01x04 - What Is Time?

[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.

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

That really is the thin blue line
that protects us.

Look at that!

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

It's 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?

CLOCK TICKS

And what is time?

HIGH-PITCHED BEEPING

Beeping's never good.

I'm back at the Royal Institution
to re-watch some of my old films.

And to explore the thing
by which we measure our lives.

Time.

It's a concept which is so familiar,

and yet recent discoveries have
revealed it to be far stranger

than we could have
possibly imagined.

I think the question,
"What is time?"

is one of the most fascinating
and evocative scientific questions.

Because it's so fundamental
to human experience.

Our experience of the world
is that the past is written,

it has happened,
and the future is yet to be written.

It's open.

CLOCK TICKS

the ticking of the clock?

It's one of the most baffling
questions in science.

JAUNTY WHISTLING

Ah, excellent, there you are.
Where am I? A bit complicated.

Sort of a spaceship/time
machine/slash/swimming pool.

Optional hat stand. I need five
minutes of your time and obviously

when I say five minutes, I'm lying. No,
no, I'm just going to go give a lecture.

I know, I've just seen it. It's
great. But I haven't given it yet.

Tricky to explain. Seen it anyway.

Right, hold on to something,
probably your sanity.

Ready?

LOUD CRACKLING

Usually it just twirls around.

It's probably this.
Shut up, Brian!

I'm often asked about time travel,

and, yeah, some of it
comes from Doctor Who.

It would be great, wouldn't it,
to have a t*rd and to be able

to go back and see some of the
great moments in history.

DOCTOR WHO THEME TUNE

It's a tantalising prospect.

But before we can even begin
to consider time travel,

we should explore
how we experience time.

METALLIC THRUMMING

The way that we experience time
and talk about it,

is actually, if you think about it,
quite strange.

Because, you know, there are
60 seconds in a minute, 60 minutes

in an hour and 24 of those in a day,

and then there are 365 days
in a year.

It's interesting, I think, that
most of us have become completely

disconnected from what
that's actually telling us.

Which is that
we live on a ball of rock,

that is spinning around on its axis

once roughly every 24 hours or so.

And in a plane that can go
that fast,

you can really illustrate that.

Because we live on a spinning
sphere,

if we turn on the after-burners
and chase the setting sun,

something
really interesting happens.

This is a Eurofighter Typhoon.

It flies at, at least Mach 1.85,
twice the speed of sound.

I can't tell you exactly
how fast, because it's classified.

They go up to at least 55,000 feet

but again, I can't tell you,
it's classified.

And you can't film
down those air intakes

because they're classified as well.

This one is BAE Systems'
development aircraft.

I'm going to get in it in a minute,

and it's got all the test software
in and the pilot told me that,

you know, it's a bit ropey so press
control, alt, delete occasionally

if it all goes funny,
and usually it comes back on.

Which is good.

Get in. Feet-wise, comfy? Good.
That's it.

That's for, if you need to
control it at any point.

It's unlikely. Unlikely.

Have you got a sick bag?

No.

Worst comes to the worst,
do it in your glove.

CONTROL TOWER: Charlie 69,
runway 07, clear take off.

Surface wind 350 degrees,
seven knots over.

PILOT: So, ready? Yeah.
Going to go for it.

ENGINES ROAR

There we go. Wahee.

It's every bit as good
as you can imagine.

It's beautiful. What a plane.

On the east side, everything's
darkening up quite nicely

as the sun starts to set.

And on the ground it's already,
it's dark on the ground now,

as far as the sun is concerned.

So, accelerating.

So that's Mach 0.78. Yeah.

And the G-suit is in play here.
Yeah.

Turning directly towards
the setting sun,

the Typhoon accelerates to catch up
with the Earth's spin.

Beneath us, a 6,000
billion, billion-tonne rock

is spinning at 650mph.

But travel faster
than the planet's surface

and the normal passage of the day
is reversed.

Right, accelerating. Accelerating.

Oh, there we go,
that's acceleration. Mach 1.

Through the sound barrier.

As the jet accelerates,

it starts to overtake
the spin of the Earth.

Causing the setting sun
to rise again.

Starting to grow a little.
It is. I can see it.

We're beating the Earth.

Absolutely terrific.

Starting to climb again,
you can see it coming up. Yeah.

That's Mach 1.4.

So, that's a thousand miles an hour.

Yeah,
almost a thousand miles an hour.

And now the sun, it's almost
the full disc over the clouds.

The sunrise. It is.

Two sunrises in one day.

And all you need is the world's
most advanced fighter aircraft.

There we go.

Beautiful.

We've done it,
we've outrun the Earth.

BRIAN CHUCKLES

Goodbye, sun! Good.

Right,
let's get ourselves on our way home.

I was told that we caused a bit
of an incident in the Isle of Man

because the sonic boom
did rattle, rattle a few windows.

So, I apologise to all residents
of the Isle of Man!

RADIO CHATTER

This hidden celestial dynamics,
the Earth's spin and its orbit

around the sun, delivers our daily
and yearly experience of time.

Spring goes into summer, and summer
into autumn, and autumn into winter.

And we experience another season
and another year.

But, of course,
they're not always the same,

because every year that passes,
we get older.

And so in that sense, I think time
connects with the central tragedy

of human experience,

the fact that inexorably
and unavoidably

we get older and ultimately we die.

So we're born and we die.

And that's a consequence
of time passing,

at least from our point of view,
as fragile human beings.

CLOCK TICKS

If you want to understand something,
you've got to be able to measure it

and observe it,
and look at it and quantify it.

And if you look back at many
ancient civilisations,

the attempts to measure the
passage of time are rarely small.

Often, they're massive.

They're huge temples
that people built.

So, clearly, there's not only
the practical need to measure

the passage of time, but there's
some ritual attached to it.

This is Chankillo on the
northwestern coast of Peru

and it's one of South America's
lesser known archaeological sites.

But, for me, it is surely
one of the most fascinating.

Around 2,500 years ago,
a civilisation

we know almost nothing about

built this fortified temple
in the desert.

Its walls were once brilliant white
and covered with painted figures.

Today, all but the smallest
fragments of the decorations

are gone.

The details of this culture and all
traces of its language are lost.

And yet, if you stand
in the right place,

you can still experience
the true purpose of Chankillo

in just the same way as you could
the day it was built.

But to do that, you have to be here
before the sun rises.

WIND WHISTLES AND HOWLS

These towers form
an ancient solar calendar.

Now, at different times of year,

the sunrise point is at a
different place on the horizon.

Actually, December 21st, which
here in the southern hemisphere

is the Summer Solstice,
the longest day,

when the sun rises just to the
right of the rightmost tower.

Then, as the year passes,
the sun moves through the towers

until on June 21st, which is the
Winter Solstice, the shortest day,

it rises just to the left
of the leftmost tower.

Actually, just in between that
mountain you can see in the distance

and the leftmost tower.

So, at any time of year,
if you watch the sun rise,

you can measure its position
and you can tell within an accuracy

of two or three days, the date.

Today's date is September 15th.

And so that means
that the sun will rise

between the fifth
and the sixth towers.

Chankillo still works as a calendar
because the sun still rises in

the same place today as it did when
these stones were first laid down.

That is a magnificent sight
as the sun burns through the towers.

You can almost feel
the presence of the past here.

Imagine what it must have been like,
thousands of citizens stood here

to greet the sun, which was
almost certainly a deity,

almost certainly their god.

What a magnificent achievement.

It's probably one of our earliest
attempts to begin to measure

the heavens.

Time has played a central role
in the drama of human experience

for thousands of years.

But it's only when we try to measure
it accurately that we start

to appreciate its mystery.

So the way that traditionally
we measure the passage of time

is to look to the heavens
and look to when the sun

reaches its highest point in
the sky, and we call that noon.

That turns out not to be sufficient
because it's not accurate enough.

The Earth doesn't spin on its axis
at a constant rate.

The Earth wobbles around
by quite a bit, actually.

The Earth's orbit
is not entirely precise.

Things change.

So if we want to measure
increasingly small time intervals

accurately, then the heavens
are not the way to do it.

And so we can build
mechanical watches,

and that's what we did initially
when we had to measure time

more accurately for
navigational purposes.

But even bits of, you know,
cogs and gears and springs,

that's not particularly
accurate either.

And so if you really want to start
dividing time up into very

small intervals, which we have to do
today for so many reasons,

then you need to look
to the subatomic world

and you find that in the world of
atoms and subatomic particles,

you can identify exquisitely
accurate natural clocks.

MILITARY BAND PLAYS

I'm going to try and answer

one of the simplest questions

you could ask - what time is it?

These are the early days, right.

This is one of the first
television programmes that I made.

If there's one place on Earth
where you can come to find the time,

it's here in Washington DC.

This is the home
of the US Naval Observatory,

one of a select bunch of time lords
dotted across the planet

who are the keepers
of time on Earth.

COMPUTERISED VOICE: Universal Time,
19 hours, 54 minutes exactly.

A time now derived
from atomic clocks.

Dennis McCarthy, Director of Time.

We defined a time in the 1950s
based on the position of the moon

with respect to the stars.

We needed a more
accurate kind of time.

Not only is it...
does it have to be more accurate

but it has to be more accessible.

That accessible kind of time
is provided by atomic clocks.

What we need to tell time
is something which repeats

with great regularity
that you can count on.

How well we can tell time
depends on which atom you're using.

So we choose certain ones to...
to use for keeping atomic time.

Inside the clocks are the atoms
of a rare metal called caesium.

The electrons in the caesium
atoms are made to jump up.

Then as they fall back down,
they give out light.

These light waves peak over
nine billion times every second

and it's this light
that drives the clock,

effectively producing nine billion
ticks for each atomic second.

This number never changes,
never alters

and that's why it's so accurate.

So the atomic clock is actually
putting out an electronic signal

which is essentially analogous
to the ticking of a pendulum clock.

You know, a pendulum clock which
might tick once every second

or once every couple of seconds.

This thing is providing us
something

which is going nine billion times
per second.

So it provides us
with a very fine definition of time.

We actually have a number of clocks
at the Naval Observatory

located all over the grounds.

Here's... here's one of them.

This is the master clock system one.
Oh, yeah.

This is the master clock system two.

So if I want an answer to the
question - what time is it?

There it is? That's it.

COMPUTERISED VOICE: Universal Time
20 hours, zero minutes exactly.

Why do I need a clock that accurate?

Well, we do.

If you're using time to measure
distance, which is how the GPS

satellite navigation system works,
then it matters.

If your clock drifts
by one nanosecond,

then your position measurement is
drifting by something like a foot.

Atomic clocks can measure
and divide time into intervals

that are so minuscule that they
are beyond human perception.

But this precision means little
when we begin to consider

the timescales that determine
our ultimate fate - cosmic time.

Timescales in the cosmos
seems so unimaginably vast,

it's almost impossible
to relate to them.

Yet there are places on Earth
where we can begin to encounter

time on these universal scales.

This is Ostional on the northern
Pacific coast of Costa Rica,

and I've come here to witness
a natural event that's been

happening long before there were
any humans here to see it.

And I suppose it really is
a window into the distant past

of life on our planet.

PEOPLE CHAT AND LAUGH

Once the sun has
dipped below the horizon

and the moon conspired
to make the tides just right,

this beach is visited
by prehistoric creatures.

Under the cover of darkness,
they emerge from the ocean.

Playa Ostional is one of the
few beaches in the world

where large numbers of sea turtles
make their nests.

But what makes this truly remarkable
is the sheer length of time

scenes like this
have been playing out.

Well, this is part one of the
oldest life cycles on Earth.

On nights like these for the
last 100 million years,

turtles like this have been hauling
themselves out of the ocean

to lay their eggs.

It's an almost incomprehensible
time span.

I mean, 100 million years ago,
there were dinosaurs roaming

the Earth, but the Earth itself
looked very different.

I mean, South America was not
connected to North America.

North America was somewhere
over close to Europe.

Australia was connected
to Antarctica.

THUNDER RUMBLES

It really is quite
wonderful to be so close

to such an ancient cycle of life.

You can hear her
breathing, actually.

TURTLE EXHALES

So, a remarkable experience.

I mean, it really is beautiful to
see that on the one night

of many hundreds of millions
of nights

stretching back into the past.

And she's gone.

To witness a moment like this
is to open up a connection

to the deep past.

To experience timespans far longer
than the history of our own species.

Yet, even the 100 million year
story of the turtles

only begins to connect us with
the vast sweep of cosmic time.

Our entire solar system
is travelling

on an unimaginably vast orbit,

spinning around the centre
of our galaxy.

It takes 250 million years to make
just one circuit of the Milky Way.

In the entire history
of the human race,

we've travelled less than
a tenth of 1% of that orbit.

The timescales of our galaxy are
almost beyond human comprehension.

But when we extend our gaze
beyond the Milky Way

and out into the universe,

we come face-to-face
with truly deep time.

The time that ticks on your watch
is the time now,

the time of the present.

But the feeling we experience
as the present time

is something we shouldn't
take for granted.

Much of what we believe is in the
present is drawn from the past.

What we feel is happening now
happened a little while ago.

We feel that we experience
a now around us.

Everything we see happened now.

But actually the light coming
from distant things into your eye

takes time to get there.

So... you look at the sun.

The sun is 93 million miles away.

That means light takes over
eight minutes to get from it

into my eyes. So I'm seeing the sun
as it was eight minutes in the past.

It could explode and I wouldn't
notice for eight minutes.

I'd just see that beautiful
image of the setting sun.

The fact that light travels
at a finite speed

offers us a unique opportunity.

It allows us to look back,
not just eight minutes but millions,

even billions of years.

I've come to Baltimore
to look back in time.

Former director of the Hubble Space
Telescope Steve Beckwith

was responsible for taking
an extraordinary photograph.

As a director, I had at my
discretion 10% of the telescope time

per year, that I could
use for anything.

One year I took all of my time,
in fact I took a little bit more

than all of my time,
and decided that we would devote it

to the deepest picture
ever taken of the universe.

In 2004, Steve pointed the
Hubble Telescope at a tiny piece

of the night sky and took a
picture called the Ultra-Deep Field.

It took a million seconds
of exposure

on the Hubble Space Telescope, the
world's most powerful telescope,

and in this image we can look
back in time 13 billion years.

It's a difficult picture
almost to comprehend, isn't it?

Because in some sense, it's...
3D is the wrong word.

But it's, it's some sense...
Oh, no, it is the right word.

It is 3D.
We are looking back in time.

Every single galaxy in this image
can be dated.

This galaxy emitted its light

when the universe
was 8.8 billion years old.

Then, as you go back in time, this
is a galaxy that emitted its light

when the universe
was 3.3 billion years old.

You can see it looks
completely different.

It's really very chaotic.

And this is one billion years
after the Big Bang.

Very red, a little tail, very small.

The most distant galaxy
in the Ultra-Deep Field

is a red one that's right over here.
This one here?

The light from that was
emitted when the universe

was 700-800 million years old.

So, really, this is one
of the first structures

that formed in the universe?

This is one of the first
structures that formed

and it's one of the first structures
we've been able to see.

In a sense you see this...

..almost, I was going
to say paradoxical,

but strange behaviour of the
universe as revealed by astronomy.

Because I'm trying to say, well,
what does that look like now?

You know, what would
that look like now?

In a sense, it's the wrong
language to use, isn't it?

That's what it looks like now.
That's right.

Steve has turned his photo into
a movie to journey back in time.

You see these little pieces coming
at us? We're going back in time,

and you can see the
three-dimensional effect.

Some of these others
are a little farther back

but here we're going back,
we're probably back now

about three billion years
from the present.

As we keep going
and you get to the smaller ones,

you get back to about
eight or nine billion years.

And then, when we get to the
very tiny most distant ones,

we'll be back probably
ten or 11 billion years in time.

And we're deep into the universe
and we're just looking

at the smallest structures back
in time to about 13 billion years,

and suddenly we run out.

The Hubble Space Telescope
can see galaxies

that are as they were about 500
million years after the Big Bang.

But it can't see
further back than that.

The reason is that the
universe is expanding.

And, so, as the light travels
through the expanding universe,

it gets stretched.

And that means that it gets redder
and redder and redder

until it gets so red
that the Hubble Space Telescope

isn't sensitive to it.

So it can't see any
further away than that.

But a new telescope
with that capability

is due to be launched
later this year.

The James Webb Telescope is going
to be much more sensitive

to those, what we call,
the infrared light,

so the very long wavelengths.

And that means it will be able
to see back past those galaxies

that Hubble can see to the very
early history of the universe.

We'll be able to watch the formation
of the first stars and galaxies.

We're able to look very
far back into the past,

but we can't see into the future.

Which raises the question,

what is the difference
between the two?

Why does time appear to flow
only in one direction?

This question has puzzled
many of science's greatest minds.

All the laws of nature that we
have today that we consider

to be fundamental, so Einstein's
equations or Newton's laws

for that matter, all of them
make no distinction

in the direction of the
flow of time, if you like.

There's only one law of physics that
has an explicit direction of time

in it, and that law is concerned
with something called entropy.

Entropy explains why,
left to the mercy of the elements,

mortar crumbles, glass shatters
and buildings collapse.

And a good way to understand how
is to think of objects

not as single things but as being
made up of many constituent parts,

like the individual grains
that make up this pile of sand.

Now, entropy is a measure
of how many ways

I can rearrange those grains and
still keep the sand pile the same.

And there are trillions and
trillions and trillions

of ways of doing that.

I mean, pretty much anything
I do to this sand pile,

if I mess the sand around
and move it around,

then it doesn't change the shape
or the structure at all.

So, in the language of entropy,

this sand pile has high entropy

because there are many, many ways
that I can rearrange

its constituents and not change it.

But now let me create
some order in the universe.

Now, there are approximately as
many sand grains in this sand castle

as there are in the sand pile.

But now, virtually anything
I do to it will mess it up,

will remove the beautiful
order from this structure.

And because of that, the sand castle
has a low entropy.

It's a much more ordered state.

So, many ways of rearranging
the sand grains

without changing the structure -
high entropy.

Very few ways of rearranging
the sand grains without changing

the structure, without
disordering it - low entropy.

Now, imagine I was to leave this
castle in the desert all day -

then, it's obvious
what's going to happen.

The desert winds are going
to blow the sand around

and this castle
is going to disintegrate.

It's going to become less ordered.

It's going to fall to bits.

But think about what's
happening on the fundamental level.

The wind is taking
the sand off the castle

and blowing it over there somewhere
and making a sand pile.

There's nothing fundamental
in the laws of physics that says

that the wind couldn't pick up
some sand from over here,

deposit it here,
and deposit it in precisely

the shape of a sand castle.

In principle, the wind could
spontaneously build a sand castle

out of a pile of sand.

There's no reason why that couldn't
happen, it's just extremely,

extremely unlikely because there
are very few ways of organising this

sand so that it looks like a castle.

It's overwhelmingly more likely

that when the wind blows
the sand around, it will take

the low entropy structure,
the castle,

and turn it into a high
entropy structure, the sand pile.

So, entropy always increases.

Why is that?

Because it's overwhelmingly
more likely that it will.

There was a tremendous
sandstorm and we all hid,

and it was just vicious.

And when we came out,

we had cameras and we were opening,
taking the lenses off

and just pouring sand out of them!

The Second Law of Thermodynamics,
for me, demonstrates everything

that is powerful and beautiful
and profound about physics.

Here's a law that entered science
as a way of talking about how heat

moves around and the
efficiency of steam engines.

But it ended up being able to
explain one of THE great mysteries

in the history of science.

Why is there a difference
between the past and the future?

You see, the Second Law says that
everything tends from order

to disorder.

That means that there
is a difference

between the past and the future.

In the past,
the universe was more ordered

and in the future,
the universe will be less ordered.

And that means that there's a
direction to the passage of time.

So, the Second Law of Thermodynamics
has introduced the concept

of an arrow of time into science.

The so-called
thermodynamic arrow of time,

this idea that entropy
always increases,

is well understood, but actually its
origin at a deep level

is not so clear.

The reason seems to be
something to do with

the origin of the universe itself
and the presence of this

strange thing called the Big Bang
way back in our past.

Why were things in the past
so beautifully ordered,

that the universe can fall to bits
gently and in the process,

in the transition from order
to disorder,

these tremendous structures,

of which we are
the most magnificent example,

can exist for a brief time
in the universe?

There are almost more opinions than
there are theoretical physicists

on this question!

The Second Law of Thermodynamics
is an explanation for the arrow

of time, and what makes the past
different from the future.

But it is silent on a question
that excites science fiction writers

and physicists alike.

If I'm in a big audience
of people and someone says,

"Is time travel possible?" there
might be someone who thinks

they know it just adds a bit
of a laugh and chuckles.

It is a very good question.

The answer is,
we don't know for sure.

I think that, ultimately,

the desire to go back in time

is tied in to the tragedy
of human experience.

We've all...

There are not only moments
that we wish we could relive,

there are people that we
wish we could see again.

People who are no longer with us.

So, the idea that somehow the past
might be accessible is extremely

enticing and powerful, emotionally.

But that's impossible if the past
only exists in our memories.

That was the accepted view.

But Einstein changed that.

If we take his Theory of Relativity
at face value,

then every moment of our past
still exists.

We feel as if we move through
space as time ticks by.

But that's an illusion.

The separation of space
and time is false.

The first person to realise that
was Albert Einstein.

He thought deeply about motion,
about the idea that we can't tell

whether we're moving or not.

And he tried to reconcile that with
our picture of the universal laws

of nature, and he found
that he could do,

at the expensive of jettisoning
space and time as separate entities

and merging them together
into a unified whole,

the fabric of the universe
called Spacetime.

As the Earth moves
through Spacetime,

its orbit traces out a spiral
as it circles the sun

and races into the future.

It never returns to the same place

because each moment is
a different location

in the fabric of the universe.

And just as the Earth
travels relentlessly onwards

on its path through Spacetime,

so must we.

So this is how Einstein asks us
to picture the sweep of our lives,

the experience of living.

Our lives are a series of moments

and they're laid out
like places on a map.

There's me as a little baby.

My dad with my grandad.

That idyllic summer some time in the
early '70s in a paddling pool

with my sister.

When I was about four years old.

And the perfect Christmas
with my grandparents

sometime back in the 1970s.

There's me when I was 20 years old
with a ridiculous haircut

playing a gig somewhere in the
middle of Europe.

In Budapest, I think.

Wedding day.

And me in Oldham, where I grew up,

with my little boy, George.

That was a very personal scene.

It was really personal because,
actually, it wasn't long

after my dad had died.

So in my mind, there was,
obviously, as with everyone

who goes through those
experiences as we all do,

there's a, you know,

a reconsideration
or a rethinking of the past.

You find your mind wandering back
into the past and past events.

This isn't exactly like a map.

See, I can return to these places
in space, to Oldham,

to central Europe, to Duluth,
Minnesota where I got married,

back to Oldham again.

But I can't return to these moments,
to these events in Spacetime.

Because of the geometry
of Spacetime itself,

we are compelled to move inexorably
into the future.

What's interesting is that in
Einstein's Theory of Relativity,

the past is not accessible

but it's still there
in a very real sense.

I think the bottom line is,
no, you can't travel into the past.

Probably!

But the explanation for that
is anything but trivial.

But time travel into the future
is a different matter.

We are all travelling
into the future all the time.

But Einstein's Theory of Relativity
tells us that we are each

doing it at different rates.

We experience time passing
because we are all travelling

along the time dimension.

But, strangely, Einstein also said

we don't all experience
the same time.

Spacetime can be pictured
as a sort of fabric

where time and space are
inextricably woven together.

As a result, the dimensions of space
and time can get mixed up.

Although we are all travelling
through Spacetime

at the speed of light, it's
the mixing of time and space

that Einstein said causes time to
tick differently for each of us.

For Einstein,
time wasn't like a metronome

that just ticks
the same for everybody.

It's different for you and me,

and everywhere in the universe, the
metronomes tick at different rates.

Einstein said that two people
will only ever agree

on the speed time ticks if they're
standing next to each other.

If I was to fly past you
incredibly fast,

I would see your time
tick much slower than mine.

This idea lies at the heart
of Einstein's Theory of Relativity.

No-one has a right to the... to
claim that their time is THE time,

the absolute time.

It just depends on
who's moving relative to who.

Because of the mixing
of space and time,

time ticks differently for you
relative to other people,

depending on how fast
everyone is moving.

When someone moves relative to me,

they use some of their speed
of light through Spacetime

to move through space.

So they haven't got as much left
to move through time,

and that means that their speed
through time is a bit slower.

They've sort of, in a very real
sense, used a bit of it up.

It's a really profound way
of understanding

Einstein's theory of Spacetime.

And the strange nature of time
doesn't stop there.

It's not just how fast you're moving

but what you're next to
that also affects time.

According to Einstein,
you should see the time tick slower

at my feet
than at the top of my head.

This is because the nearer you are
to a big object like the Earth,

the more bent and warped
is the Spacetime,

and the slower time ticks.

On our planet
the effect is minuscule

but out there in the universe, the
vast mass of the stars and galaxies

bend and warp the Spacetime so much
that time ticks all over the place.

There's a very famous experiment
in 1971 by Hafele and Keating

in which they decided to,

let's say, put Einstein to the test.

They got atomic clocks
and they flew them

around the world on civil airliners
one way, and around the world

on civil airliners the other way,
synchronised them all

before they left and compared
them when they came back.

And the clocks that went eastward
around the world

lost 59 nanoseconds,

59 thousand-millionths
of a second.

And the one that went westward
gained 273 nanoseconds.

The people on those planes
aged at different rates,

the same amount as the shift
in the time difference

is measured by the clocks.

And so we can travel
into the future,

but actually, we can travel
into the future at different rates.

Let's say I get into spacecraft now
and fly off to Alpha Centauri

and come back again.
I can arrange that journey,

and if I can travel fast enough,
such that I come back 1,000 years

in the future or 10,000 years
in the future.

Actually, the closer
I can make it to the speed of light,

the further into the future
I can get.

I think the more
that you know about time,

the more you consider the
fundamental nature of reality itself

and discover that it's nothing
like our experience of the world,

then the more astonished you become!

And at some point,
I'll probably become so astonished

that I'll no longer be
able to speak about it!

Even though physicists are still
grappling with the fundamental

nature of time, we do know
with reasonable certainty

how time will ultimately end.

There are few places on Earth
where you can get an inkling

of what the far future has in store.

HELICOPTER ROTORS WHIR

This is Namibia's Skeleton Coast,
where the cold waters

of the South Atlantic
meet the Namib Desert.

And it is one of the most
inhospitable places on Earth.

Back in the 17th century, Portuguese
sailors used to call this place

The Gates to Hell because
this dense fog that you see

pretty much every morning along this
coast, coupled with the constantly

shifting shape of the sandbanks,
meant that over the years

literally thousands of ships
were wrecked along this coastline.

And even if you made it to shore,
that wasn't the end of your problems

because the currents
are so strong here

that there is no way
of rowing back out to sea.

If you look that way,
there's just hundreds of miles

of inhospitable desert.

So, it genuinely was
a place of no return.

If you were shipwrecked here,
this was the end of your universe.

This is the Edward Bohlen.

She was once an ocean-going steamer
ferrying passengers and cargo

between here and Europe.

On 5th September 1909,
she ran aground in thick fog.

Yet, like all the vessels wrecked
along this shoreline, the time it

takes her to decay to nothing will
be far longer than her time at sea.

And in the far future, long after
our own sun has run out of fuel,

a similar destiny awaits
the universe's last stars.

A black dwarf will be the final
fate of those last stars -

white dwarfs that have become
so cold that they barely emit

any more heat or light.

Black dwarves are dark, dense,
decaying bowls of degenerate matter,

little more than
the ashes of stars.

Their constituent atoms
are so severely crushed

that black dwarves are a
million times denser than our sun.

Stars takes so long
to reach this point

that after nearly 14 billion years,

we believe there are currently
no black dwarves in the universe.

But despite never seeing one,
we can still predict

how they will end their days.

Just as the iron that makes up
this ship will eventually rust

and be carried away
by the desert winds,

so we think that the matter inside
black dwarves, the last matter

in the universe, will eventually
evaporate away and be carried off

into the void as radiation,

leaving absolutely nothing behind.

There were wild dogs everywhere,
actually, when we were filming that.

And so they... Standing there
on my own and they're all gone,

they're all in the helicopter,
and I thought,

I really hope they come back

because the dogs are circling,
you know.

With the black dwarves gone,

there won't be a single atom
of matter left.

All that will remain
of our once-rich cosmos

will be particles
of light and black holes.

After an unimaginable
length of time,

even the black holes will
have evaporated and the universe

will be nothing but a sea of photons

gradually tending towards
the same temperature,

as the expansion of the universe
cools them towards absolute zero.

And when I say unimaginable
period of time, I really mean it.

It's 10,000 trillion, trillion,
trillion, trillion, trillion,

trillion, trillion, trillion years.

How big is that number?

Well, if I were to start counting
with a single atom representing

one year, then there wouldn't be
enough atoms in the entire universe

to get anywhere near that number.

CLOCK TICKS SOFTLY

Once the very last remnants
of the very last stars have finally

decayed away to nothing,
and everything reaches the same

temperature, the story of the
universe finally comes to an end.

For the first time in its life,

the universe will be
permanent and unchanging.

Nothing happens,

and it keeps not happening forever.

People always ask me, actually,
why do you smile when you talk about

the inevitable decay of the universe
and the dissolving of everything

that we hold most dear
into a bath of radiation

that will... will fade?

So even the afterglow of our
presence in the universe

will become undetectable and
there will be no memory at all...

of everything or anything
that we've created.

There will be no imprint
left of us at all in the far future.

Why do you smile?

And I just think it's quite funny.

That's why I smile, I think!

It certainly does take us down
a peg or two, doesn't it?

You know, I want to build a statue
of myself, a grand celebration,

a permanent monument
of my achievements.

There will be no such permanent
monument of your achievements.

Don't worry about the statues.
Don't worry about them.

They're all going to dissolve
into a bath of photons!