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 2,300 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
were 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 4th 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 10 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 Earth's 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.
40 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.
Its 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 5%
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 I and II.
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 II gave us our first portrait
of the outermost planet, Neptune
where the winds roar at 1000 miles per hour
and its moon Triton
where 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.
35 years after its launch
Voyager I 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 I 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 I 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
and 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 all the way 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 two 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.
26 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, tock.
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 ticktock 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 1,000 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.
'Oh, come on now. 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 enough 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 centre 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 I 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 whoever 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 harbour 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 favourite 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.