01x03 - What Is Gravity?

[bunniefuu]

For more than a decade, I've made
programmes about the universe...

..and our place within it.

There it is! There it is!

I can see the parachutes.

I've had some remarkable
encounters...

I can't believe you can just
stand next to a spacecraft!

Release.

..memorable experiences...

They came down exactly the same.

Wow.

MUFFLED SPEECH

..and explained some beautiful
science about how our planet works.

That really is the thin blue line
that protects us.

Look at that!

CHEERING

Now I'm taking a new
look at those past programmes.

This is surely as close as I'm going
to get to being in space.

I think our knowledge really has
moved on since we made these films,

so it's really interesting
to revisit

some fundamental questions again.

Are we alone in the universe?

What really is gravity?

Where will the exploration
of space take us?

And what is time?

BEEPING

Beeping's never good.

This time at the Royal Institution,

I'm looking back at some of my old
films to explore gravity.

It's the force of nature
we are most familiar with,

but our understanding of it has been
transformed by recent discoveries.

It's one of those things
we've been studying

for hundreds of years, centuries.

We all learn at school

it's the force that pulls
you down to the ground.

If you throw a ball up in the air,
it returns to the ground

because of gravity, and that seems
to be all there is to it.

There is a lot more to it than that.

In the last 12 months or two years,
we've made profound discoveries

about the nature of gravity and the
way it behaves, and how this relates

to the origin of the universe
and strange things like black holes.

Our theory of gravity has become
the attempt to understand

the nature of space and time.

And that word, gravity,
is really a word

that's hiding the investigation

of the deepest structure of reality.

And that's what I find interesting!

Even the most basic understanding
of gravity has allowed us

to do some extraordinary things.

This is the Kazakh Steppe,

and certainly at this time of year -
in mid-March -

it's a vast, icy wilderness.

And it's just flat
as far as the eye can see,

and for around 800,000 square
kilometres, which is basically the

reason that we're here,

because we're part of a
search and rescue mission.

Tomorrow morning, we're going to
meet three astronauts out there

in the snow, because we're going to
rendezvous with a Soyuz spacecraft

as it returns from the
International Space Station.

Right here you can see in the
foreground the three departing

crew members giving
the final wave goodbye.

INDISTINCT RADIO COMMUNICATIONS

Once the hatches are closed,

the Soyuz, containing its three
human passengers, undocks...

Physical separation confirmed.

Confirmed at 7.02pm. Central Time.

..and the tiny craft gently
drifts away.

Separation.

Copy.

The technology that will bring these
three humans back to Earth

and the physics that will
guide them home

is based on a knowledge of gravity
dating back hundreds of years.

Even I, just knowing a bit of
physics, in my head can calculate

exactly what the astronauts
have to do

to re-enter the Earth's atmosphere.

All you need are the two laws
written down first

by Isaac Newton,

F=MA

and the universal
law of gravitation.

Now, what you can show from those,
really simply,

is that for a circular orbit,
which is what the

International Space Station is
basically in, the velocity flying

along there is given by the
square root of GM over R

where M is the mass of the Earth

and R is the distance from the
centre of the Earth.

And the equations tell you that
to return to Earth,

all the astronauts need to do
is reduce that velocity

by 128 metres per second

and gravity will do the rest.

And here is the important thing -

I can do that because I know
those two equations.

Why do I know them?

Because I read them in a text book
that was based on Newton's work

and published in 1687.

But if I had to do that
from scratch,

if I had to come up with
those two equations,

it'd never happen.
Newton was a genius.

He worked for decades on those
equations. I would have no chance.

BEEPING

ARCHIVE: This view from external
cameras

on the International Space Station

showing the entry of the
Soyuz vehicle

as it barrels through the
Earth's atmosphere,

streaking towards
a central steppe of Kazakhstan.

I've got to say this is one of the
most exciting things I've ever done,

waiting for a spaceship to return

from the International Space
Station. It's just...

LAUGHS

Go!

That film
was one of the most difficult

and fun things that I've ever
been involved in.

HORN BLASTS

We had 4x4 vehicles that
got stuck in the snow

and so the Russian army said to us,

"Well, two of you can get in the
back of our big Russian army truck."

So me and the cameraman got in the
back of a Russian army truck.

Erm, lots of vodka was consumed!

There it is. There it is.

Right there - you can
see the parachutes.

We're the first there. We're
the first vehicle there, actually,

other than one.
It's quite remarkable!

It's so informal.

It is just a load of people jumping
out of the back of this truck

and wandering up to a spaceship.
Look!

I couldn't believe it.

You wouldn't be able to do that
if it was Nasa!

You can smell a faint...

..faint burning smell - not
surprising when you see the...

..the, er, damage.

Well, the sacrificial heat shield
has burnt away to protect it

on its way through re-entry.

This is incredi...

I can't believe you can just
stand next to a spacecraft!

But if you turn around now, the
first astronaut's going to come out.

I think it's going to be the
captain first.

CHATTERING

CAMERA SHUTTERS CLICK

MALE VOICE: Well, Mike.

Welcome home, man.

Welcome home!

You can see what a
physical experience it must be.

I suppose not only the re-entry,
which is, you know, only an hour

and probably pulled four or five G.

But after living on the Space
Station for six months,

to feel Earth's gravity, to feel
this cold air again, it...

They look very happy.
They're all smiling.

They look absolutely knackered!

CHATTERING

Those astronauts have not
experienced gravity

for six months, or more
in some cases.

And so when you return,
when you see them

literally being pulled out of the
Soyuz and put on deck chairs,

which is what happens, in the snow,
and they sit there,

you see how strong a force
gravity actually is

to us here on the surface
of the Earth.

We've evolved for our hearts
to pump blood up against the force

of gravity into our brains, because
we're upright most of the time.

When gravity goes away then your
body will change and adapt.

Astronauts speak of the shape
of the eyes changing

and so your vision changes.

And if you've lived without it for
six months, it's very difficult

to come to terms again with this
force that we all live with

and so don't really give
a second thought.

But when you do,
things start to get interesting.

The story of our current
understanding of gravity

begins just over 300 years ago

with the man who devised the maths

that I used in the back of that army
truck - Sir Isaac Newton.

In one of the first films I made,
I visited his famous orchard.

Film about gravity...

..apple.

It's a cliche,

but the story goes that it
was in this orchard that Newton was

sat thinking about the universe,
and an apple fell on Newton's head

and got him thinking about what
it is that makes the apple fall,

what force pulls the apple towards
the ground.

Newton suggested that the apple
falls because of a force

of attraction that naturally exists
between the apple and the Earth.

It's this force that we
know as gravity.

But Newton's real genius was not
to just stop with the apple,

but to ask the question, "Is the
same force that causes the apple

"to fall here on Earth also
responsible for the movement

"of much bigger things out
there in the cosmos?"

Newton believed that gravity is
a force that acts throughout

the entire universe.

In 1686, he finally managed
to break it down

into one single
mathematical equation.

Newton's understanding
of gravity is actually incredibly

simple, that the force between two
objects depends on only two things -

the mass of the objects,
and the distance they are apart.

So the more massive the objects,
the stronger the force.

And the further the objects
are apart, the weaker the force.

With one beautiful bit of maths,
Newton had figured out gravity.

But not just here on Earth.

The Moon seemed to orbit the Earth
exactly as he predicted...

..as did the planets
orbiting around the Sun.

Newton believed we live
in a universe in which, ultimately,

the movement of everything
can be predicted.

So you don't need anything
complicated to predict

the motion of the planets or
the motion of a cannonball

if you fire it from a cannon.

The basic thing you need

is an equation.

You can predict where planets
are going to be,

and you can predict how things are
going to fire through space

and so on.

The Sun reigns over a vast empire
of worlds,

all moving like clockwork.

Everything within its realm obeys
the laws of celestial mechanics.

These laws allow us
to predict exactly

where each world will be
for centuries to come...

..and exactly where I needed to be
to experience one of the solar

system's most amazing displays.

This is Varanasi.

For Hindus, it's one of the holiest
sites in all of India.

Part of what makes it so special
is the orientation of its sacred

river as it flows past the city.

This is the one place on the Ganges
where you can bathe

in the river on this shore and you
can see the sunrise on the eastern

shore. It's the only place where
the Ganges turns around to the

north, so you can do that.
When the sun rises tomorrow,

a truly extraordinary phenomenon
will take place -

a total eclipse of the Sun.

It's an auspicious occasion
for a place that ancient Hindus

knew as the Solar City.

Science is different to all
the other systems of thought,

the belief systems that have
been practised in this

city for millennia
because you don't need faith in it.

You can check that it works.
So, for example,

I can tell you that tomorrow
morning at precisely 6.24am,

the Moon will cover the face of the
Sun and there'll be

a total solar eclipse.
I could tell you that in 2904,

there will be five solar
eclipses on the Earth.

And I could tell you
that on July 16th, 2186,

there will be the longest solar
eclipse for 5,000 years,

of seven minutes.

However, as anyone who's been to
India in June, July,

will know, the climate is not
entirely predictable.

Right? It's a cloudy, rainy place!

And so we took a huge gamble.

It's 5.28, so that's time
of first contact

and you can't see
the disc of the Sun at the moment.

It's obscured by low cloud.

The edge of the Moon is at this
point just beginning to touch

the disc of the Sun.

You can see the Sun emerging through
the clouds, see the disc.

EXCITED CHATTER

Oh, and you can see the Moon!

Can you see the moon on the top?

Oh, yeah!

CROWD CHATTERS

It's just vanished.

You can see the limb of the Moon
there - absolutely fantastic.

Yeah? See...?

You can see the
celestial mechanics,

the clockwork of the
solar system, at work.

The alignment is absolutely perfect.

Oh, there!

CROWD CHEERS

Oh-ha-ha!

CHEERING AND APPLAUSE

Look at that!

You know, if you ever needed
convincing that we live in

a solar system, that we are on a
ball of rock orbiting around the Sun

with other ba*ls of rock,
then look at that.

That's the solar system coming down
and grabbing you by the throat.

It's one of the most remarkable
things I've ever experienced.

Because this piece of rock went
across the face of the Sun,

on time, to the second,

exactly as had been predicted
using Newton's laws.

And I got this feeling that
I'm on a piece of rock in space,

and it was this totally
unexpected feeling.

Look at that!

CHEERING AND APPLAUSE

One of the most interesting
things about gravity is that it's

so weak, which you can see.

I mean, I can lift my hand
up off the ground now.

There's a whole planet
pulling that down,

and I can just lift it up.

So it's tremendously weak.

But it's additive. It only adds.

There's no negative gravity,
like with electricity.

There's plus charges and negative
charges, and they kind of

cancel each other out,
or they DO cancel each other out.

And so, because gravity is only
additive and only attractive,

then notwithstanding its weakness,
if you get a big enough

lump of matter squashed together,
gravity will dominate.

The Earth's gravity
pulls everything down,

from people to snowflakes...

..to the very rock that the
Earth is made of.

And this is ultimately why
the Earth is spherical.

So why does gravity sculpt
things into spheres?

Well, the first thing to say
is that it doesn't, necessarily.

If I pick up a snowball...

..it's not spherical.
Kind of an irregular shape.

But if I apply pressure to it

and squash it evenly
in all directions,

then I can turn that into a sphere,

and that is what's happening
with gravity.

As I start adding mass to it, that
gravitational pull becomes bigger.

So I'll get to a point where this
snowball, if I kept adding mass to

it, would be so massive that the
gravitational pull on its surface

would be so strong that it would
start to squash the material

out of which it's made -
in this case, snow,

or in the case of a planet or moon,
the rock.

That pressure exerts on the surface
equally in all directions

because gravity works
equally in all directions.

Now, you could ask the question -
how much matter do I need

for gravity to get strong
enough to start overcoming

the strength of rock
and sculpting things into spheres?

Well, that minimum size
has got a name.

It's a brilliant name.
It's called the potato radius.

You can see why. Because things
that are too small for gravity to

be strong enough to sculpt them look
like misshapen potatoes.

The great thing is you don't
even need to imagine it.

You can calculate it.

Now, I did that this morning,

and I got an answer, just roughly,
between 100 and 200km.

The brilliant thing,
the most beautiful thing,

is if you look up into space
and look at the moons of Mars

and Saturn and Jupiter and objects
out there in the solar system,

you will find that,
roughly speaking,

if their radius is bigger than about
200km, they're beautiful

spheres. And if the radius is less
than about 200km,

they look more like misshapen
potatoes. So you can calculate it.

If you're small,
spheres don't come easily.

Even asteroids or moons
don't quite manage it.

The potato shape might
be as close as you can get.

But when you're the size
of a planet,

spheres come naturally.

Four-and-a-half-billion years ago,

rocks surfing the Sun
began sticking together

until they had sufficient mass
for gravity to really get to work

turning potato shapes into one very
important sphere

suspended in space.

A universal law sculpted
the familiar, elegant,

symmetrical shape of our planet.

Gravity is sort
of an enigma in many ways.

It can squash things down.

But in the right balance,
with the right amount of stuff

and the right amount of spin
and the right conditions,

it can sculpt the most beautiful
things in the universe.

When dinosaurs roamed the Earth,
we now suspect Saturn

had a moon in orbit that no
longer exists.

A moon perhaps 400km across

and formed almost entirely of ice.

But this world was doomed.

It found itself orbiting too close

to resist the immense
forces of Saturn's gravity.

Gravity is the sculptor
of the Saturnian system

but also the instigator of change
within it.

We tend to think of gravity
as a force that pulls things

together but it can also act
to rip things apart.

Think about the tides here on Earth.

They're caused by the difference
in gravitational pull

of the Moon from one
side of the Earth to the other,

and that difference, which can be
quite subtle, is also powerful.

It can move entire oceans.
It's called tidal gravity.

But the Earth also has a tidal
effect on the Moon,

and because the Earth is an
entire planet,

that effect is much more powerful.

The pull of the Earth is enough to
deform the Moon's surface.

The effect was particularly strong
four-and-a-half-billion years ago,

when the Moon was nearly
17 times closer.

Back then, the pull from the Earth

caused a tide of solid rock
to rise and fall.

If the Moon had been any nearer,

it would have crossed what
we call the Roche limit -

a place where tidal gravitational
forces are so strong,

moons can get ripped apart.

The Romans named the planet Saturn
after their god of time and harvest,

and in one of the more gruesome
tales from classical mythology,

Saturn actually ate his
newborn babies

in order to prevent them
from taking his power.

What's sort of interesting is that,
at least metaphorically speaking,

they weren't far wrong.

Just beyond Saturn's atmosphere,

our leading theories suggest an
icy moon must have approached

close to, or even just inside,
the planet's Roche limit.

As Saturn's immense tidal
gravitational forces

acted across the moon,
it began to rupture.

Saturn began to devour its child.

Up to 15,000 trillion tonnes of ice

broke apart in orbit around Saturn.

Because of the speeds the ice
fragments were travelling,

it's likely that, in just a few
days, they spread out

to encircle the great giant.

Saturn's iconic ring had been born.

Gravity has two sides,
two faces, if you like.

On the one hand, it's the great
sculptor of the universe.

It creates the structures we see.

But also, ultimately,
gravity can be a destroyer.

If you think about what a star is,

then it's a... it's a fight.

Gravity is trying to squash
everything down

and a star resists that
by carrying out nuclear fusion

producing heat through these nuclear
reactions to hold itself up.

We found an unusual way to
illustrate this fight

in Wonders of the Universe.

There was the idea that we could
film in this prison

as though it was a star.

And because we found out they were
going to demolish the prison, we had

this opportunity for me to describe
this battle in the star

as it creates the heavy elements

and the elements of life
in the universe.

Imagine this old prison in Rio
is a dying star.

Out there is the bright surface
shining off into space.

As I descend deeper and deeper
into the prison,

the conditions would become
hotter and hotter

and denser and denser

until down there,

in the heart
of the star, is the core.

Deep in its core, the star is
fighting a futile battle

against its own gravity.

As it desperately tries to stop
itself collapsing under its

own weight, new elements are made
in a sequence of separate stages.

Stage one is while there is still
a supply of hydrogen to burn.

Whilst the star's burning hydrogen
to helium in the core,

vast amounts of energy are released
and that energy escapes, literally

creating an outward pressure which
balances the force of gravity

and, well, it holds the star up
and keeps it stable.

But eventually, the hydrogen
in the core will run out,

and at that point the fusion
reactions will stop,

no more energy will be released,

and that outward pressure
will disappear.

Now, at that point, the core will
start to collapse very rapidly,

leaving a shell...

..of hydrogen and helium behind.

Beneath this shell,
as the core collapses,

the temperature rises again.

Until, at 100 million degrees,

stage two starts

and helium nuclei
begin to fuse together.

It's kind of nerve-racking
because you're walking through

a prison that's been wired up to
explode, and you're allowed to do it

for some reason I've never really
understood! But we did it anyway.

Helium fusion does two things.

Firstly, more energy is released,
and so the collapse is halted.

But, secondly, two more elements
are produced in that process...

..carbon...

..oxygen -
two elements vital for life.

So this is where all the carbon
in the universe comes from.

Every atom of carbon in my hand,

every atom of carbon in every
living thing on the planet

was produced
in the heart of a dying star.

Compared to the lifetime of a star,
the creation process of carbon

and oxygen is over in the blink
of an eye

because, in only about a
million years,

the supply of helium in the core is
used up. And for stars as massive

as the Sun, that's where fusion
stops, because there isn't enough

gravitational energy to compress
the core any further and restart fusion.

But, for massive stars,
the fusion process can continue.

When the helium runs out,
gravity takes over again

and the collapse continues.

The temperature rises once more,

launching stage three...

..in which carbon fuses
into magnesium,

neon, sodium and aluminium.

And so it goes on.

Core collapse followed by the next
stage of fusion to create

more elements - each stage hotter
and shorter than the last.

And eventually, in a final stage
that lasts only a couple of days,

the heart of the star is transformed

into almost pure iron...

..whose chemical symbol is Fe.

And this is where the
fusion process stops.

In its millions of years of life,

the star has made all
the common elements,

the stuff that makes
up 99% of the Earth.

The core is now a solid
ball of those elements,

stacked on top of each other,
in layers.

On the outside,
there's a shell of hydrogen.

Beneath it, a layer of helium,

then carbon and oxygen

and all the other elements,
all the way down to

the very heart of the star...

..and once that has fused
into solid iron,

the star has only seconds
left to live.

When a star runs out of fuel,

then it can no longer release energy
through fusion reactions,

and then, there's only one thing
that can happen.

expl*si*n

HELICOPTER WHIRS

In about the same amount of time

it takes this prison block
to crumble,

the entire star falls in on itself.

In a battle like that,
gravity always will win.

The remarkable thing about
Newton's law of gravitation

is that we still use it today.

It's remarkably accurate.

But it turns out

that it's not perfect.

It's not a perfect model of gravity.

Newton famously said
he stood on the shoulders of giants.

He didn't work in isolation.
Scientists never do.

Galileo was extremely important

to Newton's laws.
Galileo had developed

a lot of the insights
that led to Newton.

And Galileo noticed that
things fall at the same rate

under the action of gravity.

That's really strange because
gravity is a force between things,

and if the thing is more
massive, like a big star,

it has more gravitational pull than
a little bowling ball. Right?

It's kind of obvious. But then you
might ask yourself, in that case,

why does everything fall to the
ground at exactly the same rate?

Well, that actually is a clue

that there's something much more
interesting about gravity

than just a force between things.

It took the genius of Einstein,

who started thinking
about gravity 200 years

after Newton, to realise
that the way things fall

is telling us something profound

about the nature of reality itself.

He realised that
when things are falling,

you could see that they're not
falling at all

because they're all staying in the
same place, relative to each other.

So, it led him to his
theory of relativity.

And so, we thought,

well, what better way to introduce
Einstein's theory

than to go and film that in action?

There is a place where
you can see with your eyes

what Einstein saw in his mind.

This is Nasa's Space Power Facility
near Cleveland, Ohio,

and it is the world's
biggest vacuum chamber.

It's used to test spacecraft

in the conditions of outer space,

and it does that by pumping out

the 30 tonnes of air in this chamber

until there are about 2g left.

Galileo's observation

that all objects fall at

the same rate is correct

but it's far from obvious.

HEAVY THUD

In this case, the feathers

fell to the ground
at a slower rate than

the bowling ball because of

air resistance.

So, in order to see

the true nature of gravity,

we have to remove the air.

ALARM WAILS

OK, we dropped two millitorr
in the last 30 minutes.

61-04 manual, 10% open. Station
One,

go for drop. PCB 30-1,
pressure set point at 240 PSI.

We are go for drop.

Ten, nine, eight,

seven, six,

five, four...

Cameras on... two, one, release.

Ha! Exact! Exact! Look at that!

They came down exactly the same.

Wow! Oh, look, look, look!

BRIAN LAUGHS

Holy mackerel!

Exactly...

Exactly the same.

Feathers don't move, nothing.

Look at that! That's just...

You know, and then we
all realise, you know,

we're applauding a force of nature.

So, why are we doing that?

We all knew it was going to happen,

but it was so wonderful to see it.

Isaac Newton would say that
the ball and the feather fall

because there's a force
pulling them down, gravity,

but Einstein imagined the scene
very differently.

The happiest thought
of his life was this -

the reason the bowling ball
and the feather fall together

is because they're not falling,

they're standing still.

There is no force
acting on them at all.

He reasoned that if you couldn't
see the background,

there'd be no way of knowing that
the ball and the feathers were being

accelerated towards the Earth...

..so he concluded,

they weren't.

Einstein's observation that gravity

appears to vanish for objects

in free fall is something that

astronauts are well aware of.

We're all familiar with the
International Space Station,

and it looks for all the world
as if gravity's gone.

In a Newtonian sense, it hasn't.

They're not very far
away from the Earth.

Actually, what's happening
is the Space Station

is falling towards the ground.

Newton would say it's being
pulled towards the Earth,

just like any falling object,

but because it's travelling
very fast,

it constantly falls over

the edge of Earth's horizon.

So, it's in permanent,
a permanent state of falling.

We call it free fall.

But the point is that

in that setting,

the gravitational force,

in that small space,
the Space Station,

is not detectable. It's gone.

So, that is an insight into
Einstein's genius, isn't it?

Because certainly
in Einstein's time,

at the turn of the 20th century,

you'd never seen astronauts.

The insight that
that requires, which is

key to understanding gravity,

is quite dazzling.

So, why does Einstein say that

this idea that falling,

free fall, cancels out the force

of gravity? Why is that
the happiest thought of his life?

Because it led him to
the theory of general relativity,

which is often described -
and I think rightly -

as the most beautiful
of all physical theories.

And it's tremendously accurate.

It's the theory that

gives us the science of
cosmology and black holes,

and all these exotic things
in the universe.

Einstein's theory of
general relativity introduces

a radical new way of
looking at gravity.

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 planets, 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 on to the scale of
planets and stars and 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.

So, 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.

But Einstein struggled
with just how radical

the implications of
his new theory were.

Einstein himself noticed

that his equations

of general relativity suggest

pretty strongly that the universe

is stretching, or shrinking,

but not... it's not static.

The universe does not just
sit there, it changes over time.

So, you have this picture of the
fabric of the universe that is

either stretching or shrinking.

And that tells you that
if the universe is expanding today

then, in the past,
it was closer together,

and that's a tremendous realisation

because you have a theory

that's predicting something
that looks like a beginning.

His equations, his theory,

essentially predicting the Big Bang,

and that's quite a thing.

That you've written down an equation

and created a theory that predicts

the universe had an origin.

And it shocked Einstein,

because Einstein
really felt at the time

that the universe should be eternal,

because that avoids the thorny
question of the nature of creation.

Right? If the universe has
an origin, why? What did it?

There was another prediction
of general relativity

which concerned Einstein at the time
and for many years afterwards,

and that's associated with the...
that the possibility

allowed for in the theory

that gravity could squash
something down so small

that it would form an object

from which even light can't escape.

Nearly a century later,

one such object was discovered

in the heart of our galaxy.

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.

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 supermassive 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 superheated 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 forever hidden from us.

Einstein, along with pretty much,

it should be said, everybody
else in the 1920s and 1930s,

really through to
the '50s and '60s,

felt that black holes were absurd.

Many physicists said that
nature would not produce

such absurd things. There
should be a law of nature.

There should be a law against it.

Erm, trouble is, then,

we've got a picture of one.

HE LAUGHS

So, there's one. So, now,

that black holes

exist and the absurdities

have to be faced.

It's the most remarkable image,

I think, in my career in physics.

I mean, the idea that we
can now see a black hole

and we have a photograph of one -
this one, six billion times

the mass of the Sun -

is just remarkable to me.

This characteristic shape

was predicted by Einstein's theory.

The idea that there's this

rather strange area in the middle,

which is called the shadow
of the black hole, is odd.

It's a huge region of collapsed

space and time, inside which

is the mass. It's six billion times

the mass of our Sun.

Black holes raise
further profound questions

that we're struggling to answer.

At first sight, it might seem that

anything that falls into

a black hole - atoms,
or even information -

vanishes from the universe.

How could that happen?

Now, it might seem pedantic,
but it's one of the fundamental

laws of nature. In fact, I would
say now it is THE fundamental law

that underpins all of reality,

that information is not destroyed.

It seems to be fundamental
to the universe.

So, the question is, does the
information get out of a black hole?

And if so, how?

And it turns out,
in the last few years,

there seems to be an answer.

However... However,

the way that
that information gets out

is absolutely bizarre.

We now think that the information

is returned to the universe,

imprinted in radiation, as the

black hole slowly evaporates away,

but the details of this explanation

appear to suggest that space

and time are not what they seem.

So, the current best view of gravity

is that gravity emerges from some

description of the universe

that has got no space and time
in it at all.

It's so cool!

HE LAUGHS

It's so cool.

Bonkers!

HE LAUGHS

And that realisation

has come from thinking
carefully about black holes,

which are a prediction of
this 100-year-old theory,

Einstein's theory of
general relativity,

building on the shoulders of giants,

going all the way back
to Newton and Galileo.

Next time...

Where am I? Bit complicated -
sort of a spaceship/time machine.

Time - the constant ticking that

marks the passage of our lives...

The perfect Christmas!

We could look back in time

13 billion years.

..but it could be the greatest

illusion of all.

Accelerating...

Two sunrises - one day!