook that remained in use
for 2300 years.

The Old Testament Bible
comes down to us mainly...

from the Greek
translations made here.

The original manuscripts
of the masterpieces

of Greek comedy
and drama, poetry...

science, engineering...

medicine and history...

the total work product of the awakening
of ancient civilization, was kept here.

Estimates vary on the total
number of scrolls.

They range from 500,000
to nearly a million.

And all of it...

all of this is but a tiny fraction
of the information that you have...

at your fingertips at this very moment.

The collective knowledge of our species...

our own electronic Library of Alexandria...

may be accessed by anyone who has a device
and the interest and the freedom to do so.

This was not true in Alexandria,
where knowledge belonged to the elite.

So in the fourth century A.D.,

when the mob came to destroy the library
and the genius of classical civilization...

there were not enough people to defend it.

What will happen
the next time the mob comes?

We've come a long way together...

traveling from deep inside
the heart of an atom...

clear out to the cosmic horizon...

and from the beginning of time
to the distant future.

I think we're ready
to perform an experiment.

It's not the kind of experiment
that requires a laboratory.

You can do it in your head.

It's called a thought experiment.

Pick a star, any one
of the hundreds of

billions of stars in our
Milky Way Galaxy...

which is just one galaxy out of the
hundred billion in the known universe.

How about that star?

Or that one?

Okay, this one.

It's orbited by dozens
of planets and moons.

Suppose, on one of them,
there lives an intelligent species...

one the ten million life-forms
on that planet.

And there's a subgroup of that species
who believe they have it all figured out.

Their world is the center of the universe,
a universe made for them.

And that they know everything
they need to know about it.

Their knowledge is complete.

How seriously would you take their claim?

Our ancestors believed
the universe was made for them.

It was natural to assume
that we were at the center.

After all, it looks like the sun
and stars all revolve around us.

We still speak of the sun rising.

The architecture of our language, myths and
dreams comes from that prescientific age.

This is our planet as it was known then,
just before Columbus set sail.

This first globe of the Earth
was cutting-edge...

when Martin Behaim
made it in 1492.

Like everyone else...

he believed that the jigsaw puzzle
of geography was complete.

There were three continents,
Europe, Africa and Asia.

And only the great world ocean
in between.

Behaim had no clue that North
and South America even existed.

It's easy to feel smug, right?

Well, the fact is
Martin Behaim knew

infinitely more about
his world, the Earth...

than we know about ours, the universe.

A recent lesson in humility
will serve to illustrate.

In 1912, Victor Hess made a series
of voyages into the sky above Austria...

and found the thing
that scientists love best...

a mystery that defied understanding
in terms of conventional scientific wisdom.

And even today, a century later...

we are still searching for a complete
explanation of what Hess found.

A new kind of energy had recently
been discovered, radioactivity.

It was given off by certain elements,
like radium.

But it was also found in the air,
even far away from radioactive rocks.

It was everywhere.

Where did this strange energy come from?

No one knew.

Hess suspected that it might come
from above the Earth.

To test his hypothesis, he carried
radiation detectors high into the sky.

During a risky ascent
in a hydrogen balloon...

he attained an altitude
of more than three miles.

When he reached the thin, cold,
upper half of the atmosphere...

he found that the radiation was more than
twice as strong as on the ground.

The radiation must be coming
from above.

That's why its intensity
was weaker on the ground.

The Earth's atmosphere
was absorbing most of it.

Some thought that the radiation
might come from the sun.

To test that idea, Hess timed one of his
ascents to coincide with a solar eclipse.

But the eclipse had no effect
on the radiation.

Hess also found that the radiation
was just as strong at night as in daylight.

It was coming from above
but not from the sun.

What Hess did not know
was that the solar wind...

doesn't move that quickly.

And so, for the wrong reason,
he came to the right conclusion.

Hess had discovered cosmic rays...

showers of subatomic particles
that crisscross the universe...

at nearly the speed of light.

Without the shielding effect
of the atmosphere, they would be lethal.

Some cosmic rays can carry as much energy
as a bullet fired from a rifle.

It would take decades
to trace those cosmic rays back...

to a death of unimaginable violence.

The cosmic rays that Victor Hess detected
in the skies above Austria...

posed a mystery to scientists.

Radioactivity in minerals on Earth,
like uranium ore...

comes from the disintegration of atoms.

But cosmic rays were of a different nature.

They were far more powerful
than anything known in Hess's world.

Scientists wondered for two decades
what could possibly produce cosmic rays.

Enter Fritz Zwicky, the most
brilliant man you've never heard of.

In 1933, he and a colleague discovered
that some stars flare up...

to become as bright as their entire galaxy
for a few weeks, before fading out again.

Fritz Zwicky was the first person
to understand what just happened.

He correctly surmised
that this is the way a massive star dies.

It blows its guts out into space.

He called this kind of stellar death
a supernova.

And predicted that the
dying star would shrink

from about a million
miles across to only 10.

This corpse would be so dense
that a single grain of it...

would weigh as much
as the Great Pyramid in Egypt.

It would consist almost entirely
of subatomic particles called neutrons.

So he named these bizarre objects
neutron stars.

And 35 years after Zwicky predicted
their existence...

astronomers began to find them.

We call them pulsars when they spin rapidly
and emit regular pulses of radio energy.

Supernovas and neutron stars could account
for a wide range of cosmic rays...

but not the most energetic ones.

Nothing yet known to science
can explain them and we're fine with that.

It's one of the things
I love about science.

We don't have to pretend
we have all the answers.

Zwicky also came up with the idea
that the gravity of a galaxy...

warps the fabric of space around it
to act like a lens.

This distorts and magnifies light from
any other galaxy lying directly behind it.

So astronomers on Earth...

would see multiple images
of that same distant galaxy...

deformed as in a funhouse mirror.

Forty years after this prediction,
we started finding them too.

And Zwicky made yet another discovery
back in the 1930s.

While studying the Coma cluster
of galaxies...

he noticed something funny
about the way they moved.

The galaxies were going way too fast...

so fast that they should've been
flying apart from each other...

because all the stars in all those
galaxies had far too little gravity...

to hold the cluster together.

Zwicky thought that something else
must be binding them to each other.

That mysterious missing component...

would have to weigh something like
50 times as much as the stars themselves.

But no one paid much attention
to this wild notion.

Just another one of Zwicky's crazy ideas.

In our solar system,
the innermost planet, Mercury...

moves much faster than
the outermost one, Neptune.

And that makes sense, right?

The harder you push or pull
on something, the faster it goes.

The sun's gravity weakens
with increasing distance...

so the planets that are farther
from the sun move more slowly.

Everyone expected that the outermost stars
in a galaxy would act the same way.

Most of the stars are concentrated
towards the center...

so their collective gravity
pulls on the other stars...

the same way the sun pulls on the planets.

But in the 1970s, when astronomer Vera
Rubin studied the Andromeda Galaxy...

she discovered that the outer stars
obeyed no such rule.

Unlike the outer planets
in the solar system...

the outer stars in the galaxy
were all going at the same speed...

as the stars that were closer in.

And they were moving way faster
than expected.

"That's funny," Vera thought.

"There must be something weird
about the Andromeda Galaxy."

So she looked at another galaxy.

Same story.

And another.

Vera studied 60 galaxies...

and found that all of them seemed
to be violating the Law of Gravity...

a core principle of physics.

After some initial healthy skepticism...

her colleagues looked for themselves
and found that Vera was right.

It's not that Isaac Newton
had gotten the Law of Gravity wrong.

Vera Rubin had discovered that the gravity
of something massive and invisible...

was forcing the stars to go fast.

And then, someone remembered
crazy old Fritz Zwicky...

and the unknown source of gravity
in the galaxy clusters...

that he called dark matter, back in 1933.

Vera Rubin had verified the existence
of a new, much larger cosmos.

And just like the one we thought we knew,
it was filled with mystery.

Dark matter is completely unobservable,
except for its gravitational effect...

which makes visible stars and galaxies
move faster.

It's nature is another deep mystery.

Rubin had provided the evidence
for an invisible universe...

nearly 10 times more massive
than the one we thought we knew.

It was as if we had been standing
on the seashore at night...

mistakenly believing that the froth on
the waves was all there was to the ocean.

Vera Rubin looked at the stars and realized
they were merely the foam on the waves...

and that the greatest part of the ocean
remained unknown.

But wait.

It gets crazier.

Our Milky Way galaxy,
a few hundred billion stars...

plus the clouds of gas and dust,
the stuff of once and future stars...

and about a hundred billion other galaxies,
all of that...

including those uncounted billions of
trillions of planets, moons, and comets...

amounts to only five percent
of what is actually there.

Because there's a bigger unsolved mystery
than dark matter, dark energy...

which makes up even more of the cosmos
and drives its expansion.

And it was Fritz Zwicky's supernovas
that lit the way...

to the revelation of its existence.

In one scenario,
a star consumes all of its nuclear fuel...

then cools and suddenly collapses
under its own gravity.

The star rebounds in a massive explosion...

leaving behind a neutron star
or a black hole.

Since the mass of the original star
can fall within a wide range...

its peak brightness as a supernova
can also vary widely.

So you can't tell how far away it is
just from how bright it looks.

A relatively nearby supernova...

might appear just as bright as one
that was more powerful but farther away.

But there's another kind of supernova
that comes in only one strength.

It marks the violent
grand finale of a tango...

danced by a giant star and a dwarf.

As the two stars orbit
closely around each other...

the giant sheds its outer layers of gas
onto the dwarf.

When the added weight
becomes too much for it to bear...

the dwarf detonates
like a stupendous thermonuclear bomb.

For a few weeks,
the brilliance of such a supernova...

rivals the combined light
of all the stars in its galaxy.

This kind of supernova always has
the same maximum power output...

about five billion times brighter
than our sun.

With big telescopes, we can see
them in galaxies very far away...

out toward the edge
of the observable universe.

Because all such supernovas
have the same wattage...

they're ideal tools for measuring distances
to the farthest reaches of the universe.

We call them standard candles.

In 1929, Edwin Hubble discovered
that the universe is expanding.

The distant galaxies are drifting away
from one another.

Later, we learned that the expansion
began some 14 billion years ago...

with the explosive birth of the universe,
the Big Bang.

Everybody assumed that the rate of
expansion would be slowing down...

due to the mutual pull of gravity
between all the parts of the universe.

If there is enough dark matter...

its gravity would eventually bring
the expansion to a stop.

And the universe
would then fall back on itself.

In that case, everything would eventually
collapse in a big crunch.

On the other hand,
if the universe had less dark matter...

the expansion would continue forever,
just getting slower and slower.

Two competing teams of astronomers...

were observing those supernovas
in distant galaxies.

It turned out to be another one
of those that's-funny moments.

In 1998, both teams independently came
to the same conclusion...

the expansion isn't slowing down at all,
it's speeding up.

This means the universe
will continue to expand forever.

There seems to be a mysterious force
in the universe...

one that overwhelms gravity
on the grandest scale...

to push the cosmos apart.

Most of the energy of the universe
is bound up in this unknown force.

We call it dark energy.

But that name, like dark matter,
is merely a code word for our ignorance.

It's okay not to know all the answers.

It's better to admit our ignorance than
to believe answers that might be wrong.

Pretending to know everything closes the
door to finding out what's really there.

Tonight, our ships sail
into even more exotic waters.

Come with me.

Only two of our ships have ventured into
the great dark ocean of interstellar space.

The longest odyssey in all of history...

was launched back in 1977...

NASA's Voyager 1 and 2.

The Voyagers move about 40,000 miles
an hour.

They gave us our first close-up look
at Jupiter's Great Red Spot.

A hurricane three
times the size of Earth...

and one that's been raging
since at least 1644...

when it was first observed.

For all we know,
it could be thousands of years old.

The Voyagers discovered the first active
volcano on another world...

on Jupiter's moon Io.

And an ocean beneath the icy surface
of the moon Europa...

with at least twice as much water
as we have here on Earth.

The Voyagers dared to fly
across Saturn's rings...

and revealed that they were made
of hundreds of thin bands...

of orbiting snowballs.

On Saturn's giant moon Titan...

Voyager detected an atmosphere
four times denser than Earth's.

That hinted at the existence
of hydrocarbon seas on Titan...

which we later confirmed.

Voyager 2 gave us our first portrait
of the outermost planet, Neptune...

where the winds roar
at a thousand miles per hour...

and its moon Triton...

with geysers of boiling nitrogen...

shoot five miles high.

Voyager successfully completed its mission
of discovery to the outer planets...

but its odyssey into the darkness
was just beginning.

Thirty-five years after its launch...

Voyager 1 became the first of our
spacecraft to enter an uncharted realm.

The sun is constantly shooting out streams
of charged particles in all directions...

moving at a million miles an hour.

This solar wind
blows a vast magnetic bubble...

our heliosphere
that extends beyond the outer planets.

It pushes out against the thin gas
of interstellar space.

There's a border where one ends
and the other begins.

Voyager 1 reported back to Earth...

that its detectors were being pummeled...

by more and more cosmic rays.

Until then, we didn't know
where the interstellar ocean began.

Voyager 1 pressed on past a boundary
we had never crossed before.

The heliosphere shields us
from most of the deadly cosmic rays.

When stormy solar winds blow,
this zone of protection grows...

in calm solar weather, it shrinks.

When a star goes supernova
in our galactic neighborhood...

the debris from the exploded star...

pushes the heliosphere
back towards the sun.

If it's strong enough to push it back
to Earth's orbit...

our planet gets a radioactive
bath of supernova debris.

Luckily, this doesn't happen very often.

The last one was perhaps
2 million years ago.

A neighboring star
explodes a million years...

before there's even such a thing
as the human species.

How can we possibly know this?

Because the dying star left its traces
miles below the surface of the ocean.

Manganese nodules...

small rocks like this one...

are scattered over much
of the deep sea floor.

They grow very slowly.

I'm talking a millimeter
in a million years...

layer upon layer.

These nodules grow in partnership
with bacteria...

by taking up minerals
dissolved in the seawater.

A supernova produces a radioactive form
of iron...

unlike anything made
by natural processes on Earth.

Researchers found
telltale traces of this iron...

in a thin layer below the surface
of the manganese nodules.

They used the known rate of growth
of the nodules...

to date that layer...

and to connect it to the fate of a star
that perished eons ago.

The difference between seeing nothing
but a pebble...

and reading the history of the cosmos
inscribed inside it...

is science.

The interstellar ocean is dark
and deep.

Out here, the sun is just
the brightest star in the sky.

Yet the Voyagers maintain
their regular communications...

with NASA's Jet Propulsion Laboratory...

talking back and forth
across the light-hours

that separate these
ships from their home port.

No other objects touched by human hands
have ever ventured this far from home.

Even after they lose their ability
to respond to our command...

the last and, by far, the longest phase
of the Voyager mission will begin.

Back in 1979,
when both Voyagers rounded Jupiter...

its massive gravity acted as a slingshot,
flinging them out of the solar system...

to travel among the stars of our galaxy
for a billion years.

Carl Sagan recognized
that the Voyager mission...

offered two free tickets
to something approaching eternity.

He assembled a small team
to create a message to any civilization...

that might, one day,
encounter the derelict spacecraft.

Twenty-six centuries ago,
the Assyrian king Esarhaddon wrote:

"I had monuments made of bronze
and inscriptions of baked clay.

I left them in the
foundations for future times."

These hieroglyphics continue
that ancient tradition.

They are inscribed on
the cover of a message...

designed to be read
by the beings of other worlds and times.

What could we possibly have in common
with an alien civilization...

with its own separate
evolutionary history...

and one so far advanced beyond us
that they can patrol interstellar space?

One thing at least.

A universal language...

science.

It's hard to break the bonds of gravity.

You can only sail the cosmic seas
if you speak mathematics and physics.

Hydrogen is the most common element
in the universe.

The electron in a hydrogen atom...

flips the direction of its spin
at a constant rate or frequency.

Hydrogen atoms are
like tiny natural clocks.

Tick. Took.

Now we have a unit of time in common
with the extraterrestrials.

This will come in handy when we get
to the next level of the message.

Here's our return address in space.

Pulsars are
rapidly-spinning neutron stars

that give off regular
bursts of radio waves.

You can set your clock by them.

The sun is at the center of this diagram...

and the lines point to
the 14 nearest pulsars.

A simple code labels each pulsar
with its unique frequency...

using the tick took of
the hydrogen atom as the unit of time.

So alien astronomers
could use this diagram...

to locate the home star of the Voyager
spacecraft in our galaxy.

They could also tell how long ago
the spacecraft was launched.

And that's important...

because the Voyager record has a projected
shelf life of a thousand million years.

Become an extraterrestrial archaeologist
for a few moments.

An artifact has been fished out
of the interstellar ocean.

It was made by beings
that lived about a billion years ago.

What would you make
of them and their world?

They've sent us their music...

and greetings in 59 human languages.

Hello from the children of planet Earth.

And one whale language.

And a sound essay that includes
a Saturn V rocket launch.

A mother's first words to her newborn baby.

Come on now. Be a good boy.
Be a good boy.

The brain waves of a young woman
newly fallen in love.

And the sound of a pulsar.

All of that will live for a billion years.

How long is a billion years?

If you compress all the time
since the Big Bang...

the explosive birth of the universe,
into a single Earth year...

a billion years is about
one month of that year.

What was happening on Earth
a billion years ago?

Most of Earth's land was amassed
into a supercontinent called Rodinia.

It was a barren desert,
no animals, no plants.

A billion years ago...

there wasn't oxygen in our
atmosphere to form an ozone layer...

and without it, ultraviolet radiation
prevented life from colonizing the land.

Rodinia probably looked more like Mars
than present-day Earth.

The giant world ocean produced huge
rainstorms, causing flooding and erosion.

Glaciers formed...

and their slow but relentless movements
carved the land into new shapes.

Single-celled organisms
dominated the oceans...

but some existed in colonies
called microbial mats...

and the first multicellular organisms
would soon evolve.

And a billion years from now...

what will Earth be like...

long after our cities,
the Egyptian pyramids...

the Rocky Mountains
have all been eroded to dust?

There are few things we can say with
confidence about such a far distant time.

The only thing we can say for sure...

is that Earth as we know it
will be so changed...

that we would scarcely recognize it
as home.

But even a thousand million years
from now...

something of who we were...

and the music that we made
in that long-ago spring...

will live on.

In that distant future...

our sun will have completed another four
orbits around the center of the galaxy...

and the Voyagers will have ventured
far from the sun.

Carl Sagan was a member
of Voyager's imaging team...

and it was his idea
that Voyager take one last picture.

A generation before...

an astronaut on the last Apollo flight
to the Moon...

had taken a picture of the whole Earth...

the planet as a world without borders.

It became an icon of a new consciousness.

Carl realized the next
step in this process.

He convinced NASA to turn the Voyager 1
camera back towards Earth...

when the spacecraft went beyond Neptune...

for one last look homeward...

at what he called...

the Pale Blue Dot.

That's here.

That's home.

That's us.

On it...

everyone you love, everyone you know...

everyone you ever heard of...

every human being who ever was...

lived out their lives.

The aggregate of our joy and suffering...

thousands of confident religions,
ideologies, and economic doctrines...

every hunter and forager...

every hero and coward, every creator
and destroyer of civilization...

every king and peasant...

every young couple in love,
every mother and father...

hopeful child, inventor and explorer...

every teacher of morals,
every corrupt politician...

every superstar, every supreme leader...

every saint and sinner
in the history of our species...

lived there...

on a mote of dust...

suspended in a sunbeam.

The Earth is a very small stage...

in a vast, cosmic arena.

Think of the rivers of blood...

spilled by all those
generals and emperors...

so that in glory and triumph they could
become the momentary masters...

of a fraction of a dot.

Think of the endless cruelties visited...

by the inhabitants
of one corner of this pixel...

on the scarcely distinguishable inhabitants
of some other corner.

How frequent their misunderstandings...

how eager they are to kill one another...

how fervent their hatreds.

Our posturings...

our imagined self-importance...

the delusion that we have some privileged
position in the universe...

are challenged by this point of pale light.

Our planet...

is a lonely speck...

in the great, enveloping cosmic dark.

In our obscurity...

in all this vastness...

there is no hint that help
will come from elsewhere...

to save us from ourselves.

The Earth is the only world known so far
to harbor life.

There is nowhere else,
at least in the near future...

to which our species could migrate.

Visit?

Yes.

Settle?

Not yet.

Like it or not, for the moment...

the Earth is where we make our stand.

It has been said that astronomy is a
humbling and character-building experience.

There is perhaps no better demonstration
of the folly of human conceits...

than this distant image.

To me...

it underscores our responsibility
to deal more kindly with one another...

and to preserve and cherish
the Pale Blue Dot...

the only home we've ever known.

How did we, tiny creatures
living on that speck of dust...

ever manage to figure out
how to send spacecraft...

out among the stars of the Milky Way?

Only a few centuries ago,
a mere second of cosmic time...

we knew nothing of where
or when we were.

Oblivious to the rest of the cosmos...

we inhabited a kind of prison.

A tiny universe bounded by a nutshell.

How did we escape from the prison?

It was the work of generations of searchers
who took five simple rules to heart.

Question authority.

No idea is true,
just because someone says so...

including me.

Think for yourself.

Question yourself.

Don't believe anything
just because you want to.

Believing something doesn't make it so.

Test ideas by the evidence gained
from observation and experiment.

If a favorite idea fails a
well-designed test, it's wrong.

Get over it.

Follow the evidence, wherever it leads.

If you have no evidence, reserve judgment.

And perhaps the most
important rule of all...

remember, you could be wrong.

Even the best scientists have been wrong
about some things.

Newton, Einstein...

and every other great
scientist in history...

they all made mistakes.

Of course they did, they were human.

Science is a way to keep
from fooling ourselves...

and each other.

Have scientists known sin?

Of course.

We have misused science, just as we have
every other tool at our disposal...

and that's why we can't afford to leave
it in the hands of a powerful few.

The more science belongs to all of us...

the less likely it is to be misused.

These values undermine the appeals
of fanaticism and ignorance.

And after all...

the universe is mostly dark,
dotted by islands of light.

Learning the age of the Earth...

or the distance to the stars
or how life evolves...

what difference does that make?

Well, part of it depends on how big
a universe you're willing to live in.

Some of us like it small.

That's fine.

Understandable.

But I like it big.

And when I take all of this into my heart
and my mind, I'm uplifted by it.

And when I have that feeling,
I want to know that it's real...

that it's not just something
happening inside my own head...

because it matters what's true...

and our imagination is nothing...

compared with nature's awesome reality.

I want to know what's
in those dark places...

and what happened before the big bang.

I want to know what lies beyond
the cosmic horizon and how life began.

Are there other places in the cosmos where
matter and energy have become alive...

and aware?

I want to know my ancestors, all of them.

I want to be a good, strong link
in the chain of generations.

I want to protect my children
and the children of ages to come.

We, who embody the local eyes and ears
and thoughts and feelings of the cosmos...

we've begun to learn the story
of our origins...

star stuff contemplating
the evolution of matter...

tracing that long path by which
it arrived at consciousness.

We and the other living things
on this planet...

carry a legacy of cosmic evolution
spanning billions of years.

If we take that knowledge to heart...

if we come to know and love nature
as it really is...

then we will surely be remembered
by our descendants...

as good, strong links in the chain of life.

And our children will continue
this sacred searching...

seeing for us as we have seen
for those who came before...

discovering wonders
yet undreamt of...

in the cosmos.