Wonders of the Universe (2011–…): Season 1, Episode 3 - Falling - full transcript

Why are we here?
Where do we come from?

These are the most enduring
of questions, and it's an essential

part of human nature
to want to find the answers.

And we can trace our ancestry back
hundreds of thousands of years

to the dawn of humankind,
but in reality,

our story extends
far further back in time.

Our story starts
with the beginning of the universe.

It began 13.7 billion years ago.

And today, it's filled with over
100 billion galaxies,

each containing
hundreds of billions of stars.

In this series, I want to tell
that story because, ultimately,



we are part of the universe.

So its story is our story.

The force at the heart
of this story is gravity.

This fundamental force of nature
built everything we see.

It creates shape and order,

and it initiates patterns
that repeat across the heavens.

But gravity also forges some of
the most alien worlds in the cosmos,

worlds that defy belief.

The quest to understand
this fundamental force of nature

has unleashed a golden
age of creativity,

exploration and discovery.

And it's led to
a far deeper understanding

of our place in the universe.

Every moment of our lives,



we experience a force
that we can't see or touch.

Yet this force is able to keep
us firmly rooted to the ground.

It is, of course, gravity.

But despite its intangible nature,
we always know it's with us.

If I was to ask you,

"How do you know that there's
gravity around here?"

Then you might say,
"Well, it's obvious."

You know, I can just do an
experiment, I can drop something.

Well, yes, but actually, gravity is
a little bit more subtle than that.

But to really experience it,
to understand it,

you have to do
something pretty extreme.

And this plane has been
modified to help me do it.

Thanks to its flight plan,
it's known as the Vomit Comet.

Once we've climbed to 15,000 metres,

this plane does something
no ordinary flight would do.

Its engines are throttled back,
and the jet falls to earth.

And then,
something quite amazing happens.

SCREAMS AND CHEERS

Push to me, push to me! Oh!

I'm now plummeting towards
the ground just like

someone's cut the cable in a lift,
and you see that I'm not moving.

Relative to Einstein,
we're all just floating.

By simply falling
at the same rate as the plane,

for a few fleeting moments,
we are all free of gravity's grip.

But this isn't just a joyride.

There's something
very profound here,

because although I'm falling towards
the ground, as you can see,

gravity has completely gone away.

Gravity is not here any more.

I've cancelled gravity out
just by falling.

If you understand that,
then you'll understand gravity.

So it is possible,
by the simple act of falling,

to get a very
different experience of gravity.

But this force of nature does more

than just bring us
back down to earth.

Gravity also plays a role
on the grandest of stages,

because across the universe,
from the smallest mote of dust

to the most massive star,
gravity is the great sculptor

that created order out of chaos.

Since the beginning of time, gravity
has been at work in our universe.

From the primordial cloud
of gas and cosmic dust,

gravity forged the stars.

It sculpted the planets and moons,

and set them in orbit
around the newly formed suns.

And gravity connects these star
systems together in vast galaxies,

and steers them on their journey
through unbounded space.

Over the centuries, our quest to
understand gravity has allowed us

to explain some of the true
wonders of the universe.

But at a deeper level, that quest
has also allowed us to ask questions

about the origin and evolution
of the universe itself.

To understand how gravity works
across the universe,

we need look no further
than the ground beneath our feet.

Well, the first scientist
to really think about it

was Isaac Newton back in the 1680s,
and he said this -

"Gravity is a force of attraction
between all objects".

Now, the force of attraction
between these two rocks

is obviously very small,
almost impossible to measure,

and that's because
the force is proportional

to the masses of the objects.

These things are not very massive.

But there is a more
massive rock around here.

It's the one I'm standing on,
planet Earth.

The mass of our earth
generates a gravitational pull

strong enough to sculpt
the entire surface of the planet.

It causes water
to gouge out vast canyons.

It sets the limit
for how high mountains can soar,

and it shapes whole continents.

But this invisible force
does more than just shape our world.

The skies are always changing,
and the constellations rise and fall

in different places every night,

and the planets wander across
the background of the fixed stars.

But throughout human history,

there's been one constant
up there in the night sky,

because every human that's ever
lived has gazed up at the moon

and seen one face
shining back at us.

The reason why we never see
the dark side of the moon

is all down to the subtlety
with which gravity operates.

Millions of years ago,
the moon rotated rapidly.

But from the moment it was born, our
companion felt the tug of gravity.

Just as the moon creates
great tides in our oceans,

the Earth caused
a vast tide to sweep across
the surface of the moon.

But this tide wasn't in water.

It was in rock.

Imagine that this is the moon,
and over there is the Earth.

The Earth's gravity acts on
the moon and stretches it out

into a kind of rugby ball shape.

Now, the size of that tidal bulge
facing the Earth is something like

seven metres in rock
and then, as the moon rotates,

that bulge sweeps across
the lunar surface.

I mean, imagine
what that would look like here.

You'd see a tidal wave sweep

across this landscape, with the rock
rising and falling by seven metres.

This massive wave
acted like a brake,

and gradually slowed the moon down.

Eventually, the tidal bulge
became aligned with the Earth,

locking the speed
of the moon's rotation.

So the time it takes
the moon to spin once

is almost the same as the time
it takes to orbit the Earth.

So there is no
dark side of the moon,

just a side
that gravity hides from our view.

The bond that gravity creates
between the Earth and the moon

is repeated across the cosmos.

It's the glue that holds
the planets in orbit around the sun.

And it binds our solar system

and countless other
solar systems together,

to form galaxies
like our own Milky Way.

But gravity's influence
can be felt even further

because it controls
the fate of galaxies.

When you look up into the night sky
and you see the universe

as it looks in visible light,
with the glowing

of the stars and the galaxies,
but that's only part of the story,

because the universe
is full of dust and gas

which you can't see
with a conventional telescope,

but you can see
with a telescope like this.

Radio telescopes, like the very
large array in New Mexico,

are able to peer deep into space

and reveal the incredible
attractive power of gravity.

This is Andromeda,

a spiral galaxy roughly the same
size and mass as the Milky Way.

This island of over a trillion stars

sits over 2.5 million
light years away,

but every hour that gap shrinks
by half a million kilometres.

Whilst most galaxies have been
rushing away from each other

ever since they formed
just after the Big Bang,

some galaxies formed so close
together that they are locked

in a gravitational embrace,

and the Milky Way and Andromeda
are two such galaxies.

Computer simulations suggest
that they will collide together

in around three billion years' time.

Look at that. That's a simulation
of the Milky Way galaxy

and the Andromeda galaxy
colliding together,

and all these wisps of smoke
getting thrown out are stars.

These are star systems getting
ripped out of the galaxy

and thrown off
into interstellar space.

These two islands
of hundreds of billions of suns

have flown through each other,
and gravity has exerted its grasp

and dragged them back again.

And just remember
that we are one of those dots.

You know, our sun
and the Earth and the solar system

are either going to be flung out
into interstellar space,

or they're going to be in here,

in this maelstrom
of hundreds of billions of suns

swirling around each other and
forming the core of a new galaxy.

Just imagine what it would be like

to gaze up at the sky
as Andromeda approached.

The sky would be ablaze with the
light of hundreds of billions

of suns, and the imminent collision
would provide the energy

to generate the births
of hundreds of millions more.

What a magnificent
sight it would be.

But far more magnificent is the
immense scale of gravity's embrace.

It holds galaxies together across
hundreds of billions of kilometres

and, in doing so, it creates
the most magnificent structures.

Our own Milky Way is part of
one of these, the Virgo cluster.

Every point of light in this image
is not a star, but a galaxy.

There are 2,000 galaxies
in this cluster,

and they're all
bound together by gravity,

making it the largest structure
in our intergalactic neighbourhood.

There seems to be no limit
to the reach or power of gravity.

Its influence can be felt across
the vast expanses of space and time.

But there's something
very interesting about gravity,

because it is by far the weakest
force of nature. I mean, look.

I can...pick this rock up
off the ground even though

there's an entire planet,
planet Earth, trying to pull it down.

So if gravity is so weak,

how come it's so influential?

Gravity may be weak here on Earth,

but it's not so weak
across the cosmos.

This invisible force varies on
all the planets in the solar system

and on the exo-planets we've
discovered orbiting other suns.

To experience what gravity
feels like on these worlds,

I need to go for a spin.

This is a centrifuge.

It was built in the 1950s
to test whether fighter pilots

had the right stuff,
but it's going to allow me to

feel what it'd be like to stand
on the surface of any of the planets

in the solar system that are more
massive than the Earth,

and, in fact, also what it would be
like to stand on some of the planets

that we've found
around distant stars.

Right, I'll have to
strap you in, first of all.

This is an emergency switch
in case something happens.

When you release it,
the centrifuge will stop.

I was just told by the F-16 fighter
pilot, who's just been in here,

that it's a hundred times
more uncomfortable

than being in a jet fighter.

I was kind of confident
because I've been in jet fighters

and didn't find it too
uncomfortable, but apparently,

this is a hundred times worse!

Doors closed again.
Profile is there. Doctor is ready.

We'll start up the centrifuge,
Brian, and bring you in orbit,

and it happens in three...two...one
second from now.

'The first planet
I'm travelling to is Neptune.

'Its gravity is just fractionally
stronger than here on Earth'.

So this is
the gravitational field

on Neptune and you feel,
you know what?

I could probably get used to this.

I could probably live
on the surface of Neptune.

Can you lift your hands a little?

There we go. Yeah, and down.

And it is actually quite an effort.
It is noticeably heavier.

It's like having a reasonably
heavy weight in your hand.

Are you ready to go to 2.5G?

Yes, so now we'll move,
move from Neptune to Jupiter.

Let's go there.

Jupiter is over 1300 times
more massive than the Earth,

but because it's mostly gas,
it's not very dense, so its gravity

is just over twice
as strong at its surface.

Well, now actually, it is quite
difficult to lift my hand.

And that's 2.5G. I wouldn't
want to sit here for half an hour.

Can you lift, lift both
of your hands above your head?

See what happens there. Let's see,
so actually...just about,

but actually,
it's an immense amount of hard work.

So it would be hard work living
on Jupiter. Let's go to 4G.

Actually, this is heading to
a planet around,

a planet called Ogle-2TRL9B,

which is around a star
in the constellation of Carina.

It's one of the exo-planets
we've discovered.

Oh, and there we go.

Now, that is actually

beginning to feel quite unpleasant.

Can you describe what you're feeling?

Very heavy face.

My head is extremely heavy.

How about your lungs,
inhaling, exhaling, breathing?

It's much harder work.

I can't lift my hand off my leg.

OK. And that's at 4G? Yeah.

Well, my head and my face
feel very, very heavy.

It's quite an unpleasant feeling.

We'll go to five, and let me know
if you have any visual disturbances.

'I'm now en route to a newly
discovered exo-planet, Wasp-8B'.

4.4.

'This world sits in the small
and faint constellation of Sculptor'.

Quite hard to speak.

'It has a gravitational force
nearly five times that of the Earth'.

Right, we'll go to 5G.

Very foggy. OK.

Very foggy. Very foggy?

Still foggy? Yeah.

Right.

Take it down.
OK, we'll take you down.

Very interesting.

It was, wasn't it?

My face felt a bit saggy, though.

Well, you looked a little different.

It was quite unpleasant
that time, actually.

It went very quickly up to 5G
and what happens is -

for me, anyway -
vision becomes very, very foggy.

The whole thing just
blurs and blurs and blurs.

So you realise that we're,
obviously, very finely tuned to live

on a planet that has an acceleration
due to gravity of 1G.

When you go to 2G, it's difficult.

When you go to 3G and 4G,
it becomes unpleasant

and 5G anyway, for me,
was on the border of being

so unpleasant that you pass out.

So, although gravity feels weak
here on Earth,

it certainly isn't weak everywhere
across the universe,

and that's because gravity
is an additive force.

It scales with mass, so the more
massive the planet or star,

the stronger its gravity.

The body with the strongest gravity
in our solar system is the sun.

Our star has so much mass packed
inside a relatively small space that

it has a gravitational pull at its
surface 28 times that of the Earth.

If I were able to set foot on
this world, all the blood would be

poled out of my upper body, and I
would die in less than a minute.

But our sun's gravitational force
is nothing compared to the extreme G

found at the surface of one of
the strangest places in the universe.

Imagine the gravity on a world
with more mass than our sun,

crammed into a sphere
just 20 kilometres across.

We first detected such a wonder
just 40 years ago, but the story

of its discovery begins
over a thousand years earlier.

This is Chaco Canyon in New Mexico
in the south western United States,

and it was home to what's become
known as the Chacoan civilisation.

Well, this is Pueblo Bonito, one of
the so-called Chacoan great houses.

Back in the 1100s,
this place had over 600 rooms.

It's thought that this building
must have been ceremonial

or religious,
a cathedral, if you like.

The Chacoan great houses are aligned
with interesting objects in the sky,

so the points
at which the sun and moon rise

at important times of the year.

So it seems that by constructing
these grand buildings,

the Chacoans were not only
trying to place themselves

at the heart of local culture,

but also to place themselves
at the heart of the cosmos.

Very little is known
about the Chacoan culture,

because no written text
has ever been discovered.

But in another part of the canyon,
there is a record of a spectacular

event that they witnessed
in the sky in 1054.

Now, I've known about this place
since I was 12 or 13 years old,

and the reason is this book,
and the television series Cosmos,

Carl Sagan's masterpiece,

probably the most important reason
that I got interested in astronomy.

And on page 232, there's a picture
that's always fascinated me

and captured my imagination and it's
a photograph of that wall of rock,

and in particular a painting
that's on the overhang.

Because it's thought that
that painting is a record of one of

the most spectacular
and magical events in the cosmos.

On 4th July 1054AD,
a bright new star appeared,

and it outshone every other star in
the night sky for over three weeks.

It was so bright that it
was visible in the daytime,

and it's thought that this painting
is the Chacoan people's record

of that astronomical event.

The reason we think that is that
using modern computer techniques,

you can wind back
the night sky and say,

"Where would the moon have been?
Where would the stars have been?"

And you find that in that direction,

the moon would have risen
and tracked across the night sky,

and the new star would have been
very, very close

to the crescent moon.

We now know that that new star
was in fact the explosive death

of an old star,
a supernova explosion,

a star, literally, blowing itself
apart at the end of its life.

Throughout a star's life, there is
a constant battle between energy

pushing out and gravity pushing in.

As long as the star burns, the
two forces balance each other out.

But when it runs out of fuel,
gravity wins and the star collapses,

and then explodes with the
brightness of a billion suns.

We can no longer see the supernova
the Chacoans saw,

but we can still marvel
at what it left behind.

This is the Crab Nebula,
the remains of that

exploding star that the Chacoans saw
in these skies a thousand years ago.

It's an expanding cloud
of gas and dust, the remains

of that dying star, and the colours
are different chemical elements,

so the orange is hydrogen,
the red is nitrogen

and those filaments of green
are oxygen.

While the explosion blew most of the
stellar material out into the cosmos

to form this vast nebula,

we now know that this wasn't
the end of the story.

At the centre of the nebula lies
the remnant of the star, its core,

crushed by the force of gravity.

That is a neutron star,

an image taken by the
Chandra X-ray satellite.

The central blob there
is only about 20 kilometres across,

but it's got the mass of our sun,
a star the size of a city.

It's spinning at a rate
of over 30 times a second,

1,800 revolutions per minute,

and it really is an
astonishingly alien world.

As the neutron star spins,
jets of particles

stream out from the poles
at almost the speed of light.

These jets are powerful beams that
sweep around as the star rotates.

When the beams sweep across
the Earth,

they can be heard as regular pulses,
so we call them pulsars.

But it's not this rhythmic noise
that makes the Crab Pulsar a wonder.

It's the extraordinary nature of
gravity on this alien world.

If I were to be on its surface,
then the gravitational pull on me

would be a hundred thousand million
times that that I feel on Earth.

That means that if I were
to jump from the top of that

projection screen, by the time
I hit the ground,

I'd be travelling at over
four million miles an hour.

That's a lot of gravity.

Pulsars have such extreme gravity

because they're made of incredibly
dense matter.

To understand why, we have to look
at what gravity can do to matter

at the very smallest scales.

Everything in the universe
is made of atoms,

and until the turn
of the 20th century,

it was thought that they were the
smallest building blocks of matter.

I mean, the word itself comes from
the Greek "atomos",

which means indivisible.

But we now know that atoms
are made of much smaller stuff.

Atoms consist of an atomic nucleus
surrounded by a cloud of electrons.

And whilst almost all of the
mass is contained in the nucleus,

it is incredibly tiny compared to
the size of an atom.

If this were a nucleus, then the
cloud of electrons would stretch out

to something like a kilometre away.

I mean, that's from here
to that rock.

And electrons on this scale
are incredibly tiny.

They're just like specks of dust
and they're aren't many of them.

So imagine a giant sphere centred
on the atomic nucleus stretching out

all the way to that rock
and beyond,

with just a few points of dust
in it.

That's an atom.

So that means that matter
is almost entirely empty space.

I'm full of empty space.
The Earth is full of empty space.

Everything you can see
in the universe

is pretty much just empty space.

So if everything in the
universe is made up of atoms,

and atoms are 99.9999% empty space,
then most of the universe is empty.

But in the Crab Pulsar,
the force of gravity is so extreme

that the empty space inside the
atoms is squashed out of existence,

so all you're left with
is incredibly dense matter.

Imagine this was matter taken
from a neutron star -

then it would weigh more than
Mount Everest.

Or to put it another way, if I
took every human being on the planet

and squashed them so they were as
dense as neutron star matter,

then we would all fit inside that.

And if I were to drop my
neutron star stuff to the ground,

then it would slice
straight through the earth

like a knife through butter.

Wherever we look in the universe,
we see gravity at work.

It creates shape and structure.

It governs the orbits of every
planet, star and galaxy

in ways we thought we were able
to predict.

But there was a flaw in our
understanding of this force,

and it was exposed
by one of our close neighbours.

This is Mercury.

For thousands of years,
we've marvelled

as this fleet-footed planet
races across the face of the sun.

But 150 years ago,

astronomers noticed something
strange about Mercury's orbit.

Imagine that this rock is the sun,
and this is Mercury.

Now Mercury has
quite a complex orbit.

For one thing it's not
a perfect circle,

it's quite an elongated ellipse.

So at its closest approach
to the sun,

it's around
46 million kilometres away,

and then it drifts out to something
just under 70 million kilometres.

But you can calculate Mercury's
orbit very precisely

using only Newton's laws of gravity.

So astronomers used to predict the
exact time when you could look up

into the sky, look at the sun

and see the tiny disc of Mercury
pass across its face.

But the thing was,
they never got it right.

They predicted it time and time
again, and every time it happened,

they got it slightly wrong,
which was an immense embarrassment.

So what they did was that,
rather than question Newton,

they invented another planet,
and they called it Vulcan,

and they said that there must be
another planet somewhere

in the solar system, which is
always invisible from Earth

but which perturbed
Mercury's orbit a bit,

and so that was the reason
their calculations were wrong.

For decades, astronomers
searched and searched for Vulcan.

But they never found it,
because Vulcan didn't exist.

The explanation,
the real explanation,

was even more interesting than
inventing the planet Vulcan,

because it required a modification,

in fact, a complete re-writing
of Newton's law of gravity.

Gravity is NOT a force pulling us
towards the centre of the Earth

like a giant magnet.

In a sense, gravity isn't really
a force at all.

Describing the nature of gravity
turned out to be one of the great

intellectual challenges,

but almost 200 years after Newton's
death, a new theory emerged.

The new theory, called
general relativity,

was published in 1915 by Albert
Einstein after ten years of work,

and it stands to this day
as one of the great achievements

in the history of physics.

You see, not only was it able to
explain with absolute precision

the strange behaviour of Mercury,

but it explains to this day
everything we can see

out there in the universe that has
anything to do with gravity.

And, most importantly of all, it
explains how gravity actually works.

Gravity is the effect that the
stars, planets and galaxies

have on the very space that
surrounds them.

According to Einstein, space
is not just an empty stage -

it's a fabric called space-time.

This fabric can be warped,
bent and curved

by the enormous mass
of the planet's stars and galaxies.

You see, all matter
in the universe bends.

The very fabric of the universe
itself - matter - bends space.

I bend space, these mountains
bend space,

but by the tiniest
of tiniest of amounts.

But when you get onto the scale of
planets and stars, galaxies,

then they bend and curve
the fabric of the universe

by a very large amount indeed.

And here is the key idea.

Everything moves in straight lines

over the curved landscape
of space-time.

So what we see as a planet's
orbit is simply the planet

falling into the curved space-time
created by the huge mass of a star.

This is able to explain
Mercury's erratic orbit.

Because of the planet's
proximity to our sun,

the effects of the curvature of
space-time matter far more

for Mercury than for any other
planet in the solar system.

This idea of curved space
is difficult to imagine,

but if you could only
step outside of it,

if we could only float above
space-time and look down on it,

this is what our universe
would look like.

You would see the
mountains and valleys.

You would see the
little peaks and troughs

created by planets and moons,

and you would see these vast, deep
valleys created by the galaxies.

And you would see planets and moons
and stars circling the peaks

as they follow their
straight-line paths

through the curved landscape
of space-time.

So one way to think about gravity

is that everything in the universe
is just falling through space-time.

The moon is falling into the valley
created by the mass of the Earth.

The Earth is falling
into the valley created by the sun,

and the solar system is falling
into the valley in space-time

created by our galaxy.

And our galaxy is falling towards
other galaxies in the universe.

Einstein's theory of general
relativity is so profound

and so beautiful that it can
describe the structure and shape

of the universe itself.

But remarkably, the theory can
also predict its own demise,

because it predicts the existence
of objects so dense and so powerful

that they warp and stretch and bend
the structure of space-time so much

that they can stop time,
and that they can swallow light.

These are objects so powerful

that they can tear all the other
wonders of the universe apart.

Since the dawn of civilisation,
we've peered at the stars

in the night sky and tracked
the movements of the planets.

We see these familiar patterns
repeated across the whole universe.

But when we train our telescopes
to the stars that orbit around

the centre of our galaxy,
we see something very unusual.

Well, this is one of the most
fascinating and important movies

made in astronomy over the last
ten or 20 years. This is real data.

Every point of light
in this movie

is a star orbiting around
the centre of our galaxy.

They're known as the S stars.

Our sun takes around 200 million
years to make its way

around the Milky Way.

One of these S stars takes
only 15 years to go around

the centre of the galaxy.

It's travelling at 3,000 or
4,000 kilometres per second.

Now, by tracking the orbits,

it's possible to work out the mass
of the thing at the centre.

The answer took astronomers by
surprise, I think it's fair to say,

because the object in the centre
of our galaxy

is four million times as massive
as the sun,

and it fits into a space smaller
than our solar system.

Now there's only one thing that
anyone knows of that can be so small

and yet so massive,
and that's a black hole.

So what we're looking at here
is stars swarming like bees

around a super-massive black hole at
the centre of the Milky Way galaxy.

We think black holes can be
smaller than an atom,

or a billion times more massive
than our sun.

Some are born when a star dies.

When a star around 15 times
the mass of our sun collapses...

..all the matter in
its core is crushed

into an infinite void of blackness
known as a stellar mass black hole.

Black holes are the most extreme
example of warped space-time.

They have such enormous mass
crammed into such a tiny space

that they curve space-time more than
any other object in the universe.

The immense gravitational pull of
these monsters can rip a star apart.

They tear matter from its surface
and drag it into orbit.

This super-heated matter spins
around the mouth of the black hole,

and great jets of radiation
fire from the core.

Although these jets can be
seen across the cosmos,

the core itself remains a mystery.

Black holes curve space-time so much
that nothing,

not even light, can escape.

So their interior is for ever
hidden from us.

But because we understand how
matter curves the fabric of space,

it is possible to picture
what is happening.

Near a black hole, space and time
do some very strange things,

because black holes are probably
the most violent places

we know of in the universe.

This river provides a beautiful
analogy for what happens

to space and time as you get
closer and closer to the black hole.

Now, upstream, the water
is flowing pretty slowly.

Let's imagine that it's flowing
at three kilometres per hour,

and I can swim at four,

so I can swim faster than the flow
and can easily escape.

But as you go further and further
downstream towards the waterfall

in the distance,
the river flows faster and faster.

Imagine I was to decide to jump into
the river just there,

on the edge of the falls -

the water is flowing far faster
than I could swim.

So no matter what I did,
no matter how hard I tried,

I would not be able to swim
back upstream.

I would be carried
inexorably towards the edge,

and I would vanish over the falls.

Well, it's the same close to
a black hole, because space

flows faster and faster and
faster towards the black hole.

Literally, this stuff,
my space that I'm in,

flowing over the edge
into the black hole.

And at the very special point
called the event horizon,

space is flowing at the speed
of light into the black hole.

Light itself, travelling at
300,000 kilometres per second

is not going fast enough
to escape the flow,

and light itself will plunge into
the black hole.

Well, as you fall into a black hole,
across the event horizon,

then if you were going feet first,

your feet would be accelerating
faster than your head,

so you would be stretched,

and you would be quite literally
spaghettified.

Now as you get right to the centre,

then our understanding of the
laws of physics breaks down.

Our best theory of space and time,

Einstein's theory
of general relativity,

says that space and time
become infinitely curved,

that the centre of the hole
becomes infinitely dense.

That place
is called the singularity,

and it is the place where our
understanding of the universe stops.

Gravity is the great creator,
the constructor of worlds.

That's because it's the only force
in the universe

that can reach out across the vast
expanses of space

and pull matter together
to make the planets,

the moons, the stars
and the galaxies.

But gravity is also the destroyer,
because it's relentless,

and for the most massive objects
in the universe,

for the most enormous stars,
and the centres of galaxies,

gravity will eventually crush matter
out of existence.

Now, the word beautiful
is probably over-used in physics.

I probably over-use it.

But I don't think there is any
scientist who would disagree

with its use in the context of
Einstein's theory of gravity.

Because here is a theory that
describes a universe that

is bent and curved out of shape
by every moon, every star

and every galaxy in the sky.

And everything in the universe
has to follow those curves,

from the most massive black hole
to the smallest mote of dust,

even to beams of light.

But the most tantalising thing about
Einstein's theory of gravity

is we know that it's not complete.

We know that it's not the
ultimate description

of the structure and shape
of the universe.

And that, for a scientist, is
the most beautiful place to be,

on the border between the known
and the unknown.

That is the true wonder
of the universe -

there's so much more left of it
to explore.

Subtitles by Red Bee Media Ltd