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 balls 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.
EXPLOSION
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!