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The dream of becoming a citizen

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of the cosmos was born here

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more than two millennia ago,
in the city of Alexandria

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named after and conceived

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by its dead conqueror,
Alexander the Great.

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The Ptolemys, the Greek kings

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who inherited the Egyptian portion

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of Alexander's empire,
built this library

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and its associated
research institution.

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Rarely, if ever, before or since

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has there been a government
that was willing to spend

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so much of its gross
national product

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on the acquisition of knowledge.

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And it paid off. Big time.

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Every ship entering
Alexandria's harbor was searched

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not for contraband, but for books
that might be copied

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and stored here, in what was then

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the greatest library on Earth.

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Here, Eratosthenes,
one of the chief librarians

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accurately calculated the size
of the Earth

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and invented geography.

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Pythagoras.

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Hypatia.

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Euclid.

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Euclid set forth the precepts
of geometry

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in a textbook that remained
in use for 2,300 years.

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The Old Testament Bible
comes down to us

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mainly from the Greek
translations made here.

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The original manuscripts
of the masterpieces

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of Greek comedy and drama

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poetry, science, engineering

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medicine and history

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the total work product
of the awakening

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of ancient civilization

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were kept here.

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Estimates vary
on the total number of scrolls.

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They range from 500,000

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to nearly a million.

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And all of it

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all of this is but a tiny
fraction of the information

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that you have at your fingertips
at this very moment.

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The collective knowledge
of our species

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our own electronic Library
of Alexandria

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may be accessed by anyone
who has a device

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and the interest
and the freedom to do so.

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This was not true in Alexandria

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where knowledge belonged
to the elite.

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So, in the 4th century A.D.

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when the mob came
to destroy the library

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and the genius
of classical civilization

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there were not enough people
to defend it.

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What will happen
the next time the mob comes?

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We've come a long way together

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traveling from deep inside
the heart of an atom

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clear out to the cosmic horizon

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and from the beginning
of time to the distant future.

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I think we're ready
to perform an experiment.

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

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You can do it in your head.

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It's called a thought experiment.

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Pick a star, any one
of the hundreds of billions

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of stars in our Milky Way Galaxy

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which is just one galaxy

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out of the hundred billion
in the known universe.

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How about that star?
Or that one?

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Okay, this one.

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It's orbited by dozens
of planets and moons.

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Suppose on one of them

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there lives an intelligent species

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one the 10 million life-forms
on that planet

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and there's a subgroup
of that species

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who believe they have
it all figured out.

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Their world is the center
of the universe

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a universe made for them

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and that they know everything
they need to know about it

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their knowledge is complete.

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How seriously
would you take their claim?

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Our ancestors believed
the universe was made for them.

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

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

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We still speak of the sun rising.

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The architecture of our language

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myths and dreams comes
from that prescientific age.

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This is our planet
as it was known then

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just before Columbus set sail.

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This first globe of the Earth
was cutting-edge

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when Martin Behaim
made it in 1492.

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Like everyone else

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he believed that the jigsaw puzzle

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of geography was complete.

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There were three continents.
Europe, Africa and Asia

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and only the great
world ocean in between.

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Behaim had no clue

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that North and South America
even existed.

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It's easy to feel smug, right?

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Well, the fact is Martin Behaim

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knew infinitely more
about his world, the Earth

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than we know about ours,
the universe.

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A recent lesson in humility
will serve to illustrate.

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In 1912, Victor Hess

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made a series of voyages
into the sky above Austria

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and found the thing
that scientists love best

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a mystery that defied understanding

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in terms of conventional
scientific wisdom.

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And even today, a century later

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we are still searching
for a complete explanation

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of what Hess found.

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A new kind of energy
had recently been discovered

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radioactivity.

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It was given off
by certain elements, like radium.

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But it was also found in the air

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even far away
from radioactive rocks.

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It was everywhere.

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Where did this strange energy
come from?

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No one knew.

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Hess suspected that it might
come from above the Earth.

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To test his hypothesis

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he carried radiation detectors
high into the sky.

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During a risky ascent
in a hydrogen balloon

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he attained an altitude
of more than three miles.

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

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he found that the radiation

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was more than twice
as strong as on the ground.

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The radiation must be
coming from above.

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That's why its intensity
was weaker on the ground

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the Earth's atmosphere
was absorbing most of it.

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Some thought that the radiation
might come from the Sun.

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To test that idea,
Hess timed one of his ascents

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to coincide with a solar eclipse.

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But the eclipse had no effect
on the radiation.

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Hess also found that the radiation

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was just as strong at night
as in daylight.

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It was coming from above,
but not from the sun.

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What Hess did not know
was that the solar wind

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doesn't move that quickly.

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

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Hess had discovered cosmic rays

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showers of subatomic particles
that crisscross the universe

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at nearly the speed of light.

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Without the shielding effect
of the Earth's atmosphere

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they would be lethal.

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Some cosmic rays
can carry as much energy

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as a bullet fired from a rifle.

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It would take decades
to trace those cosmic rays back

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to a death
of unimaginable violence.

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The cosmic rays
that Victor Hess detected

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in the skies above Austria

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posed a mystery to scientists.

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Radioactivity in minerals
on Earth, like uranium ore

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comes from the disintegration
of atoms.

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But cosmic rays
were of a different nature.

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

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Scientists wondered for two decades

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what could possibly
produce cosmic rays.

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Enter Fritz Zwicky,
the most brilliant man

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you've never heard of.

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In 1933, he and a colleague
discovered that some stars

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flare up to become as bright
as their entire galaxy

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for a few weeks,
before fading out again.

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

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He correctly surmised

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that this is the way
a massive star dies.

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It blows its guts out into space.

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He called this kind
of stellar death a supernova

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and predicted that
the dying star would shrink

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from about a million miles
across to only 10.

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This corpse would be so dense

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that a single grain of it

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would weigh as much
as the Great Pyramid in Egypt.

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It would consist almost entirely

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of subatomic particles
called neutrons

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so he named these bizarre objects
neutron stars.

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And 35 years after
Zwicky predicted their existence

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astronomers began to find them.

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We call them pulsars
when they spin rapidly

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and emit regular pulses
of radio energy.

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Supernovas and neutron stars
could account

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for a wide range of cosmic rays

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but not the most energetic ones.

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Nothing yet known
to science can explain them

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and we're fine with that.

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

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

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Zwicky also came up
with the idea that

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the gravity of a galaxy warps
the fabric of space around it

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to act like a lens.

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This distorts and magnifies light

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from any other galaxy
lying directly behind it.

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So, astronomers on Earth
would see multiple images

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of that same distant galaxy

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deformed, as in a funhouse mirror.

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40 years after this prediction

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we started finding them, too.

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

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While studying the Coma Cluster
of galaxies

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he noticed something funny
about the way they moved.

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The galaxies were
going way too fast.

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

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because all the stars
in all those galaxies

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had far too little gravity
to hold the cluster together.

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Zwicky thought that something else

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must be binding them to each other.

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That mysterious missing component

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would have to weigh
something like

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50 times as much
as the stars themselves.

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But no one paid much attention
to this wild notion.

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Just another one
of Zwicky's crazy ideas.

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In our solar system,
the innermost planet, Mercury

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moves much faster than
the outermost one, Neptune.

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And that makes sense, right?

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

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The sun's gravity weakens
with increasing distance

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

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Everyone expected
that the outermost stars

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in a galaxy would act the same way.

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Most of the stars are
concentrated towards the center

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so, their collective gravity

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pulls on the other stars
the same way

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the sun pulls on the planets.

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But in the 1970s

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when astronomer Vera Rubin
studied the Andromeda Galaxy

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she discovered that the outer stars
obeyed no such rule.

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Unlike the outer planets
in the solar system

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

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as the stars that were closer in

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and they were moving
way faster than expected.

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"That's funny," Vera thought.

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

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So she looked at another galaxy.

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Same story. And another.

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Vera studied 60 galaxies
and found that all of them

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seemed to be violating
the Law of Gravity

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a core principle of physics.

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After some initial
healthy skepticism

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her colleagues looked
for themselves

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and found that Vera was right.

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

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Vera Rubin had discovered

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that the gravity of something
massive and invisible

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was forcing the stars to go fast.

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And then, someone remembered

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crazy old Fritz Zwicky

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and the unknown source
of gravity in the galaxy clusters

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that he called dark matter

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back in 1933.

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Vera Rubin had verified
the existence of a new

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much larger cosmos.

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And just like the one
we thought we knew

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it was filled with mystery.

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Dark matter is completely
unobservable

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except for its gravitational effect

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which makes visible stars
and galaxies move faster.

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Its nature is another deep mystery.

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Rubin had provided the evidence
for an invisible universe

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

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

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mistakenly believing
that the froth on the waves

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was all there was to the ocean.

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Vera Rubin looked
at the stars and realized

260
00:16:10,460 --> 00:16:13,460
they were merely the foam
on the waves

261
00:16:13,740 --> 00:16:17,620
and that the greatest part
of the ocean remained unknown.

262
00:16:17,900 --> 00:16:21,300
But wait. It gets crazier.

263
00:16:22,500 --> 00:16:24,300
Our Milky Way galaxy

264
00:16:24,340 --> 00:16:26,780
a few hundred billion stars

265
00:16:26,820 --> 00:16:29,020
plus the clouds of gas and dust

266
00:16:29,300 --> 00:16:31,940
the stuff of once and future stars

267
00:16:31,980 --> 00:16:34,940
and about a hundred billion
other galaxies

268
00:16:35,140 --> 00:16:38,820
all of that, including
those uncounted billions

269
00:16:38,860 --> 00:16:42,740
of trillions of planets,
moons, and comets

270
00:16:43,260 --> 00:16:45,740
amounts to only 5%

271
00:16:45,780 --> 00:16:47,780
of what is actually there.

272
00:16:47,860 --> 00:16:51,700
Because there's a bigger unsolved
mystery than dark matter

273
00:16:52,100 --> 00:16:55,580
dark energy, which makes up
even more of the cosmos

274
00:16:55,660 --> 00:16:57,620
and drives its expansion.

275
00:16:58,060 --> 00:17:01,620
And it was Fritz Zwicky's
supernovas that lit the way

276
00:17:01,700 --> 00:17:04,460
to the revelation of its existence.

277
00:17:06,740 --> 00:17:08,020
In one scenario

278
00:17:08,060 --> 00:17:11,260
a star consumes
all of its nuclear fuel

279
00:17:14,660 --> 00:17:18,620
then cools, and suddenly
collapses under its own gravity.

280
00:17:18,820 --> 00:17:22,460
The star rebounds
in a massive explosion

281
00:17:22,740 --> 00:17:26,700
leaving behind a neutron star
or a black hole.

282
00:17:28,220 --> 00:17:30,260
Since the mass
of the original star

283
00:17:30,340 --> 00:17:32,540
can fall within a wide range

284
00:17:32,660 --> 00:17:34,460
its peak brightness as a supernova

285
00:17:34,580 --> 00:17:36,180
can also vary widely.

286
00:17:36,340 --> 00:17:38,700
So, you can't tell
how far away it is

287
00:17:38,820 --> 00:17:40,660
just from how bright it looks.

288
00:17:40,740 --> 00:17:44,180
A relatively nearby supernova
might appear just as bright

289
00:17:44,300 --> 00:17:47,220
as one that was more powerful,
but farther away.

290
00:17:48,020 --> 00:17:50,060
But there's another
kind of supernova

291
00:17:50,180 --> 00:17:52,540
that comes in only one strength.

292
00:17:52,740 --> 00:17:55,860
It marks the violent
grand finale of a tango

293
00:17:55,980 --> 00:17:59,660
danced by a giant star and a dwarf.

294
00:18:01,140 --> 00:18:04,020
As the two stars orbit
closely around each other

295
00:18:04,180 --> 00:18:07,860
the giant sheds its outer layers
of gas onto the dwarf.

296
00:18:09,100 --> 00:18:12,260
When the added weight
becomes too much for it to bear

297
00:18:12,580 --> 00:18:14,020
the dwarf detonates

298
00:18:14,060 --> 00:18:16,940
like a stupendous
thermonuclear bomb.

299
00:18:18,460 --> 00:18:21,420
For a few weeks, the brilliance
of such a supernova

300
00:18:21,460 --> 00:18:25,420
rivals the combined light
of all the stars in its galaxy.

301
00:18:25,860 --> 00:18:29,820
This kind of supernova always has
the same maximum power output

302
00:18:30,100 --> 00:18:33,340
about five billion times
brighter than our sun.

303
00:18:34,020 --> 00:18:37,980
With big telescopes, we can see
them in galaxies very far away

304
00:18:38,100 --> 00:18:41,420
out toward the edge
of the observable universe.

305
00:18:50,220 --> 00:18:53,380
Because all such supernovas
have the same wattage

306
00:18:53,540 --> 00:18:55,980
they're ideal tools
for measuring distances

307
00:18:56,020 --> 00:18:58,500
to the farthest reaches
of the universe.

308
00:18:58,540 --> 00:19:01,580
We call them standard candles.

309
00:19:05,940 --> 00:19:09,100
In 1929, Edwin Hubble discovered

310
00:19:09,140 --> 00:19:11,340
that the universe is expanding.

311
00:19:11,740 --> 00:19:15,020
The distant galaxies are
drifting away from one another.

312
00:19:15,460 --> 00:19:17,340
Later, we learned that
the expansion began

313
00:19:17,420 --> 00:19:19,660
some 14 billion years ago

314
00:19:19,700 --> 00:19:22,500
with the explosive birth
of the universe

315
00:19:22,540 --> 00:19:24,300
the big bang.

316
00:19:26,820 --> 00:19:28,620
Everybody assumed that
the rate of expansion

317
00:19:28,700 --> 00:19:31,380
would be slowing down
due to the mutual pull of gravity

318
00:19:31,460 --> 00:19:33,820
between all the parts
of the universe.

319
00:19:33,980 --> 00:19:35,500
If there is enough dark matter

320
00:19:35,660 --> 00:19:39,060
its gravity would eventually
bring the expansion to a stop

321
00:19:39,180 --> 00:19:42,100
and the universe
would then fall back on itself.

322
00:19:42,420 --> 00:19:45,180
In that case, everything
would eventually collapse

323
00:19:45,260 --> 00:19:47,060
in a big crunch.

324
00:19:47,100 --> 00:19:48,060
On the other hand

325
00:19:48,140 --> 00:19:50,300
if the universe
had less dark matter

326
00:19:50,380 --> 00:19:52,500
the expansion
would continue forever

327
00:19:52,740 --> 00:19:54,940
just getting slower and slower.

328
00:19:55,620 --> 00:19:57,620
Two competing teams of astronomers

329
00:19:57,780 --> 00:20:00,860
were observing those
supernovas in distant galaxies.

330
00:20:00,980 --> 00:20:04,940
It turned out to be another one
of those "that's funny" moments.

331
00:20:05,540 --> 00:20:06,660
In 1998

332
00:20:06,700 --> 00:20:09,980
both teams independently came
to the same conclusion.

333
00:20:10,060 --> 00:20:12,340
The expansion isn't
slowing down at all

334
00:20:12,420 --> 00:20:13,940
it's speeding up.

335
00:20:14,460 --> 00:20:18,100
This means the universe
will continue to expand forever.

336
00:20:18,620 --> 00:20:21,660
There seems to be
a mysterious force in the universe

337
00:20:21,980 --> 00:20:24,860
one that overwhelms gravity
on the grandest scale

338
00:20:24,940 --> 00:20:27,140
to push the cosmos apart.

339
00:20:27,700 --> 00:20:29,060
Most of the energy of the universe

340
00:20:29,220 --> 00:20:31,540
is bound up in this unknown force.

341
00:20:31,620 --> 00:20:34,020
We call it dark energy.

342
00:20:34,300 --> 00:20:36,940
But that name, like dark matter

343
00:20:37,140 --> 00:20:39,980
is merely a code word
for our ignorance.

344
00:20:40,620 --> 00:20:42,940
It's okay not to know
all the answers.

345
00:20:43,020 --> 00:20:44,540
It's better to admit our ignorance

346
00:20:44,660 --> 00:20:47,020
than to believe answers
that might be wrong.

347
00:20:47,420 --> 00:20:49,220
Pretending to know everything

348
00:20:49,540 --> 00:20:52,780
closes the door to finding out
what's really there.

349
00:20:54,020 --> 00:20:56,500
Tonight, our ships sail

350
00:20:56,580 --> 00:20:59,340
into even more exotic waters.

351
00:20:59,660 --> 00:21:01,220
Come with me.

352
00:21:08,260 --> 00:21:09,660
Only two of our ships

353
00:21:09,740 --> 00:21:12,060
have ventured
into the great dark ocean

354
00:21:12,140 --> 00:21:13,780
of interstellar space.

355
00:21:14,300 --> 00:21:17,340
The longest odyssey
in all of history

356
00:21:17,780 --> 00:21:20,780
was launched back in 1977

357
00:21:21,340 --> 00:21:24,460
NASA's Voyager I and II.

358
00:21:25,780 --> 00:21:29,740
The Voyagers move about
40,000 miles an hour.

359
00:21:30,300 --> 00:21:32,540
They gave us
our first close-up look

360
00:21:32,580 --> 00:21:34,740
at Jupiter's Great Red Spot

361
00:21:34,780 --> 00:21:38,180
a hurricane three times
the size of Earth

362
00:21:38,260 --> 00:21:41,820
and one that's been raging
since at least 1644

363
00:21:41,900 --> 00:21:43,460
when it was first observed.

364
00:21:43,580 --> 00:21:44,700
For all we know

365
00:21:44,780 --> 00:21:47,220
it could be thousands of years old.

366
00:21:47,620 --> 00:21:50,820
The Voyagers discovered
the first active volcano

367
00:21:50,940 --> 00:21:52,220
on another world

368
00:21:52,420 --> 00:21:54,580
on Jupiter's moon Io

369
00:21:56,340 --> 00:22:00,300
and an ocean beneath
the icy surface of the moon Europa

370
00:22:03,860 --> 00:22:07,820
with at least twice as much water
as we have here on Earth.

371
00:22:10,460 --> 00:22:14,140
The Voyagers dared
to fly across Saturn's rings

372
00:22:14,220 --> 00:22:17,740
and revealed that they were
made of hundreds of thin bands

373
00:22:17,820 --> 00:22:19,940
of orbiting snowballs.

374
00:22:23,300 --> 00:22:25,620
On Saturn's giant moon Titan

375
00:22:25,740 --> 00:22:27,740
Voyager detected an atmosphere

376
00:22:27,780 --> 00:22:30,180
four times denser than Earth's.

377
00:22:30,300 --> 00:22:34,260
That hinted at the existence
of hydrocarbon seas on Titan

378
00:22:34,820 --> 00:22:37,100
which we later confirmed.

379
00:22:38,660 --> 00:22:41,140
Voyager II gave us
our first portrait

380
00:22:41,220 --> 00:22:44,300
of the outermost planet, Neptune

381
00:22:45,220 --> 00:22:48,900
where the winds roar
at 1000 miles per hour

382
00:22:50,260 --> 00:22:51,940
and its moon Triton

383
00:22:52,140 --> 00:22:55,180
where geysers of boiling nitrogen

384
00:22:55,340 --> 00:22:57,940
shoot five miles high.

385
00:23:01,060 --> 00:23:04,140
Voyager successfully completed
its mission of discovery

386
00:23:04,260 --> 00:23:05,500
to the outer planets

387
00:23:05,620 --> 00:23:09,620
but its odyssey into the darkness
was just beginning.

388
00:23:11,900 --> 00:23:13,780
35 years after its launch

389
00:23:13,900 --> 00:23:16,500
Voyager I became
the first of our spacecraft

390
00:23:16,740 --> 00:23:19,540
to enter an uncharted realm.

391
00:23:20,260 --> 00:23:22,580
The Sun is constantly
shooting out streams

392
00:23:22,660 --> 00:23:24,900
of charged particles
in all directions

393
00:23:25,020 --> 00:23:27,460
moving at a million miles an hour.

394
00:23:27,540 --> 00:23:29,300
This solar wind

395
00:23:29,340 --> 00:23:31,940
blows a vast magnetic bubble

396
00:23:32,140 --> 00:23:36,100
our heliosphere, that extends
beyond the outer planets.

397
00:23:36,420 --> 00:23:40,380
It pushes out against
the thin gas of interstellar space.

398
00:23:40,860 --> 00:23:43,500
There's a border where one ends

399
00:23:43,540 --> 00:23:45,380
and the other begins.

400
00:23:45,500 --> 00:23:47,700
Voyager I reported back to Earth

401
00:23:47,780 --> 00:23:49,860
that its detectors
were being pummeled

402
00:23:49,980 --> 00:23:52,420
by more and more cosmic rays.

403
00:23:52,580 --> 00:23:56,500
Until then, we didn't know where
the interstellar ocean began.

404
00:23:56,580 --> 00:23:59,580
Voyager I pressed on
past a boundary

405
00:23:59,660 --> 00:24:01,900
we had never crossed before.

406
00:24:01,980 --> 00:24:05,940
The heliosphere shields us from
most of the deadly cosmic rays.

407
00:24:06,020 --> 00:24:07,940
When stormy solar winds blow

408
00:24:08,060 --> 00:24:10,220
this zone of protection grows

409
00:24:10,300 --> 00:24:12,460
and calm solar weather,
it shrinks.

410
00:24:12,540 --> 00:24:16,100
When a star goes supernova
in our galactic neighborhood

411
00:24:17,620 --> 00:24:19,660
the debris from the exploded star

412
00:24:20,300 --> 00:24:23,300
pushes the heliosphere
back towards the sun.

413
00:24:23,460 --> 00:24:26,700
If it's strong enough to push it
all the way back to Earth's orbit

414
00:24:26,980 --> 00:24:30,940
our planet gets a radioactive
bath of supernova debris.

415
00:24:32,020 --> 00:24:34,740
Luckily, this doesn't happen
very often.

416
00:24:34,900 --> 00:24:37,900
The last one was perhaps
two million years ago.

417
00:24:38,260 --> 00:24:39,940
A neighboring star explodes

418
00:24:40,060 --> 00:24:42,740
a million years before
there's even such a thing

419
00:24:43,180 --> 00:24:45,060
as the human species.

420
00:24:45,500 --> 00:24:47,420
How can we possibly know this?

421
00:24:47,740 --> 00:24:49,180
Because the dying star

422
00:24:49,340 --> 00:24:52,820
left its traces miles below
the surface of the ocean.

423
00:24:53,300 --> 00:24:55,180
Manganese nodules

424
00:24:55,380 --> 00:24:57,340
small rocks like this one

425
00:24:57,580 --> 00:25:00,780
are scattered over much
of the deep sea floor.

426
00:25:02,300 --> 00:25:04,580
They grow very slowly.

427
00:25:04,740 --> 00:25:06,740
I'm talking a millimeter

428
00:25:07,260 --> 00:25:09,180
in a million years

429
00:25:10,020 --> 00:25:11,780
layer upon layer.

430
00:25:13,540 --> 00:25:16,740
These nodules grow
in partnership with bacteria

431
00:25:16,820 --> 00:25:20,300
by taking up minerals
dissolved in the seawater.

432
00:25:21,460 --> 00:25:24,900
A supernova produces
a radioactive form of iron

433
00:25:25,380 --> 00:25:28,940
unlike anything made
by natural processes on Earth.

434
00:25:29,260 --> 00:25:31,820
Researchers found
telltale traces of this iron

435
00:25:32,100 --> 00:25:35,860
in a thin layer below the surface
of the manganese nodules.

436
00:25:36,180 --> 00:25:38,780
They used the known rate
of growth of the nodules

437
00:25:38,980 --> 00:25:40,300
to date that layer

438
00:25:40,380 --> 00:25:44,340
and to connect it to the fate
of a star that perished eons ago.

439
00:25:47,140 --> 00:25:50,060
The difference between
seeing nothing but a pebble

440
00:25:50,380 --> 00:25:54,340
and reading the history
of the cosmos inscribed inside it

441
00:25:54,740 --> 00:25:56,420
is science.

442
00:25:59,020 --> 00:26:02,900
The interstellar ocean
is dark and deep.

443
00:26:03,500 --> 00:26:07,100
Out here, the sun is just
the brightest star in the sky.

444
00:26:07,180 --> 00:26:08,140
Yet the Voyagers

445
00:26:08,340 --> 00:26:10,020
maintain their
regular communications

446
00:26:10,100 --> 00:26:12,580
with NASA's
Jet Propulsion Laboratory

447
00:26:12,660 --> 00:26:14,740
talking back and forth
across the light-hours

448
00:26:14,860 --> 00:26:18,420
that separate these ships
from their home port.

449
00:26:18,700 --> 00:26:21,860
No other objects
touched by human hands

450
00:26:21,980 --> 00:26:24,700
have ever ventured
this far from home.

451
00:26:30,900 --> 00:26:34,220
Even after they lose their
ability to respond to our command

452
00:26:34,420 --> 00:26:35,900
the last and, by far

453
00:26:35,980 --> 00:26:39,340
the longest phase
of the Voyager mission will begin.

454
00:26:46,580 --> 00:26:49,980
Back in 1979, when both
Voyagers rounded Jupiter

455
00:26:50,100 --> 00:26:52,620
its massive gravity
acted as a slingshot

456
00:26:52,700 --> 00:26:54,340
flinging them out
of the solar system

457
00:26:54,420 --> 00:26:56,900
to travel among
the stars of our galaxy

458
00:26:57,180 --> 00:26:58,860
for a billion years.

459
00:26:59,060 --> 00:27:00,500
Carl Sagan recognized

460
00:27:00,580 --> 00:27:03,220
that the Voyager mission
offered two free tickets

461
00:27:03,300 --> 00:27:06,140
to something approaching eternity.

462
00:27:06,500 --> 00:27:09,220
He assembled a small team
to create a message

463
00:27:09,340 --> 00:27:12,140
to any civilization that might,
one day

464
00:27:12,380 --> 00:27:14,900
encounter the derelict spacecraft.

465
00:27:18,620 --> 00:27:20,420
26 centuries ago

466
00:27:20,500 --> 00:27:23,420
the Assyrian king
Esarhaddon wrote

467
00:27:23,540 --> 00:27:26,020
"I had monuments made of bronze"

468
00:27:26,100 --> 00:27:28,500
"and inscriptions of baked clay."

469
00:27:28,700 --> 00:27:32,300
"I left them in the foundations
for future times."

470
00:27:32,860 --> 00:27:36,820
These hieroglyphics continue
that ancient tradition.

471
00:27:36,940 --> 00:27:38,500
They are inscribed on the cover

472
00:27:38,580 --> 00:27:40,260
of a message designed to be read

473
00:27:40,340 --> 00:27:44,300
by the beings of other
worlds and times.

474
00:27:45,340 --> 00:27:48,620
What could we possibly have in
common with an alien civilization

475
00:27:48,820 --> 00:27:51,620
with its own separate
evolutionary history

476
00:27:51,700 --> 00:27:53,820
and one so far advanced beyond us

477
00:27:54,060 --> 00:27:56,980
that they can patrol
interstellar space?

478
00:27:57,700 --> 00:27:59,100
One thing at least

479
00:27:59,260 --> 00:28:00,940
a universal language

480
00:28:01,580 --> 00:28:02,820
science.

481
00:28:04,140 --> 00:28:06,180
It's hard to break the bonds
of gravity.

482
00:28:06,460 --> 00:28:08,460
You can only sail the cosmic seas

483
00:28:08,580 --> 00:28:11,540
if you speak mathematics
and physics.

484
00:28:11,900 --> 00:28:14,780
Hydrogen is the most common
element in the universe.

485
00:28:15,060 --> 00:28:16,780
The electron in a hydrogen atom

486
00:28:16,860 --> 00:28:20,820
flips the direction of its spin
at a constant rate, or frequency.

487
00:28:21,180 --> 00:28:24,500
Hydrogen atoms are like
tiny natural clocks

488
00:28:24,940 --> 00:28:26,860
tick, tock.

489
00:28:27,100 --> 00:28:28,620
Now we have a unit of time

490
00:28:28,700 --> 00:28:31,180
in common with
the extraterrestrials.

491
00:28:31,700 --> 00:28:34,740
This will come in handy when we get
to the next level of the message.

492
00:28:34,940 --> 00:28:37,540
Here's our return address in space.

493
00:28:37,820 --> 00:28:40,300
Pulsars are rapidly-spinning
neutron stars

494
00:28:40,380 --> 00:28:43,500
that give off regular bursts
of radio waves.

495
00:28:44,020 --> 00:28:46,220
You can set your clock by them.

496
00:28:46,340 --> 00:28:48,940
The sun is at the center
of this diagram

497
00:28:49,100 --> 00:28:52,540
and the lines point
to the 14 nearest pulsars.

498
00:28:52,820 --> 00:28:56,300
A simple code labels each pulsar
with its unique frequency

499
00:28:56,540 --> 00:28:58,340
using the ticktock
of the hydrogen atom

500
00:28:58,460 --> 00:28:59,780
as the unit of time.

501
00:29:00,300 --> 00:29:03,020
So, alien astronomers
could use this diagram

502
00:29:03,100 --> 00:29:07,060
to locate the home star of the
Voyager spacecraft in our galaxy.

503
00:29:08,180 --> 00:29:11,340
They could also tell how long ago
the spacecraft was launched.

504
00:29:11,500 --> 00:29:12,460
And that's important

505
00:29:12,500 --> 00:29:13,620
because the Voyager record

506
00:29:13,700 --> 00:29:17,580
has a projected shelf life
of a 1,000 million years.

507
00:29:20,340 --> 00:29:23,860
Become an extraterrestrial
archaeologist for a few moments.

508
00:29:24,060 --> 00:29:27,620
An artifact has been fished
out of the interstellar ocean.

509
00:29:27,700 --> 00:29:31,660
It was made by beings that
lived about a billion years ago.

510
00:29:31,860 --> 00:29:34,460
What would you make of them
and their world?

511
00:29:34,980 --> 00:29:37,780
They've sent us their music

512
00:29:39,020 --> 00:29:42,860
and greetings
in 59 human languages.

513
00:29:46,540 --> 00:29:49,140
'Hello from the children
of planet Earth.'

514
00:29:49,260 --> 00:29:50,900
And one whale language.

515
00:29:51,660 --> 00:29:55,620
And a sound essay that includes
a Saturn-V rocket launch.

516
00:29:57,580 --> 00:30:01,060
A mother's first words
to her newborn baby.

517
00:30:01,460 --> 00:30:03,980
'Oh, come on now. Be a good boy..'

518
00:30:04,940 --> 00:30:08,420
The brain waves of a young woman
newly fallen in love.

519
00:30:11,380 --> 00:30:14,300
And the sound of a pulsar.

520
00:30:19,420 --> 00:30:23,380
All of that will live
for a billion years.

521
00:30:36,060 --> 00:30:39,140
How long is a billion years?

522
00:30:44,420 --> 00:30:47,380
If you compress all the time
since the big bang

523
00:30:47,460 --> 00:30:49,220
the explosive birth of the universe

524
00:30:49,300 --> 00:30:51,100
into a single Earth year

525
00:30:51,300 --> 00:30:54,580
a billion years is about
one month of that year.

526
00:30:54,860 --> 00:30:58,620
What was happening on Earth
a billion years ago?

527
00:31:01,780 --> 00:31:03,660
Most of Earth's land was amassed

528
00:31:03,740 --> 00:31:06,380
into a supercontinent
called Rodinia.

529
00:31:06,580 --> 00:31:10,140
It was a barren desert,
no animals, no plants.

530
00:31:10,180 --> 00:31:12,300
A billion years ago,
there wasn't enough oxygen

531
00:31:12,380 --> 00:31:14,500
in our atmosphere
to form an ozone layer

532
00:31:14,660 --> 00:31:15,540
and without it

533
00:31:15,780 --> 00:31:17,300
ultraviolet radiation

534
00:31:17,340 --> 00:31:20,220
prevented life
from colonizing the land.

535
00:31:20,460 --> 00:31:22,500
Rodinia probably looked
more like Mars

536
00:31:22,580 --> 00:31:24,100
than present-day Earth.

537
00:31:25,060 --> 00:31:28,100
The giant world ocean
produced huge rainstorms

538
00:31:28,180 --> 00:31:30,420
causing flooding and erosion.

539
00:31:30,500 --> 00:31:31,620
Glaciers formed

540
00:31:31,820 --> 00:31:33,860
and their slow
but relentless movements

541
00:31:33,900 --> 00:31:36,340
carved the land into new shapes.

542
00:31:37,140 --> 00:31:40,220
Single-celled organisms
dominated the oceans

543
00:31:40,340 --> 00:31:43,940
but some existed in colonies
called microbial mats

544
00:31:44,020 --> 00:31:47,980
and the first multicellular
organisms would soon evolve.

545
00:31:48,540 --> 00:31:50,660
And a billion years from now

546
00:31:50,900 --> 00:31:52,420
what will Earth be like

547
00:31:52,860 --> 00:31:55,300
long after our cities,
the Egyptian pyramids

548
00:31:55,500 --> 00:31:58,540
the Rocky Mountains have
all been eroded to dust?

549
00:31:58,860 --> 00:32:01,020
There are few things
we can say with confidence

550
00:32:01,260 --> 00:32:03,660
about such a far distant time.

551
00:32:08,020 --> 00:32:10,380
The only thing we can say for sure

552
00:32:10,580 --> 00:32:12,300
is that Earth as we know it

553
00:32:12,540 --> 00:32:13,820
will be so changed

554
00:32:14,100 --> 00:32:17,180
that we would scarcely
recognize it as home.

555
00:32:17,780 --> 00:32:20,900
But even a thousand million years
from now

556
00:32:21,140 --> 00:32:23,060
something of who we were

557
00:32:23,180 --> 00:32:26,100
and the music that we made
in that long-ago spring

558
00:32:26,860 --> 00:32:28,380
will live on.

559
00:32:34,060 --> 00:32:35,460
In that distant future

560
00:32:35,580 --> 00:32:38,260
our sun will have completed
another four orbits

561
00:32:38,380 --> 00:32:40,580
around the centre of the galaxy.

562
00:32:48,460 --> 00:32:51,060
And the Voyagers
will have ventured

563
00:32:51,180 --> 00:32:53,020
far from the sun.

564
00:32:56,140 --> 00:32:58,820
Carl Sagan was a member
of Voyager's imaging team

565
00:32:59,340 --> 00:33:03,140
and it was his idea that
Voyager take one last picture.

566
00:33:04,020 --> 00:33:05,300
A generation before

567
00:33:05,420 --> 00:33:08,020
an astronaut on the last
Apollo flight to the moon

568
00:33:08,180 --> 00:33:10,740
had taken a picture
of the whole Earth

569
00:33:10,980 --> 00:33:14,180
the planet as a world
without borders.

570
00:33:14,300 --> 00:33:17,420
It became an icon
of a new consciousness.

571
00:33:17,900 --> 00:33:21,620
Carl realized the next step
in this process.

572
00:33:21,740 --> 00:33:22,940
He convinced NASA

573
00:33:23,060 --> 00:33:25,860
to turn the Voyager I camera
back towards Earth

574
00:33:25,980 --> 00:33:28,300
when the spacecraft went
beyond Neptune

575
00:33:28,580 --> 00:33:30,860
for one last look homeward

576
00:33:31,020 --> 00:33:34,980
at what he called
the Pale Blue Dot.

577
00:33:41,300 --> 00:33:42,780
That's here.

578
00:33:42,860 --> 00:33:44,060
That's home.

579
00:33:44,260 --> 00:33:45,660
That's us.

580
00:33:45,980 --> 00:33:48,900
On it, everyone you love

581
00:33:49,060 --> 00:33:50,340
everyone you know

582
00:33:50,660 --> 00:33:52,260
everyone you ever heard of

583
00:33:52,580 --> 00:33:55,380
every human being whoever was

584
00:33:55,460 --> 00:33:57,540
lived out their lives.

585
00:33:57,980 --> 00:34:01,140
The aggregate of our joy
and suffering

586
00:34:01,460 --> 00:34:04,380
thousands of confident religions,
ideologies

587
00:34:04,460 --> 00:34:06,420
and economic doctrines

588
00:34:06,460 --> 00:34:08,460
every hunter and forager

589
00:34:08,540 --> 00:34:10,420
every hero and coward

590
00:34:10,460 --> 00:34:13,740
every creator and destroyer
of civilization

591
00:34:13,860 --> 00:34:15,940
every king and peasant

592
00:34:16,060 --> 00:34:18,100
every young couple in love

593
00:34:18,180 --> 00:34:21,340
every mother and father,
hopeful child

594
00:34:21,380 --> 00:34:23,780
inventor and explorer

595
00:34:23,820 --> 00:34:27,220
every teacher of morals,
every corrupt politician

596
00:34:27,300 --> 00:34:29,140
every superstar

597
00:34:29,220 --> 00:34:31,380
every supreme leader

598
00:34:31,540 --> 00:34:35,380
every saint and sinner
in the history of our species

599
00:34:35,540 --> 00:34:37,020
lived there

600
00:34:37,260 --> 00:34:38,860
on a mote of dust

601
00:34:38,980 --> 00:34:42,380
suspended in a sunbeam.

602
00:34:42,420 --> 00:34:46,380
The Earth is a very small stage

603
00:34:46,660 --> 00:34:50,620
in a vast, cosmic arena.

604
00:34:50,660 --> 00:34:53,540
Think of the rivers of blood

605
00:34:53,620 --> 00:34:57,340
spilled by all those generals
and emperors

606
00:34:57,460 --> 00:34:59,660
so that in glory and triumph

607
00:34:59,780 --> 00:35:02,220
they could become
the momentary masters

608
00:35:02,780 --> 00:35:06,300
of a fraction of a dot.

609
00:35:06,460 --> 00:35:09,220
Think of the endless
cruelties visited

610
00:35:09,340 --> 00:35:12,580
by the inhabitants
of one corner of this pixel

611
00:35:12,740 --> 00:35:15,940
on the scarcely
distinguishable inhabitants

612
00:35:16,020 --> 00:35:17,580
of some other corner.

613
00:35:17,700 --> 00:35:20,700
How frequent
their misunderstandings

614
00:35:20,860 --> 00:35:23,540
how eager they are
to kill one another

615
00:35:23,740 --> 00:35:26,740
how fervent their hatreds.

616
00:35:27,380 --> 00:35:29,780
Our posturings

617
00:35:29,900 --> 00:35:32,660
our imagined self-importance

618
00:35:32,780 --> 00:35:35,540
the delusion that we have
some privileged position

619
00:35:35,620 --> 00:35:36,860
in the universe

620
00:35:36,900 --> 00:35:39,580
are challenged by this point

621
00:35:39,620 --> 00:35:41,180
of pale light.

622
00:35:42,220 --> 00:35:46,180
Our planet is a lonely speck

623
00:35:46,220 --> 00:35:49,980
in the great,
enveloping cosmic dark.

624
00:35:50,060 --> 00:35:52,380
In our obscurity

625
00:35:52,860 --> 00:35:55,300
in all this vastness

626
00:35:55,580 --> 00:35:57,260
there is no hint

627
00:35:57,300 --> 00:35:59,300
that help will come
from elsewhere

628
00:35:59,380 --> 00:36:02,260
to save us from ourselves.

629
00:36:02,380 --> 00:36:06,340
The Earth is the only world
known so far to harbour life.

630
00:36:06,980 --> 00:36:08,460
There is nowhere else

631
00:36:08,580 --> 00:36:10,260
at least in the near future

632
00:36:10,380 --> 00:36:13,220
to which our species could migrate.

633
00:36:13,300 --> 00:36:15,980
Visit, yes.

634
00:36:16,060 --> 00:36:18,900
Settle, not yet.

635
00:36:19,020 --> 00:36:20,180
Like it or not

636
00:36:20,380 --> 00:36:21,940
for the moment

637
00:36:21,980 --> 00:36:25,180
the Earth is where
we make our stand.

638
00:36:25,460 --> 00:36:28,380
It has been said that
astronomy is a humbling

639
00:36:28,420 --> 00:36:31,980
and character-building experience.

640
00:36:32,420 --> 00:36:34,940
There is perhaps
no better demonstration

641
00:36:35,020 --> 00:36:37,660
of the folly of human conceits

642
00:36:37,820 --> 00:36:40,900
than this distant image.

643
00:36:41,420 --> 00:36:44,780
To me, it underscores
our responsibility

644
00:36:44,940 --> 00:36:47,740
to deal more kindly
with one another

645
00:36:47,780 --> 00:36:50,700
and to preserve and cherish

646
00:36:50,740 --> 00:36:52,220
the pale blue dot

647
00:36:52,340 --> 00:36:55,820
the only home we've ever known.

648
00:37:02,580 --> 00:37:04,780
How did we, tiny creatures

649
00:37:04,860 --> 00:37:06,860
living on that speck of dust

650
00:37:06,940 --> 00:37:08,540
ever manage to figure out

651
00:37:08,580 --> 00:37:12,540
how to send spacecraft out
among the stars of the Milky Way?

652
00:37:13,020 --> 00:37:14,740
Only a few centuries ago

653
00:37:14,860 --> 00:37:17,020
a mere second of cosmic time

654
00:37:17,300 --> 00:37:20,820
we knew nothing of where
or when we were.

655
00:37:20,900 --> 00:37:23,340
Oblivious to the rest of the cosmos

656
00:37:23,380 --> 00:37:25,620
we inhabited a kind of prison

657
00:37:25,700 --> 00:37:29,660
a tiny universe
bounded by a nutshell.

658
00:37:33,980 --> 00:37:36,780
How did we escape from the prison?

659
00:37:37,940 --> 00:37:40,700
It was the work of
generations of searchers

660
00:37:40,780 --> 00:37:44,460
who took five simple rules
to heart.

661
00:37:48,860 --> 00:37:50,820
Question authority.

662
00:37:50,860 --> 00:37:52,460
No idea is true

663
00:37:52,540 --> 00:37:56,140
just because someone
says so, including me.

664
00:37:57,500 --> 00:37:59,620
Think for yourself.

665
00:38:03,180 --> 00:38:05,700
Question yourself.

666
00:38:06,740 --> 00:38:10,180
Don't believe anything
just because you want to.

667
00:38:10,300 --> 00:38:13,500
Believing something
doesn't make it so.

668
00:38:14,020 --> 00:38:16,180
Test ideas by the evidence gained

669
00:38:16,260 --> 00:38:18,860
from observation and experiment.

670
00:38:20,820 --> 00:38:24,780
If a favourite idea fails
a well-designed test, it's wrong!

671
00:38:25,420 --> 00:38:26,740
Get over it.

672
00:38:28,060 --> 00:38:31,460
Follow the evidence,
wherever it leads.

673
00:38:33,140 --> 00:38:36,900
If you have no evidence,
reserve judgment.

674
00:38:37,100 --> 00:38:39,620
And perhaps the most
important rule of all

675
00:38:40,260 --> 00:38:42,900
remember, you could be wrong.

676
00:38:42,980 --> 00:38:46,740
Even the best scientists have
been wrong about some things.

677
00:38:46,940 --> 00:38:49,060
Newton, Einstein

678
00:38:49,140 --> 00:38:52,540
and every other
great scientist in history

679
00:38:52,620 --> 00:38:54,300
they all made mistakes.

680
00:38:54,380 --> 00:38:57,500
Of course they
did, they were human.

681
00:38:57,860 --> 00:39:01,620
Science is a way
to keep from fooling ourselves

682
00:39:02,100 --> 00:39:03,620
and each other.

683
00:39:04,780 --> 00:39:07,300
Have scientists known sin?

684
00:39:10,300 --> 00:39:11,340
Of course.

685
00:39:11,900 --> 00:39:13,940
We have misused science

686
00:39:14,020 --> 00:39:17,540
just as we have every
other tool at our disposal

687
00:39:17,780 --> 00:39:20,380
and that's why we can't
afford to leave it

688
00:39:20,500 --> 00:39:22,900
in the hands of a powerful few.

689
00:39:23,700 --> 00:39:26,300
The more science belongs
to all of us

690
00:39:26,540 --> 00:39:29,220
the less likely it is
to be misused.

691
00:39:31,940 --> 00:39:33,660
These values undermine

692
00:39:33,700 --> 00:39:36,820
the appeals of fanaticism
and ignorance.

693
00:39:36,860 --> 00:39:38,780
And, after all

694
00:39:38,860 --> 00:39:41,220
the universe is mostly dark

695
00:39:41,300 --> 00:39:43,860
dotted by islands of light.

696
00:39:45,060 --> 00:39:48,380
Learning the age of the Earth
or the distance to the stars or

697
00:39:48,420 --> 00:39:50,060
how life evolves

698
00:39:50,180 --> 00:39:52,140
what difference does that make?

699
00:39:52,340 --> 00:39:53,980
Well, part of it depends on

700
00:39:54,020 --> 00:39:56,620
how big a universe
you're willing to live in.

701
00:39:57,020 --> 00:39:58,820
Some of us like it small.

702
00:39:59,020 --> 00:40:00,420
That's fine.

703
00:40:00,500 --> 00:40:02,180
Understandable.

704
00:40:02,220 --> 00:40:03,780
But I like it big.

705
00:40:03,860 --> 00:40:06,900
And when I take all of this
into my heart and my mind

706
00:40:06,940 --> 00:40:08,580
I'm uplifted by it.

707
00:40:08,660 --> 00:40:10,180
And when I have that feeling

708
00:40:10,260 --> 00:40:12,460
I want to know that it's real

709
00:40:12,500 --> 00:40:16,060
that it's not just something
happening inside my own head

710
00:40:16,300 --> 00:40:18,140
because it matters what's true

711
00:40:18,220 --> 00:40:19,780
and our imagination

712
00:40:19,900 --> 00:40:23,860
is nothing compared
with nature's awesome reality.

713
00:40:28,300 --> 00:40:31,620
I want to know what's
in those dark places

714
00:40:32,860 --> 00:40:36,140
and what happened
before the big bang.

715
00:40:38,020 --> 00:40:41,220
I want to know what lies beyond
the cosmic horizon

716
00:40:41,300 --> 00:40:43,540
and how life began.

717
00:40:43,620 --> 00:40:45,580
Are there other places
in the cosmos

718
00:40:45,780 --> 00:40:49,500
where matter and energy
have become alive

719
00:40:50,740 --> 00:40:52,860
and aware?

720
00:40:59,940 --> 00:41:03,460
I want to know my ancestors,
all of them.

721
00:41:03,500 --> 00:41:05,580
I want to be a good, strong link

722
00:41:05,660 --> 00:41:07,780
in the chain of generations.

723
00:41:07,900 --> 00:41:09,820
I want to protect my children

724
00:41:09,860 --> 00:41:12,740
and the children of ages to come.

725
00:41:12,980 --> 00:41:16,180
We, who embody
the local eyes and ears

726
00:41:16,260 --> 00:41:18,940
and thoughts and feelings
of the cosmos.

727
00:41:19,020 --> 00:41:21,820
We've begun to learn
the story of our origins

728
00:41:22,220 --> 00:41:26,100
star stuff contemplating
the evolution of matter

729
00:41:26,340 --> 00:41:30,220
tracing that long path by which
it arrived at consciousness.

730
00:41:30,540 --> 00:41:32,860
We and the other living things
on this planet

731
00:41:32,900 --> 00:41:35,100
carry a legacy of cosmic evolution

732
00:41:35,220 --> 00:41:37,740
spanning billions of years.

733
00:41:37,780 --> 00:41:39,940
If we take that knowledge
to heart

734
00:41:39,980 --> 00:41:43,620
if we come to know
and love nature as it really is

735
00:41:43,660 --> 00:41:46,540
then we will surely be
remembered by our descendants

736
00:41:46,580 --> 00:41:49,540
as good, strong links
in the chain of life.

737
00:41:49,620 --> 00:41:52,820
And our children will continue
this sacred searching

738
00:41:53,020 --> 00:41:56,700
seeing for us as we have seen
for those who came before

739
00:41:56,780 --> 00:41:59,940
discovering wonders
yet undreamt of

740
00:42:00,100 --> 00:42:02,220
in the cosmos.