All language subtitles for 649E46E7CE908628BE8F2FFC5478473F_eng

Seeing is not believing.

Our senses can deceive us.

Even the stars
are not what they appear to be.

The cosmos, as revealed by science...

is stranger than we ever
could have imagined.

Light, and time, and space, and gravity...

conspire to create realities
which lie beyond human experience.

That's where we're headed.

Come with me.

Back in 1802, on a night like this...

the astronomer William Herschel...

strolled a beach on the English coast
with his son John.

Herschel was the first person ever...

to see into the deeper waters
of the cosmic ocean.

There, he glimpsed the magic trick
that light does with time.

Father, do you believe in ghosts?

Why, yes, my son.

You... You do?

I would not have thought so.

Oh, no. Not in the human kind of ghost.

No. Not at all.

But look up, my boy.

And see a sky full of them.

The stars, Father?

I do not follow.

Every star is a sun as big, as bright,
as our own.

Just imagine, how far away from us
you'd have to move the sun...

to make it appear as small and faint
as a star.

The light from the stars travels very fast.

Faster than anything.

But not infinitely fast.

It takes time for their light to reach us.

For the nearest ones, it takes years.

For others, centuries.

Some stars are so far away...

it takes eons for their
light to get to Earth.

By the time the light
from some stars gets here...

they are already dead.

For those stars, we see only their ghosts.

We see their light...

but their bodies perished long, long ago.

John, I have seen further back in time...

than any man before me.

Millions of years into the past.

William Herschel
was the first person to understand...

that a telescope is a time machine.

We cannot look out into space...

without seeing back in time.

In one second,
light travels 300,000 kilometers...

or 186, 000 miles.

That's nearly the distance
from the Earth to the moon.

So the moon is about one light-second away.

The next time you look at the moon,
you'll be seeing one second into the past.

That sun?

It's not really there.

It won't be actually be above the horizon
for another two minutes.

The sunrise is an illusion.

Earth's atmosphere
bends the incoming rays of sunlight...

like a lens or a glass of water.

So we see the image of the sun
projected above the horizon...

before the physical sun is actually there.

That sun behind me is a mirage.

No more real than a shimmering image
that hovers in the distance...

over at a desert road on a hot day.

Sunlight takes about eight minutes
to reach Earth.

So the sun is eight light-minutes away.

From Earth, we can only ever see the sun
as it was eight minutes ago.

And another thing,
the sun doesn't really rise at all.

The Earth turns and we turn with it.

It may not look like it, but right at this
moment, I'm moving faster than a jet plane.

So are you and everyone on Earth.

And while I'm at it...

that horizon?

It's not really there at all.

There's no edge.

The horizon is just another illusion.

The distance between Earth
and the outermost planet, Neptune...

varies as the planets orbit the sun.

But on average, the light makes that trip
in four hours.

So for us on Earth, the Neptune we see...

is always four hours in the past.

Four light-hours away.

But the distances to the planets,
even the farthest one...

are mere baby steps
on a much grander scale...

of the stars and the galaxies.

As soon as we leave
the sun's immediate neighborhood...

we need to change the unit of distance
from light-hours to light-years.

A light-year is the
yardstick of the cosmos.

A single one is nearly
10 trillion kilometers.

Or about 6 trillion miles.

It's a unit of distance just like a meter
or a mile.

It's the distance light travels in a year.

The nearest star to the sun,
Proxima Centauri...

is a little more than four light-years
away from Earth.

How far away is four light-years?

NASA's Voyager spacecraft moves at
more than 56,000 kilometers an hour.

Even at that astonishing speed...

it would take Voyager
more than 80,000 years...

to reach the nearest star.

And the stars of the Pleiades cluster?

Four hundred light-years away.

The ship of the imagination is equipped
with a highly unusual capability.

One of a kind, actually.

It makes it possible for us
to see what was happening...

when the light
from a distant star or galaxy...

first set out on its long journey to Earth.

When that light left the Pleiades
about 400 years ago...

Galileo was taking his first look
through a telescope.

A few years later, he tried to measure
the speed of light.

But he couldn't do it.

He had a very clever plan.

But the technology of that era
just wasn't good enough...

to measure the motion of anything
that moves as fast as light.

When we look at the Crab Nebula
from Earth...

we're seeing much farther back in time.

The Crab Nebula was once a giant star,
10 times the mass of the sun.

Until it exploded in a supernova.

At its heart is a pulsar:

A collapsed star the size of a city
spinning 30 times a second.

This pulsar's whirling magnetic field...

whips nearby electrons into a frenzy.

Accelerating them
to almost the speed of light.

They shine with a blue glow that lights up
the tendrils of gas...

still unraveling from the supernova.

The Crab Nebula is about 6500 light-years
from Earth.

According to some beliefs...

that's the age of the whole universe.

But if the universe
were only 6500 years old...

how could we see the light from anything
more distant than the Crab Nebula?

We couldn't.

There wouldn't have been enough time...

for the light to get to Earth
from anywhere farther away...

than 6500 light-years in any direction.

That's just enough time...

for light to travel through a tiny portion
of our Milky Way galaxy.

To believe in a universe as young
as 6 or 7000 years old...

is to extinguish the light
from most of the galaxy.

Not to mention, the light from all
the hundred billion other galaxies...

in the observable universe.

The center of our own galaxy
is about 30,000 light-years from Earth.

The light we see today...

coming from the core of the Milky Way...

left there when our ancestors
were perfecting a way to vanquish death...

by making art...

with the power to inspire those
who would come long after they were gone.

The light we see
coming from the Sombrero Galaxy...

is 30 million years old.

Our ancestors were living in trees
when that light started out.

They weighed about 5 kilos
and had long tails.

But even 30 million light-years away...

is still in our own cosmic backyard.

That galaxy is part of the Coma Cluster...

320 million light-years away.

What was going on back home when the light
you were seeing began its trip to Earth?

No familiar continents, oceans or rivers.

Our distant ancestors
were just leaving the water for the land.

It's pretty old light.
But not nearly the oldest light we can see.

The oldest light is very faint.

A pale ghost in the night.

See that red blob inside the circle?

That's one of the oldest galaxies
we've ever seen.

You're looking
at 13.4 billion-year-old starlight...

as captured
by the Hubble Space Telescope.

It's coming from the very first
generation of stars.

What was happening on Earth back then?

Absolutely nothing.

There was no Earth. No sun. No Milky Way.

They would not come to be
for billions of years.

When we try to look even farther
into the universe...

we come to what appears to be
the end of space.

But actually...

it's the beginning of time.

Earth pulls on us.

Our lives are a relentless struggle
with gravity.

That little girl is trying her best
to climb out of a gravitational well.

From our first efforts to stand
to our final surrender...

we are struggling to overcome
the Earth's pull.

We are born, live and die in a force field.

One that is almost as old
as the universe itself.

And how old is that?

To visualize the 13.8 billion-year-age
of the universe...

we've compressed all of cosmic time
into a single year-at-a-glance calendar.

Midnight on December 31st...

is this very moment, right now.

And January 1st is the beginning of time.

See that glowing fog out there?

It's radiation left over
from the Big Bang...

the explosion that made the universe...

13.8 billion years ago.

Right now...

we're at the very edge
of known space and time.

So, what happened before the Big Bang?

Nobody knows.

No evidence survives
from before that moment.

We've got some pretty crazy ideas
about where the universe came from.

Which we'll get to, in time.

Where are we in the universe?

At the very center.

In the observed universe,
everyone gets to feel special.

No matter which galaxy
you happen to live in...

when you look out to the universe...

you'll find yourself at the center
of the cosmic horizon.

But this is just an illusion.

In reality, there is no center.

And the cosmic horizon is no more real
than the horizon at sea.

It's what you get
when you have finite speed of light...

in a universe that had a beginning in time.

A few hundred million years
after the Big Bang...

vast clouds of
hydrogen and helium

condensed into the
first stars and galaxies.

With these new sources of light...

the long dark ages of the universe ended.

As space continued to expand...

cosmic evolution unfolded
on grander scales.

As the first generation of stars died...

they seeded space with heavier elements...

making possible the formation of planets...

and ultimately life.

Matter and energy were formed
in the Big Bang.

But that's not all.

Space and time were created too.

And all the forces that
bind matter together.

Including gravity.

Isaac Newton discovered a mathematical law
that describes how gravity works.

With that law, he could explain
the motions of the planets.

More than 100 years later...

William Herschel realized
gravity could do much more.

John, can you keep a secret?

Yes, father.

I've made a discovery...

and have yet to tell another soul.

That gravity that holds us to the Earth...

the same gravity that Newton showed
keeps the planets in their orbits...

I've discovered that it also rules
the distant stars.

Father, but how can you know this?

Can you find the constellation of the lion?

There.

Well done.

Can you now find star that joins
the lion's head to his body?

That one.

That star is really two stars.

So close together
that they appear to be one.

I've been watching them
through my telescope...

since long before you were born.

They dance around each other
very slowly.

More slowly than any planet
moves around the sun.

Many of the stars we see tonight...

Perhaps, most of them.
...Dance with invisible partners.

Gravity's empire governs all the heavens.

A century earlier...

Isaac Newton had been haunted by the same
absence of a mechanism for gravity.

How could distant bodies affect each other
across empty space...

without actually touching?

This "action at a
distance" as he called it...

baffled him.

In the 19th century,
Michael Faraday discovered...

that we were surrounded
by invisible fields of force...

that explained how gravity works.

The apple and the Earth
don't touch each other...

but the fields between them do.

He imagined
those lines of gravitational force...

radiating out into space
from every massive body.

The Earth. The moon. The sun. Everything.

Here was the answer to that question
that had stumped Newton.

In 1865...

James Clerk Maxwell
translated Faraday's idea

about fields of electricity
and magnetism...

into mathematical laws.

He discovered that these fields
moved through space in waves.

When he calculated how fast they move...

it turned out to be the speed of light.

We were beginning to discover
the threads of the cosmic tapestry.

But we were not yet able to discern...

the rich pattern
that time, light, space and gravity weave.

As Albert Einstein worked in Berlin
on his theory of gravity...

he kept the portraits of these three men
before him.

He knew
he was standing on their shoulders.

Years before, as a teenager...

he had an insight that was as earthshaking
as any idea of theirs.

And it happened one summer
while he was daydreaming in Italy.

In the summer of 1895...

Einstein's father's business in Germany
had failed...

and the family had moved here
to northern Italy.

Young Einstein loved wandering these roads
and giving his mind free reign to explore.

There's something timeless
about this place.

Nothing here has really changed since
the time of Einstein's early daydreams.

One day, he began to think about light.

And how fast it travels.

In everyday life we've always measured
the speed of a moving object...

with respect to something else.

Something that's presumably not moving.

Something in the cosmos
that's not in motion.

For example,
I'm moving about 10 kilometers per hour...

relative to the ground.

But as I mentioned earlier,
the ground is moving.

Earth is turning at more than
1600 kilometers per hour...

while it orbits the sun at more than
100, 000 kilometers per hour.

And the sun is moving through the galaxy
at a half a million miles per hour.

The Milky Way is moving
through the universe...

at nearly one-and-a-half
million miles an hour.

There is no fixed place in the cosmos.

All of nature is in motion.

It was hard,
even for the young Einstein...

to imagine some absolute standard to
measure all those relative motions against.

This is the very book
that inspired Einstein as a young boy.

Give a kid a book
and you change the world.

And in a way, even the universe.

Look at this. The very first page.

It describes the astonishing speed
of electricity through wires...

and light through space.

Einstein remembered what he'd learned
as a child from this book.

And perhaps, for the first time,
right here...

wondered what the world would look like
if you could travel at the speed of light.

The more Einstein thought about it,
the more troubled he became.

If you imagine traveling
at the speed of light...

paradoxes seem to pop up everywhere.

Einstein was shocked to realize that so
much of what had been accepted as truth...

by even the authorities on the subject
was just plain wrong.

When traveling at high speeds, there
are certain rules which must be obeyed.

Einstein called these rules
the Principles of Relativity.

Imagine that young woman
who just blew past us on a motorbike.

Imagine she was riding her bike
through the cosmos.

Light from a moving object travels
at the same speed...

no matter whether the object is at rest
or in motion.

Her speed is not added
to the speed of light.

The light from her motorbike still travels
at the speed of light.

Nature commands, "Thou shalt not
add my speed to the speed of light."

Also, no material object can travel at
or faster than the speed of light.

Nothing in physics
prevents you from traveling

as close to the speed
of light as you like.

Ninety-nine-point-nine percent
of the speed of light is just fine.

But no matter how hard you try,
you'd never gain that last decimal point.

For reality to be logically consistent,
there must be a cosmic speed limit.

The crack of that whip is due to its tip
moving faster than the speed of sound.

It makes a shockwave, a mini sonic boom
in the Italian countryside.

A thunderclap works the same way.

So does the sound
of a passing supersonic jet.

So why is the speed of light
any more a barrier than the speed of sound?

The answer is not just that light travels
about a million times faster than sound.

And it's not merely an engineering problem,
like building the first supersonic jet.

Instead, the light barrier
is a fundamental law of nature.

As basic as gravity.

Einstein found his absolute framework
for the world.

This sturdy pillar
among all the relative motions...

within the motions of the cosmos.

Light travels just as fast...

no matter how fast or slow
its source is moving.

The speed of light is constant,
relative to everything else.

Nothing can ever catch up with it.

The thing about the laws of nature
is that they're unbreakable.

The job of physicists
is to discover these commandments...

the ones that do not vary
from culture to culture and time to time...

and hold true throughout the cosmos.

That's why, as Einstein showed...

funny things happen
close to the speed of light.

Traveling close to the speed of light...

is kind of an elixir of life...

because your biological clock slows down,
relative to those you leave behind.

This phenomena may provide us humans,
who only live for a century or so...

a practical means to travel to the stars...

where the magic show of space-time
really gets crazy.

The 19th century astronomer
William Herschel...

loved to share the wonders of the universe
with his son, John.

I once had a friend, a very clever fellow,
an astronomer...

and a parson at Leeds,
by the name of John Michell.

Poor man died when you were a babe,
God rest his soul.

He held that some stars are invisible.

They really exist,
but we shall never see them.

Dark stars, Michell called them.

With all due respect, Father...

surely your friend was mistaken.

If no one can see them,
then how can we possibly know they exist?

Did you see the man
who left those footprints, John?

Why, no, Father, I did not.

But do you know that he exists?

John Michell was one of the greatest
scientists you've probably never heard of.

He lived and worked in England
in the 18th century.

If he ever sat for a portrait,
it no longer exists.

He was once described by an acquaintance
as a short little man...

of black complexion, and fat.

Michell imagined a
star so big, so massive...

that nothing, not even light,
could escape its gravitational grip.

Can you find the dark star'?

You can't see it with
your eyes, not directly.

But it may leave a kind of footprint
on the cosmic shore.

Michell realized that
we might detect some of these dark stars...

because of their extreme gravity.

If one happened to be near a smaller,
luminous companion star...

that star would appear to travel
in a tight orbit, around nothing.

Even though we can't see it...

we know something with a lot of mass
has to be right there.

A dark star.

Or what today we call a black hole.

What does a black hole look like?

And what would it be like inside?

We'll get there.

But first, let's make a pit stop
in my hometown:

New York City...

where it's always seemed to me
that everything is in constant motion.

I've lived here most of my life.

There's always something new to see.

But one thing never changes: gravity.

Gravity on Earth has been the same
for the past four and a half billion years.

But what if today, we could alter it?

Gravity is a distortion
in the shape of space-time...

as Einstein has showed.

Space can expand or contract and warp
without limit.

If the Earth's size or density
were even a little different...

its gravity would be too.

There's an infinite range of possibilities.

New Yorkers feel right at home
with the gravitational pull of the Earth...

called 1g.

Suppose we turn off the gravity
on one of its streets.

People and objects that were already
in motion are launched into flight.

Now, what if I turned the gravity up
to, say, eight or nine g's?

Out of compassion, let's evacuate the area.

This is about the same g-force...

that a fighter pilot
in a high-speed turn would feel.

A few minutes of this wouldn't hurt you,
but it wouldn't be comfortable.

At 100,000 g's...

even fire hydrants become crushed
by their own enormous weight.

But at millions of g's...

even light bows to gravity.

The light still moves at its
constant speed, but it cannot escape.

Michell's dark star...

our black hole.

And the nearest one
may be closer than you think.

Not every star can become a black hole.

Only about one in a thousand
is massive enough.

The nearest one could be
within 100 light years of Earth.

Black holes aren't the mythic,
cosmic vacuum cleaners of science fiction.

They don't go around
gobbling up unsuspecting worlds.

You've gotta come to them.

But if you do,
it might be the last thing you ever see.

That was us
resisting a few million g's of gravity.

Don't forget. That thing swallows light.

We'll keep our distance.

When giant stars
exhaust their nuclear fuel...

they can no longer stay hot enough to fend
off the inward pull of their own gravity.

The most massive stars collapse
into darkness...

leaving only their gravity behind.

This black hole enshrouds
the shrunken corpse of a supergiant star.

The star itself has shriveled into
something even smaller than this darkness.

Only 64 kilometers wide.

This is the first black
hole ever discovered.

Cygnus x-1.

How did we on Earth ever find
something so small and dark and far away?

We looked at it in another kind of light.

X-rays.

In X-ray light, we lost
sight of the blue star...

because its surface
is a tepid 30,000 degrees.

But the disc of gas around the black hole
glowed brilliantly in X-rays...

at 100 million degrees.

As William Herschel discovered...

many stars have close companions,
forming a binary star system.

But if one member of such a pair is
enormous, and the other is compact...

the smaller star can drain and consume
the atmosphere of its larger sibling.

This neurotic relationship
can last for millions of years.

The atmosphere of the larger star...

was being siphoned on to
a glowing hot accretion disc...

that revolves around, and spirals into,
a black hole.

The overwhelming gravity was accelerating
the blue star's gas into a death spiral...

crossing the space-time boundary,
never to be seen again.

The fateful boundary that separates a
black hole from the rest of the universe...

is called an event horizon.

From our point of view...

the substance in the disc slows down
as it approaches the event horizon...

never quite reaching it.

But if you were riding on that
spiraling gas, and I don't advise it...

you would sail past the event horizon
in a matter of seconds...

into the undiscovered country...

from which no traveler returns.

We have searched the hearts
of dozens of galaxies...

and in every case,
we have found a super massive black hole.

Our own galaxy is no exception.

The stars nearest
to the center of our galaxy...

whip around at more than
40 million kilometers an hour.

What can make them move so fast?

The only logical explanation...

is that something with a mass
of 4 million suns lies at the center.

So where are the blazing light
of 4 million suns?

Since we can't see it, it must
be imprisoned inside a black hole.

Earth is far enough
away to be perfectly safe.

Other worlds might not be so lucky.

If you somehow survived the perilous
journey to cross the event horizons...

you'd be able to look back out and see...

the entire future history of the universe
unfold before your eyes.

How?

Because when space-time is warped
by the extreme gravity of a black hole...

its time is stretched to the limit.

But what would be in front of you?

Before we go there...

I should warn you that we are entering
uncharted scientific territory.

For all we know, there may be
undiscovered laws of physics...

that govern events
at the center of the black hole.

But until the next Einstein comes along...

let's perform a thought experiment.

That's how John Michell first imagined
dark stars in the 18th Century...

and how Einstein conceived
his theory of rela...

Father, do you believe in ghosts?

Oh, no. Not in the human
kind of ghost. No, not at all.

But look up, my boy...

and see a sky full of them.

If you could survive
the trip into a black hole...

you might emerge in another place and time
in our own universe...

circumventing the first commandment
of relativity:

Thou shalt not travel faster than light.

Nothing can move through space
faster than light.

But space is not near emptiness.

It has properties. It can stretch
and shrink. It can be deformed.

And when that happens,
time is deformed too.

Einstein discovered that space and time
are just two aspects of the same thing.

Space-time.

Space-time itself can deform enough
to carry you anywhere at any speed.

Black holes may very well be
tunnels through the universe.

On this intergalactic subway system...

you could travel
to the farthest reaches of space-time.

Or you might arrive in some place
even more amazing.

We might find ourselves
in an altogether different universe.

But how can a whole universe fit
inside of a black hole...

which is only a small part of our universe?

That's another magic trick of space-time.

The phenomenal gravity of a black hole...

can warp the space
of an entire universe inside it.

Our local gravity may be a drag to us...

but it's really feeble compared with
what goes on inside a collapsed star.

As far as we know...

when a giant star collapses
to make a black hole...

the extreme density and pressure
at the center mimic the Big Bang...

which gave rise to our universe.

And a universe inside a black hole
might give rise to its own black holes...

and those could lead to other universes.

Maybe that's how our cosmos came to be.

For all we know...

if you wanna see what it's like
inside a black hole...

just look around you.

William Herschel went on to discover...

that the sun and its planets
are moving through the Milky Way.

And whatever became of his son John?

He grew up to become a great scientist.

His deep space observations
built on those of his father...

to become the basis for the standard
catalog of galaxies we use today.

When William was in failing health...

John stayed with him
through the long nights

at his telescope, to
help him sweep the stars.

"And when his father died,
John wrote his epitaph."

"He broke through the walls of heaven."

John often reminisced
about those summer nights with his father.

Maybe that's why
he sought a way to preserve the past.

John Herschel was one of the founders
of a new form of time travel...

a means to capture light and memories.

He actually coined a word for it:

Photography-

When you think about it,
photography is a form of time travel.

This man is staring at us
from across the centuries...

a ghost preserved by light.

It's not hard to imagine that
in the near future...

we'll be able to capture the past
in all three dimensions.

We'll be able to step inside a memory.

It may not be possible
to travel backward in time...

but perhaps one day,
we can bring the past to us.

Here's a moment from my past.

Like John Herschel, I'm remembering
a younger version of myself.

December 20th, 1975.

A snowy day in Ithaca, New York.

A branchpoint on the road that brought me
to this moment with you.

It was the day I met Carl Sagan.

Reminds me of those ghost stars in the sky.

You know, the ones that
still shine their light upon us...

long after they're gone.