All language subtitles for gravity.and.me.the.force.that.shapes.our.lives.2017.480p.hdtv.x264.rmteam

Would you like to lose some
weight without doing any exercise

or dieting?

Would you like to age just a bit
more slowly than your friends?

Well, you might
be surprised to hear,

the laws of physics can help.

The key to unlocking these
everyday questions is gravity.

It sculpts the universe.

It warps space and time.

It's a fundamental force of nature.

But gravity's strange powers,
discovered by Albert Einstein,

also affect our daily lives
in the most unexpected ways.

In this film, we'll be using
cutting edge scientific techniques

to investigate how gravity
changes your weight...

It's gone up.

...your height...

I really have shrunk.

...and even your posture.

And, with the help
of thousands of volunteers,

I'll show you how gravity makes us
all age at different rates.

Throughout the day I've just been
logging on the phone,

logging on to the app.

As a physicist,
gravity is central to my work.

Oh, wow!

And, in exploring it,

I'll be challenged on how I
understand this mysterious force.

Wow, OK. I need to go
and write this one down.

And I'll have to tackle the
very nature of reality itself.

Gravity.

It binds together
all the matter in the universe

and it makes our existence
here possible.

But in the end, it all boils down
to one simple question.

What happens if I drop an object?

Gravity's many mysteries are all
contained in this single action.

How an object falls.

Here's the first puzzle.

Why does a hammer fall faster
than a feather?

You might think it's because
the hammer is heavier.

But that's not the real reason.

The answer is air resistance.

It's not the weight of the objects
that matters, it's their shape.

And I can demonstrate this very
easily with these two umbrellas.

They both have exactly the same
weight, but if I open one of them,

you can be pretty sure it will drop
more slowly than the other one.

In fact, all objects would fall
at the same rate

if you could only remove the air.

The first person to realise this
was the 16th century mathematician,

Galileo Galilei.

Famously, it's said he worked it out

by dropping objects
off the Leaning Tower of Pisa.

And he was spectacularly proven
right

in an experiment carried
out on the moon in 1971.

In my left hand I have a feather.

In my right hand, a hammer.

I'll drop the two of them here,

and hopefully they'll
hit the ground at the same time.

It worked perfectly.

How about that?

It proves that Mr Galilei
was correct in his findings.

Now, Galileo was obsessed
with a second question, too.

When you drop an object,

it's actually quite hard to tell
if it falls at a constant speed,

or picks up speed as it drops.

Even in slow motion,
it's pretty hard to tell.

But Galileo realised this.

First, drop an object
a very short distance.

It lands with very little impact.

But, of course,
drop it from higher up...

...this time, the ball
easily breaks the tile,

which means it must have
accelerated,

gaining in speed and momentum
as it dropped.

Galileo had identified something
fundamental to all falling objects -

they accelerate.

He realised there might be a way
to measure

how much falling objects
gain in speed.

What he devised was the first-ever
attempt to measure gravity itself.

He built a long wooden ramp,
rather like this,

that he had sloping
at a shallow angle.

The idea was to roll balls down the
ramp and measure their acceleration.

The crucial thing is that the ramp
had to be at this shallow angle

to reduce the effects
of wind resistance.

It also meant the balls would roll
down slowly enough to give him time

to measure their speed.

But the big problem was this -
how do you measure time accurately

in an age when there
were no accurate timepieces,

let alone stopwatches?

Well, Galileo came up with an
ingenious idea

involving the flow of water -

essentially, measuring time from the
amount of water collected in a cup.

So, we're going to try and repeat
Galileo's experiment.

I say we, because I have
a couple of willing volunteers,

Gavin and Johanna.

Three, two, one, go.

And, stop.

OK, there's one.

Now, if you come down a quarter
of the way down the ramp.

Go.

Stop. OK.

So, now half of the way down.

Go.

Stop.

Just in time.

OK, and then three-quarters
of the way down.

Go.

And, stop.

Right, turn the tap off.

OK, so we have our four
measurements.

And I can see a progression
from fuller to emptier,

but what we need to do now is find
the mathematical pattern

by weighing carefully
the water in each glass.

Weighing the water should give us
an idea of how long each roll took.

And in our experiment,
these were the results.

Now, there's one immediate thing
you can tell.

The ball really sped up
the longer it rolled.

In fact, our results seem to show

that the time it took to cover
the first quarter of the ramp

was about the same time it took
to cover the next three-quarters.

So, we have a strong hint
of a mathematical pattern.

Now, we'll see if we're right,
by placing bells along the ramp,

at intervals which
are based on the results.

This arrangement looks a bit strange

because the gap between the
first two bells is much shorter

than the gap between the third
and fourth bells.

But that's OK, because if we've
got our calculations right,

the ball starts off slowly,
so it covers a shorter distance,

and as it picks up pace, it'll cover
longer and longer distances.

So, we should hear the bells ringing
at equal intervals in time.

Go.

Beautiful.

So, what does this all mean,
what's the mathematical formula?

Well, this is something
that Galileo worked out.

Let's say, from the start, the ball
covers a distance of one metre

in the first second.

After two seconds, it will have
covered four metres.

After three seconds, nine metres.

After 4 seconds,
16 metres, and so on.

If you recognise this progression,

you'll see that distance goes like
the square of time.

Galileo had found the rates at
which gravity speeds up objects.

And he'd found another
fundamental principle -

you can measure
the strength of gravity

by how much it causes
falling objects to accelerate.

Detecting gravity has become
exceptionally sophisticated

these days, but still uses
exactly the same principle.

This is Herstmonceux Castle
in Sussex,

and in its grounds lies
the Space Geodesy Facility.

Here, Vicky uses an astonishingly
sensitive instrument

to detect the exact strength
of gravity on this one spot.

Vicky, tell me about this incredible
gravity meter that you work with.

OK, so this is the dropping chamber
in a stripped down version.

Essentially what happens

is you've got a cart that gets
raised to the top,

and then the cart accelerates away
from a mass in the middle,

and so this section lifts off and as
it drops, it drops under freefall.

So, this component
in the middle as it drops

is basically just Newton's apple
falling to the ground?

Yes. So this is a stripped down
version, but that's the real thing?

This is the real thing.
How does that actually work?

In here, it's a vacuum. So there's
no wind resistance as it falls.

There's no wind resistance.

Inside, a laser is used

to measure exactly how fast
the mass is accelerating.

This is the 21st-century version
of Galileo's ramp

and the balls rolling down.
So, can we get it going?

Of course, if you'd just like
to press the button on the laptop.

This one? Yep.

OK. So it's now communicating
with it.

Oh, here we go. Here we go.

It waits five seconds and then takes
the measurement of gravity.

And again. Repeats.

And you can see
the results appearing now.

Yup, each of those green dots
is a measurement of gravity

with the actual number that
it's getting for each one.

The unit Vicky uses
has a familiar ring.

I see that the number up
at the top here,

you've got this unit, micro Gal?

Yes, a Gal is essentially one
centimetre per second squared.

The Gal was named after Galileo.

So, we've just taken the measurement
of gravity here today

and it's this highly
accurate number,

micro Gals.

The reading means that the Earth's
gravity speeds up a falling object

by around 9.81 metres per second
for every second it drops.

Vicky tells me something intriguing.

She takes a reading here every week
and she's found that

the strength of gravity changes
by tiny amounts over time.

Heavy rainfall, for example, can
cause gravity to increase slightly.

Presumably, if gravity
is changing here in one spot,

it'll have different values all
around the world

and so you can have a gravity map
of the entire planet?

That's right, yes.

So what's the reason
for these strange fluctuations?

That's what I want
to investigate next.

So, gravity changes as we move
across the surface of the Earth.

This is at the heart of a challenge
that I've set two young volunteers.

I've given them a task to try
and find the place in Britain

where gravity is at its weakest.

So, where objects
would weigh the least.

I've given them just three days
to try and find it.

The volunteers are Astraya,
a PhD student.

I've been living in London
for five or six years,

and I'm originally
from Seville in Spain.

I'm very interested in taking part
in this project

because I would really like to know
more about how this world works.

And Poppy, a journalist
who lives in London.

I did my degree
in biomedical science.

And I did biology and chemistry
for my A-levels,

but I haven't done any physics
since I left school.

I'm fascinated to find out more
about gravity

and I actually enjoy a puzzle,
I like a challenge.

The team just can't weigh themselves
to see changes in gravity.

Body weight fluctuates by a couple
of kilos over the course of a day.

Whereas, changes due to gravity
as they travel around the country

are going to be tiny in comparison,
the matter of a few grams.

So, they're going to have to use
sophisticated scientific methods

if they want to measure
gravity accurately.

And that's why the volunteers will
be joined by three specialists

in gravity science.

PhD student Sonak.

He'll be in charge of some very
sensitive measuring apparatus

from the
National Physical Laboratory.

Sean, a geologist, who will
be using a portable gravity meter.

And Andrew, a cosmologist
at University College London,

who will help interpret the results.

We've taken a collective weight
for the team before they set off.

It's 380 kilograms.

So, can they find the place in
Britain where that will decrease?

They're setting out in Snowdonia
National Park in North Wales.

The railway climbs from here to the
1,000 metre summit of Snowdon.

Sean takes
his first gravity reading.

The inside is a mass on a beam
and you turn this counter,

this dial, until you get
the beam central.

By counting the
number of turns of the dial,

Sean can calculate
the downward pull of gravity

acting on the mass
inside the machine.

Sonak has a simpler method.

So, inside the box
is a two kilogram mass,

and it's supposed to be sort of
as perfectly two kilograms

as it's possible to get.

All right. And place it here.

Oh, it's just coming under,
isn't it?

1998.2 grams.

It was two kilos in the laboratory,
but now here it's a bit less.

It's the first puzzle.

Why does a two kilo mass tip the
scales at just under two kilos?

And it's one which
gets straight to the heart

of what the challenge
is really about.

Mass is often confused with
the related quantity, weight.

The mass of these dumbbells
is fixed, it doesn't change.

It's a measure of how much stuff
they contain.

Weight is different.

It's a measure of the effects
of gravity on these dumbbells.

The downward force
pulling them to the ground

in the same way that it's keeping
my feet firmly stuck to the ground.

The crucial difference is this,

if I was holding these dumbbells
on the moon,

they'd still have
exactly the same mass,

but they'd weigh six times less

because the moon's gravity is
so much weaker than the Earth's.

So that's why Sonak is bringing
along the two kilo mass.

If it changes weight
then this should mean

that gravity itself has changed.

Ahead of them is the summit
of the highest mountain

in England and Wales,
famed for its stunning scenery.

Or it would be stunning
if you could see it.

And this is what we came
all the way up here for,

this amazing view
at the top of Snowdon.

You wouldn't know it,
but honestly, we are here.

We're now near the summit of Snowdon
and I've set up the gravimeter,

and we're going to see what
the difference in the reading is.

He has to turn the dial again and
again to try and get a reading.

It's clear gravity has changed,
but which way?

Has it got stronger, or weaker?

The team leave Sean
to work out his results,

and tries to position the scales
as close as possible to the summit.

But the reading
is all over the place.

Oh! It's gone up.

It's fluctuating
quite a lot due to the wind.

I have to say, this is what science
is always like, isn't it?

It's never quite
what you want it to be.

So, they head inside to the cafe
next to the summit.

The wind was being a bit naughty,
but hopefully...

Now it's in 00,
so it should be all right.

1998.2 down there, 1997.8!

There you go. We've got it!
That's 0.4 of a gram off.

The mass weighs a tiny bit less.

It's lost about
one 5000th of its weight.

And Sean has found that gravity
itself has reduced.

At the top of the mountain
we took the measurement

and we discovered that the
pull of gravity had gone down.

It had gone down the equivalent
of 206 turns of the dial.

And we worked out that that's
equivalent to 219 milligals.

So it's clear from
the team's measurements,

gravity weakens as you go higher,
and you get a bit lighter.

It's just an excuse to say where
are we, like, the lightest.

Who cares? Yes, who does care?

It's actually really interestingly,
it's like an illustrative example

of seeing how this
is actually fluctuating,

depending on different factors.
Yeah, absolutely.

And that we could measure it
and see it with our own eyes,

it actually makes you think about
gravity in a very active way.

It's such a fundamental
force phenomenon in nature,

but we don't know much about it.

But why does gravity change
with altitude?

To understand that question,

you've to get to grips with the
extraordinary discoveries

of the next scientific giant
in our story -

Isaac Newton.

Born in England in the middle
of the 17th century,

he spent his life wrestling with so
many apparently separate questions,

from why things fall to the ground,
to why planets orbit the sun.

It took the genius of Newton
to realise

there was one single equation that
could answer all these questions.

And here it is,
his famous law of gravity.

It might look complicated,

but this is one of the most
important equations

in the whole of science.

F here is the force.

Newton said there's an attractive
force between any two objects

in the universe.

On this side of the equation, G, we
call the gravitational constant.

Newton knew it had to be there, but
he didn't know what its value was.

M1 and M2 represent the two objects,
and R is the distance between them.

Now, the equation tells us that
the more massive the objects are,

the bigger M1 and M2,
the greater the attractive force.

But the further apart they are,
the bigger the value of R here,

the weaker the gravitational force.

With Newton, what was once
mysterious now became clear.

Newton's equation describes
why an object falls to the ground,

including his famous apple.

But its true genius is
that it applies to any object,

anywhere in the universe.

So, it's a very simple
and elegant way of describing

some of the seemingly most
complicated phenomena in the cosmos.

His law of gravitation can still be
used today -

to explain how orbits work,

to predict when a comet will return,

to describe why galaxies spin.

Or to slingshot spacecraft
around planets.

Newton tells us to look for the
underlying simplicity

in natural phenomena. For instance,
how the moon orbits the Earth.

If I let go of this apple,

it'll fall straight down because
of the pull of Earth's gravity.

But if I throw it, to begin with,

it travels in
a horizontal direction,

that's the direction of travel,

but Earth's gravity
is still pulling it downwards,

so it ends up following
a curved path.

Now, if I throw it harder,

it'll travel further before it hits
the ground and, in principle,

if I could throw it hard enough,
I could put it into orbit.

That's exactly what's happening with
the moon in orbit around the Earth.

It's a combination of wanting
to travel in a straight line,

but also being pulled down
by the Earth's gravity.

So, it ends up constantly falling

around the Earth
and constantly missing.

Newton's famous equation

also explains the strange effects

which the road-trip team
has discovered.

That objects get lighter
as you gain in altitude.

When I weigh myself, I'm represented
by the first mass, M1.

The second mass,
M2, is the Earth itself.

And the force pulling me down,
my weight,

depends on the distance between me
and the centre of the Earth.

And that's the secret
of the road trip.

If you want to find the place
where you weigh the least,

then you have to get as far away as
you can from the Earth's core.

So, it's the afternoon of day one,

and the road-trip team have to work
out where to go next.

Poppy and Astraya have a good idea,

find somewhere higher
than Mount Snowdon.

From the measurements that you guys
did at Mount Snowdon,

altitude clearly plays
an important part in gravity.

So, with that in mind, we've got to
go to the highest point in the UK,

which is Ben Nevis.

OK, BUT there's just one thing that
we haven't shown you so far.

We actually brought
along an extra experiment,

so can we please show you this first
before you make the final decision?

Yes. Sonak actually has the other
part of this experiment.

We always carry around...

Some power tools,
as physicists always do.

Let's start it off nice and gentle.

OK.

And then, try and pick up some pace.

Pizza.

You've got some pizza there.
OK. Point proven.

The point is that
when something is spinning,

it kind of gets flung outwards
and you can actually use that

to make a nice, flat piece of pizza,
but this also applies to the Earth.

The Earth isn't perfectly round.

It's what's known
as an "oblate spheroid".

It bulges at the equator

where the spin is greatest.

We've kind of got
two competing effects now.

We're trying to get away from the
centre,

the actual core of the Earth,

the point at the very centre
of this ball.

But now, we can do it in two ways.

We can either go up something tall,

or we can just
go down towards the equator.

This is what we find
when we're doing gravity surveys,

as you move south, there tends
to be an effect from latitude

which is often usually larger
than the effect from altitude.

So, the closer
to the equator you go,

the further you get from the Earth's
core and the lighter you get.

So, guys, the sun's setting just
behind me here. This is north.

From the conversations
we've just had,

it sounds like we've
got to go that way,

down south, is that right?

Yes, OK. Let's go.

The team is starting to uncover
the reasons why gravity changes

as you cross the surface
of the Earth.

Our planet is defined and shaped

by the complicated forces
which act upon it.

And detecting tiny fluctuations in
its gravity field

can give us important clues.

It can help us understand
how our world is changing.

The Space Geodesy Facility at
Herstmonceux is one small part

of an enormous global network

which uses satellites to detect the
tiniest of changes

in the Earth's gravity field.

Tell me what exactly your job
is here?

What we're doing
with this telescope

is measuring very accurately

the distances of satellites
from here,

so we're using very short laser
pulses

which we direct towards
the satellite.

On the satellite,
there are reflecting cubes,

which return some
of that light to us.

We measure how long it takes
the light

to go to the satellite and back.

And how far away
is the satellite typically?

The one we're tracking now
is one of the Galileo satellites,

which is about 20,000 kilometres.

20,000 kilometres away? Yes.

OK, so, we've got it aimed at the
Galileo satellite

and you're going to turn the laser
on now? Yes.

Oh, wow!

And that laser beam that's being
fired up towards the satellite,

the time it'll take to get
there and come back again,

it's a fraction of a second,
isn't it? It is.

It's about 150 thousandths
of a second, 150 milliseconds.

And we're sending about
1,000 of those per second.

This strange-looking object
is based on satellite readings.

It's a highly exaggerated
representation

of how Earth's gravity field
varies over time.

Fluctuations like these can give us
important insights

into climate change,
icecaps melting,

sea levels rising,
changes in ground water.

All of these have an effect

on the local strength of gravity.

So, something as important
as climate change,

in order to understand it
and do something about it,

we need to know the distribution

of the gravitational field
of the Earth very accurately?

Absolutely, yes. And it's a global
measure that we need.

For the road trippers,
it's the start of day two...

...and they're heading
for the south coast.

They're stopping off
in Herefordshire,

it's a good location
as it's the same altitude

as the base of Snowdon,

but they've moved about 80 miles
further south.

So, if they find gravity changes
here, it must be due to latitude.

It's not a huge difference,
but it's noticeable.

Our counter reading at the bottom
of the mountain was 4,840.

Yes. Our counter reading
here's 4,717.

Oh, right, so, we do get to see
a difference.

So, we're at the same altitude
as the base of Mount Snowdon,

but because we've travelled
further down south overnight,

gravity's less here? Yes.

They push on.

And by sunset they reach
Sidmouth on the south coast.

Sean takes the second gravity
reading of the day

and Poppy improvises a map.

Well, sort of a map.

Can we write "not to scale"
at the top there.

So, I drew this map.

Scotland's a bit squashed.

Wales is quite high up and Cornwall
is there, but you get the idea.

Sean, we've been travelling
with you,

you've done quite a few
gravity meter readings,

can you plot them on this
not-to-scale,

badly-drawn map, please? Sure.

So, if you remember we started off
in Mount Snowdon, here,

and that was the zero measurement
for our survey.

Then we've come all the way
down here to the south coast.

The difference from the base
of Snowdon is -212 milligals. Wow.

So, the difference between going
and measuring gravity

at the base of the mountain
and the top of the mountain

is about the same as
here at this latitude

and down here at this latitude.

They're quite clearly at sea level,

yet gravity here is roughly the same
as it is at the top of Snowdon.

But where next?

We are here.

If we want to find out
where we are the lightest,

why don't we travel all the way to
the most southerly point in the UK,

which is here? But altitude
can also help us,

so why not find a place
in the country

that is both low in latitude but
also as high in altitude

in terms of height above sea level,
because that will get us somewhere

that is really far away
from the core of the Earth,

whilst staying
within the country?

So, the answer to the puzzle lies
in a combination of two factors.

How much further south should
they go and how much higher?

At the end of day two,
Sean's results show that the team

weighs about 80 grams
lighter in total

than back at the base of Snowdon.

The way that weight changes
is just one example

of Newton's famous
equation in action.

But Newton had left
his masterpiece incomplete.

He didn't know the value of G,

the gravitational constant,

which sets the size of the force.

To harness the full power
of the equation, you need to know G.

And the vital clue came
within an incredible experiment

conducted in London at
the end of the 18th century.

It was an attempt to work out
the mass of the Earth itself.

And it was carried out
by an eccentric,

extravagantly rich aristocrat,

Henry Cavendish.

Cavendish was a chronically shy,

deeply solitary man living in total
isolation in his house in Clapham.

The story goes that, one day,

he accidentally bumped into a female
servant on his staircase.

He was so traumatised by this event

that he had a new
staircase built just for him

so that this horrible incident
could never happen again.

Cavendish had inherited
vast fortunes

and was able to dedicate his life

to devising pioneering experiments -

including one particularly
extraordinary piece of equipment.

He set up something a bit like this.

It's called a "torsion balance".

It involves four lead spheres,

two large heavy ones which are held
fixed in place,

and suspended by a very
thin wire is a wooden rod,

six-feet-long, with two smaller
balls on either end.

Now, the crux of the experiment

is the relationship between
the large ball and the small ball.

Now, of course, there's
a gravitational pull downwards

on both of the balls due to the
Earth's gravity.

But Newton also tells us

that there should be a very weak
gravitational pull between the balls

and this is effectively what
Cavendish was trying to measure.

Any slight movement of the small
ball towards the large one

should cause a twist
in the torsion wire

and that's what Cavendish
was trying to detect.

Of course, this is all
much easier said than done.

The experiment
was incredibly sensitive.

The tiniest of vibrations,

the slightest breeze,
changes in temperature

could all influence the
measurements.

So, Cavendish had to isolate the
apparatus inside a box

and the box within a shed.

He even realised that his mere
presence next to the apparatus

could influence things, so he had to
remove himself outside the shed.

What he then did was sit outside
the shed,

and through a small hole
in the shed wall,

look through a telescope to detect
the tiniest of twists in the wire.

It was an incredibly difficult
process, but after many months,

he finally felt confident enough
that he had a reliable result.

Cavendish found that the
small balls did move...

...a tiny four millimetres.

He calculated his results

by comparing the density
of the balls

with the density of water.

In the end, the result of
Cavendish's experiment

and subsequent calculations

was that the density of the Earth

was about five and a half times
that of water.

Or, put another way,

the mass of the Earth was
5.9 trillion trillion kilograms.

What's most remarkable is that
Cavendish got this number right

to within an accuracy of 1%.

With Cavendish's astonishing result,

scientists were able to work out G.

Then the equation could be used

to determine the mass of any
celestial body

in orbit around another.

So, astronomers were able to
calculate the mass of the sun

and the planets, and the moon,

and, eventually,
even distant galaxies.

At the end of day two, the team were
in Sidmouth on the south coast,

looking for the place in Britain
where they'll weigh the least.

They've worked out the answer lies
in a combination of two factors -

the right mix of going south
and being higher up.

For the final leg of the journey,
I'm going to meet up with them.

I asked them to drive a short
distance west

to one of the most remote
areas in mainland Britain.

Dartmoor National Park.

'It's only 40 miles from the
southernmost tip of Britain.'

Hello. Hi, Andrew.

Good to see you. Nice to see you.

'And it's very high,
very hilly territory.'

Jim, the team got to the
south coast yesterday...

Yes... to find gravity
at its weakest.

But we haven't quite figured out
whether it's altitude or latitude.

Do we go further south
or do we go higher up?

You're right to ask, "Do we go as
far south as possible"

"or as high as possible?"

That's why I've brought
you here to Dartmoor.

We've charted the most important
points on this map here.

Right. Let's have a look.

So, we are here, Two Bridges.

Yes. These four dots represent
these hills up there behind us,

which are at about 500 metres
above sea level.

That's what we want to check out.

'These hills are close
to the south coast

'and they're also the highest in
the whole of the south of England.

'So, logic suggests they must be the
right combination

'of latitude and altitude.'

Well, there's another
reason why this makes perfect sense,

one which we haven't looked at yet,

and that's the effect of the
underlying rocks on gravity.

And I've got a map here that
shows...

You're going to trump my map
with yours, aren't you? I am!

Here we are, down here, now these
blue areas are the lowest areas

according to the density
of the rocks underneath.

'The rocks around
here are made of granite,

'which will make gravity
weaker still.'

So, that's helping -
as well as the altitude

and the fact that we're further
south.

Yes, it's also playing a part.

'Well, we have a plausible theory.

'But now we need to test it.'

'If I'm right, then, at the top,
our gravity reading

'should be by far
the lowest reading of the trip.'

'Of course, there's another effect
of gravity to deal with now -

'it's knackering
when you head uphill.'

OK, I think this is pretty much
the start of the hills

we've located on the map. So, let's
see if this is the lightest place.

Sean, if you want to get
the gravity meter out,

and we'll take another
reading here. Yep. OK.

'Sean sets up his equipment
one more time.'

What's the news?

Well, the bottom of Mount Snowdon
was our zero for this test.

We found we lost a certain amount

by going up to the top
of Mount Snowdon.

We found we lost a certain amount
coming south to the south coast.

Not only have we beaten that,
we've smashed it.

Brilliant.
We were -219 milligals

lower at the top of Mount Snowdon.

Here on Dartmoor,

we're -347 milligals lower.
Wow! Brilliant!

So, it is a combination
of three things.

We're far south, so it's the
latitude, we're at altitude,

we're quite high up, and we're
surrounded by all this granite rock,

which is low-density anyway.

I hope you all think it was worth
the climb up here anyway?

Yes, absolutely. There you go.
Boom, science!

Now, we already know that the
altitude of these hills

takes us much further
from the Earth's core

than anywhere else
further south in Britain,

so gravity must be weakest here.

There's extra evidence, too.

The British Geological Survey

has compiled tens of thousands
of gravity readings made in the UK

and the lowest readings ever
recorded were all taken around here

on the high hills of Dartmoor.

What do we do to celebrate?

We weigh ourselves, of course.

I bet you don't weigh that much.

Whoa!

It's all them Nutella pancakes
for breakfast!

74, 75. I need to lose weight!

I can tell you that you should weigh
something like 20 grams less

than you did
at the base of Mount Snowdon.

Guys, I'm guessing something
like 25 to 30 grams less.

So, if you want to weigh
as little as possible,

this is the place
in Britain to come.

But in any case,
it's such a tiny amount

that it's going to be wiped out
entirely

by whatever it was you had
for breakfast this morning.

Gravity.
What goes up must come down.

All of our lives,
we abide by its rules.

It dominates our every action.

But there's one select group of
humans

who know what it's like
to live free of gravity.

'Two, one... 'zero.

'Lift-off!'

Everybody's used to gravity.

We're used to the oppression of it.

Gravity is the ultimate oppressor.

It grinds us under its heel 24/7
with no release,

until you're in space and then,
suddenly, you're free from gravity.

You're weightless in orbit.

Canadian astronaut Chris Hadfield

spent five months on board
the International Space Station.

You can pull your knees up
to your chest and just tumble.

Or, if you take a wet cloth,

and you get it dripping wet,

and everybody on Earth knows what'll
happen when you wring it out.

All the water will fall, inevitably.

If you do that in weightlessness,

the water stays there
and it, actually,

because of the surface tension,
starts crawling up your arms.

It's a little bit mesmerising and
hypnotic to be in weightlessness.

If you're weightless,
you don't need a bed,

you don't need a mattress,

you don't need a pillow.

Your body is floating completely
suspended, like magic.

Movement becomes effortless.

You can push off with one finger
and fly, and it's humble.

You don't need to hold yourself
where you are with muscle.

You can just... With a delicate
fingertip pressure,

you can stay where you are.

But there is a price to pay.

Astronauts' bones atrophy
and their muscles wither away.

One of the things we do on board
a space station is exercise,

purely to simulate gravity.

If we don't do something,
then our heart will shrink,

our ability to pump blood
to our head will diminish,

our bones will start to dissolve
and our muscles will waste away.

'OK. Separation confirmed.
Timer's on.'

'Backing away at a rate

'of just a little over one tenth
of a metre per second.'

Re-entering gravity
is a punishing experience.

To come back to Earth is violent.

It can be five times
the force of gravity

or eight times
the force of gravity,

crushing you down into the floor
of the ship for quite a long time.

Then, of course, you hit the ground
and tumble

and roll to a stop and now
you are the victim of your past.

You're the victim of your
decision-making, lying there,

trying to shake your head and get
used to being in gravity again.

I remarked, at the time, that I had
forgotten that my lips have weight

and my tongue has weight.

You don't think about it. But
if you try and talk articulately,

standing on your head, you'll notice
that you have to sort of control

your lips and your tongue
a little differently,

just because gravity's pushing
them the other way.

And it's the same sort of thing,

raising your arm,
holding your head up,

turning your head when everything
wants to tumble,

just keeping your balance,
all of those things.

It's a little bit like relearning
to walk again like an infant.

'Gravity on Earth
grinds us all down.

'Over the course of the day,
it actually squeezes your spine,

'an effect you can see for yourself
if you use a measuring rod.'

OK, so it's 7:30 in the morning.

I've just got up and I'm going to
see how tall I am

before gravity drags me down.

That's 178 centimetres
or just over 5'10.

Over the course of the day, gravity
compresses the fluids in your spine.

Right, it is just past 11pm.

I've been standing up
for most of the day

so let's see if gravity
has had an effect on my height.

That's 176 centimetres,

so I really have shrunk by
just over half an inch

over the course of today.

In the longer term, gravity can
affect your posture permanently,

but there are exercises you can do
to counteract this effect.

Part of my research has been looking
at the effects of gravity

on the human body. So people might
not be aware

or they might not always think
about the effect of gravity

on our physical state,

on our health and, particularly,
on our posture.

However, because it's such
a constant force,

gravity has a massive impact
over the course of our lifetime.

As you get older,
you can develop a stoop,

which is damaging to your mobility.

Gokun here has actually
got very good posture

but I'd like you to just show
not so good posture.

So when...

Poor posture is really rounded
shoulders

and then loss of the curve in the
back, as well.

Can I just ask you to raise up
your arms

when you're in that posture?
I can't go any higher.

No, and then, just come back down,
shoulders back,

and then raise your arms.

You can see the effect
of posture on function.

Ironically, the exercises
which many gym-goers do

actually make your posture worse.

That's if you only exercise
the frontal muscles,

like the chest and abdominals.

So, it's recommended you exercise
the back muscles just as much,

to straighten you out and counteract
the effects of gravity.

Gravity shapes our bodies
and moulds our planet.

Nothing happens on Earth without its
power and influence.

Sir Isaac Newton explained so many
of its effects

using one simple equation.

And, in the centuries that followed,

his laws of physics led to
breakthrough after breakthrough,

spurring on the
Industrial Revolution.

But in the first decade
of the 20th century,

the next genius in our story

challenged the very foundations of
our understanding of gravity.

A young German scientist
called Albert Einstein

was churning something
over in his mind.

He thought that something in
Newton's laws didn't quite add up.

Imagine I'm the sun

and this tennis ball is the Earth
in orbit around me.

Newton's laws can describe,
very precisely,

the path the Earth takes
around the sun

in terms of the mutual gravitational
attraction between the two bodies.

But what Newton can't explain
is what connects them.

In reality, of course,

there is no invisible string
between the Earth and the sun,

holding the two together.

There's just empty space,
a complete void.

And yet, according to Newton,

the Earth and sun pull
on each other instantaneously

across a vast distance.

How can gravity act in this way

when there's nothing to connect
it or transmit it?

After years puzzling over this,

Einstein had a blinding flash
of inspiration.

Just like Galileo and his ramp...

...or Newton with his apple,

Einstein's breakthrough came

because he was thinking about one
simple action...

...what happens when something falls.

To explain, I'm visiting

this 400-foot-high tower
in Northampton...

built to safety-test lifts.

One day in 1907,

Einstein had what he called
the "happiest thought of his life".

What if I were standing
in a stationary lift,

completely isolated
from the outside world,

not feeling anything apart from
the pull of gravity on my feet?

What if, then, the lift cable breaks

and I start falling?

What are the forces that I will feel
as I'm plummeting to the ground?

Well, I'm not going to try that.

Fortunately, there's another way
to test this

without me having to plunge
down a lift shaft.

Sorry to disappoint you!

This little device here that I have
strapped to this plastic toy

is an industrial accelerometer.

So, it measures acceleration.

Now, I've got it connected
to my laptop

and it's showing a measurement
of 1G.

Now, that's the
downward acceleration

due to the pull of Earth's gravity.

So, basically, it works
just like a gravity meter.

But what happens
if I were to drop it?

Presumably,
it'll carry on measuring 1G

because it's falling
in Earth's gravity.

OK, well, let's try that and see.

So, you can see here,
along this line at the bottom,

that's when I was holding it still

and it's measuring an acceleration
of 1G.

These oscillations here is when
I stood up

and there's a bit of disturbance,

but this spike along here
is the moment I released it.

And this short duration along
here is the time it was falling.

And you see, while it was falling,

it was registering
an acceleration of zero.

Now, if you think about it,
this is really odd.

The accelerometer
is accelerating downwards.

It's plummeting in the full grip of
Earth's gravity

and yet it's measuring
no acceleration at all.

It's as though gravity
has completely disappeared.

Einstein's insight was
that when something falls,

it no longer feels the pull of
gravity.

In fact, falling is like floating
in empty space.

This is the essence of Einstein's
"happy thought"

and what we now call
his "principle of equivalence".

Einstein's point is that,
when the man in the lift falls,

he doesn't just feel weightless,
he is weightless.

Einstein said the man feels no force
pulling on him

because there is no
force pulling on him.

Gravity doesn't act on him,

it acts on the space
and time around him,

what we now call
the "geometry of space-time".

This was a radical redefinition.

Einstein says to forget the idea
of gravity as a force,

acting mysteriously
between two objects.

Now we have to think of it as the
shape of space-time changing.

You see, Newton saw space and time
as independent,

fixed and immutable,

that three-dimensional space is
the stage in which things happen,

but time is separate,

it ticks by at the same rate

everywhere in the universe.

According to Newton,
an object would travel through space

in a straight line unless acted upon
by a force like gravity

that would cause it to deviate
from that path.

But Einstein said that space and
time aren't fixed and immutable,

they're interconnected,
meshed together

in what is known as space-time.

And he said that space-time
can be warped -

that matter curves
space and time around it.

So, after Einstein,
we no longer see gravity

as an invisible string
pulling objects together.

Instead, a body like the Earth

warps the structure of
space and time around it.

And an object in orbit

follows a path which
is as straight as possible

through that space-time.

It's a fundamental part of
Einstein's vision of reality.

Space and time
can't be disentangled.

You can't talk about space
separately from time.

So, matter warps time
as well as space.

It's known as
"gravitational time dilation",

and it's possibly the strangest
of all of Einstein's discoveries.

I've got two identical clocks here.

Now, because the clock lower down

is closer
to the centre of the Earth,

it feels ever so slightly a stronger
gravitational pull

than the clock higher up.

Einstein's theory says that the
lower clock will tick by

at a slightly slower rate
than the higher clock.

Basically, gravity slows time down.

It's an extraordinary conception
of reality that Einstein describes.

Space is being curved
and time is being distorted.

So, why can't we perceive this
in our everyday lives?

Einstein had a rather nice way
of explaining it.

Most of us have had the experience,
as children,

of trying to work out
what our parents do for a living.

Well, imagine your father
is Albert Einstein.

When he was about 12 years old,

young Eduard Einstein asked his
father why he was so famous,

what he'd discovered. Well, this
put Einstein Sr on the spot,

but he came up with
a beautifully simple analogy.

Einstein told his son,

"When a blind beetle crawls over
the surface of a curved branch,"

"it doesn't notice that the track
it has covered is curved."

"I was lucky enough to notice
what the beetle didn't notice."

This is what Einstein meant.

The beetle is free to move
in any direction on the branch.

It can move forwards,
backwards, left and right,

but it has no concept of a direction
up off the branch.

It's as though, for the beetle,

the universe is missing
the third dimension.

The beetle may think it's moving in
a straight line along the branch,

but we can see that the surface
it's walking on

is itself curving and twisted.

Einstein's point was that what we
see as the twists and curves

of the branch feel, to the beetle,
like forces pushing and pulling it.

OK, so, consider this rather
strange example.

Imagine we have two beetles perched
on this pumpkin and,

for whatever reason, they want to
walk up towards the top.

Now, if they start at the equator,
pointing due north,

as they walk, they will begin
by moving parallel to each other.

That means their paths should never
meet.

But, as they get closer to the top,
their paths get closer together.

Now, if they're clever beetles,

they might try and figure out
what's going on,

and they could imagine
that there's some mysterious force

that's pulling
them closer together.

But, for us, from our perspective,

we can see there is no such force.

All they're doing

is following straight paths
over a curved surface.

Just as the beetles have no sense

that the surface of the branch
is curved,

we completely fail to perceive

the bizarre ways that gravity

shapes the reality we live in.

Einstein's problem was proving
that he was right.

After years more thought,
he realised that there WAS a way...

by looking far out
into the solar system.

Incredibly, here in the grounds
of Herstmonceux Castle

is housed one of
the original telescopes

that were used
to prove Einstein was correct.

In 1915, when Einstein developed
his general theory of relativity,

it was just that - it was a theory,
it had no proof.

In fact, many people found it
completely outlandish.

But then, just four years later,

in 1919, this telescope,
and allow me to geek out a bit here

and I'll give it its correct name,

this is the 13-inch
astrographic refractor,

this telescope proved
that Einstein was, in fact, right.

That gravity does curve
space itself.

Marek Kukula is the public
astronomer

at the Royal Observatory in London,

and he's recently rediscovered

a neglected treasure
in their archives.

This is, perhaps,
one of the most important

scientific artefacts we have in
the collection here in Greenwich

and, for an astrophysicist like me,

it's almost a holy relic.

It's a glass plate photo of a solar
eclipse taken in 1919

as part of a famous
scientific expedition.

British astronomers had travelled
all the way to Brazil

and the West Coast of Africa

to take photographs which they
hoped would prove Einstein right.

What we're seeing here
is the eclipse of 1919.

You can see the black disc of the
moon silhouetted against the sun,

blocking its light. Around it is
the solar corona,

the sun's outer atmosphere,

and this spectacular prominence
of gas leaping off the surface.

But it's not the sun
that we're really interested in.

The fundamental point that this
photo

and others from the expedition show

is that the positions,
the apparent positions,

of the stars in the sky
are altered and shifted

from where we would expect
them normally to be,

and that proves this very strange
thing

that general relativity predicts -

that the mass of the sun

bends the space and time around it,

and that distortion is gravity.

This is a negative
of one of the photos.

It has markings showing
where the stars' positions

seem to have shifted.

Since then, observation after
observation

have confirmed that matter
curves space

and slows down time.

So, the simple question of why
things fall the way they do

has led us deeper and deeper

into the very nature of space and
time itself.

Gravitational science shows us how
galaxies, stars and planets form.

By measuring gravity, we've
discovered the existence

of dark matter, that 80% of the mass
of our universe is invisible

and we don't know what
it's made of.

And we've detected exotic objects
with extreme gravity...

...like neutron stars,

which have more mass than our sun

yet are only 20 kilometres across.

But it's another mysterious aspect
of Einstein's universe

that I want to explore in
my next gravity project.

Here at the University of Surrey,

some colleagues and I have been
working on it for months.

What we're doing is devising a
nationwide citizen science project.

We're developing a smartphone app

that uses the GPS
contained on your phone

to explore one of the strangest
properties of gravity -

how it affects the rate
at which we age.

'I formulated the equations
myself...

'..and a small team of computer
scientists and software developers

'is using them to devise the app.'

Einstein discovered that,
as gravity changes,

so does the rate that time ticks.

This means the strength
of gravity you feel

affects how quickly
or slowly you age.

The aim of my app
is to demonstrate this effect.

It works by using a phone's GPS data

to estimate your local gravity.

And it also calculates the average
speed at which you move

because this, too, affects
the rate at which you age.

It then uses the equations
I've written,

which are based on
Einstein's theory of relativity,

to calculate, overall, how
fast or slowly you're ageing.

Once the app is ready,
I tweet about it.

Thousands of people download it

and we start to gather results
from across the country.

Some people send me videos,
giving me their results,

how fast they are ageing

compared with how time ticks out
in space in zero gravity.

Over the past day, I have aged less
by about 172 microseconds.

I have aged less by 10.02
milliseconds.

So, since downloading the app, I
have aged less by 1.14 milliseconds.

Since opening Time Warper,

I have aged less
by 2.6 milliseconds.

Our aim is to use their results

to build up a map of how time flows

because of gravity.

My smartphone project provides just
one insight into the space and time

which Einstein's theories describe.

Gravity and its strange ways

have given us astonishing insights

into the dark secrets
of our universe.

Perhaps the weirdest objects
in the universe are black holes,

collapsed stars whose gravity
is so strong

that not even light can escape
their grip.

Now, for the first time ever, their
effects have been felt on Earth

and they've been detected through
the medium of gravity itself.

It's a story that has revolutionised
the study of modern cosmology.

1.3 billion years ago,

in a galaxy far, far away,

two black holes
swirled around each other,

drew closer and closer together,

until they finally collided
with incredible violence.

In that final fraction of a second,

at the precise moment that they
merged,

a disturbance was created

that sent ripples
out through the universe.

Gravitational waves are a key
prediction of Einstein's theory.

Matter doesn't just curve space
time, it can cause waves,

ripples which expand outwards,

exactly like a stone dropped
in water.

This particular wave
was unimaginably large.

The energy released was greater than
all the light being given out

by all the stars in the universe.

The wave rippled through space
at the speed of light.

In 1.3 billion years,

it covered a distance of over
10 billion trillion kilometres.

Until, on the morning of the 14th of
September, 2015,

it arrived here.

The streets and cafes
of New Orleans.

In fact, everything in America -
and on Earth -

expanded and contracted very, very
slightly

as the wave passed through.

No-one noticed as,
by the time it arrived here,

the distortion
was phenomenally tiny.

Except that one science
laboratory did notice...

...and I'm going to see it.

1,000 scientists across the world
are collaborating on it.

It's the culmination of over 50
years of effort

and is one of the most
sophisticated experiments

ever devised by humanity.

So, I'm pretty excited to see it.

It's a rather unusual setting.

Here I am, in the middle of
rural Louisiana,

about an hour's drive
outside New Orleans.

I don't expect to find such a
multi-million dollar,

cutting-edge research facility as
this,

and yet, this is the place
where, recently,

one of the most important scientific
discoveries

in human history was made.
This is LIGO.

The Laser Interferometer
Gravitational Wave Observatory

is an enormous construction
shaped like an L...

...with a sophisticated laser system

bouncing up and down the two arms.

So, we're standing on top of one of
LIGO's two arms.

This is the first LIGO arm.

And in that tube, there's a laser
beam that we bounce back and forth

between a mirror and the end station
and a mirror in this building.

And the other bit goes that
way four kilometres,

perpendicular
to the arm we first saw.

So, this is the L shape?
It's a big L on the ground.

So, the light bounces back and forth

in that arm and bounces back
and forth in this arm,

and what we actually measure with
LIGO is the length of this arm

as measured by the light between the
two mirrors,

and the length of that arm as
measured by the light

between two mirrors.
And then the laser interferometer

measures the difference
between those two arm lengths.

So, as the gravitational wave passed
through, the lasers picked it up.

They detected that LIGO's two arms
changed in length

to a very, very tiny degree.

The signal that we saw

was just a few thousandth
of the size of the atomic nucleus.

It's the biggest the signal
ever got.

So far, far smaller
than the size of a single atom?

Oh, much, much smaller, yeah.

And you need something this huge
to pick that up?

That's right. This is one
of the biggest sources of energy

in the universe, one of the biggest
events you'd ever measure,

and we just barely saw it.

The LIGO scientists turned
the gravitational waves

into sound waves,

so what you're about to hear is,
in a very real sense,

the sound of two black holes
colliding.

It was the first observation
of any kind

of pairs of stellar mass
black holes.

"Stellar mass" means, you know,

several or a bunch of suns
in weight.

And so we learned that they exist,

we learned that there are enough
of them that, occasionally,

they run
into each other and coalesce.

And...

we also learned, by comparing
the waveform we observed

with the general relativity
calculations,

that general relativity is,
as far as we know, dead-on right.

The long concrete bunker to my left
houses the beam line,

one of the LIGO's laser arms.

The detail and the effort that's
gone into isolating the beam

from the outside environment

reminds me very much of Cavendish's
famous experiment.

He, too, had to worry about
isolating his experiment

from external disturbances.

Only, of course, LIGO takes things
to a far, far greater degree.

Inside the arm is one of the largest
and purest vacuums in the world.

Atmospheric pressure in there

has been reduced to one trillionth
of the pressure outside.

The mirrors inside are so reflective

that they only absorb
one in three million photons.

And at the end of my little trip,
lies a British success story.

Well, I made it all the way to the
end of one of the LIGO arms.

To be honest, it took me a bit
longer than I thought,

especially in that thing,

but housed inside this building is
one of the reflecting mirrors

that bounces the laser beam

all the way back down the
four kilometre arm

to the main control centre.

And the technology that went
into developing these mirrors

is quite remarkable.

It was developed in the UK
at the University of Glasgow.

This is what the mirror looks like.

Its surface
is extraordinarily smooth,

no bump bigger than a few billionths
of a metre high.

Equally amazing are these...

...fused silica fibres, a few times
the thickness of a human hair...

...designed by the University
of Glasgow

in conjunction with scientists
from other British universities.

They isolate the mirror completely

so it hangs perfectly still.

You could say that in there

is the quietest place on Earth.

Despite this, outside events do
sometimes interfere

with the work here,
as I witnessed for myself.

I've wandered into the control room
here at LIGO because I'm told

something kicked off a few hours ago
and they're all very busy.

The image that's flickering up there

is not meant to be like that.

Essentially, what they picked up

is a seismic disturbance,
an earthquake.

Now, that's not an earthquake
down the road.

It started on the other side
of the planet, in Japan.

So, it just gives us a sense

of the tremendous challenges
faced by LIGO

and the team here and the level of
sensitivity needed

that an earthquake on
the other side of the Earth

can disrupt their measurements
and they have

to reset everything all over again.

One of the scientists involved in
developing this extraordinary place

put it quite succinctly.

"Once we were blind,
but now we can see."

Throughout the entire history
of astronomy,

we've studied gravity and how it
affects matter in the universe

and how it warps space-time,

but only by looking at the light
that enters our telescopes,

now, for the first time,

we can study the universe
in a different way.

The discovery of gravitational waves
means we can see objects

that cause extreme warping of
space-time

and its effect on gravity directly.

This essentially opens up
a new era in astronomy,

it gives us a new way of looking out
at the universe.

Professor Sheila Rowan was one of
the scientists

who spearheaded the
British effort for LIGO.

For her and her colleagues,

gravitational wave detection
is just in its infancy.

New instruments - even more
sensitive than LIGO -

are now being developed.

There's so much
that we don't understand

about the universe that we live in,

and this has suddenly given us
a new tool, a new way,

to probe the dark processes
in the universe,

because every time we make the
observatories more sensitive,

we can sense gravitational wave
signals from further away,

from further out in the universe,
from further back in cosmic history.

Things like supermassive black holes
spiralling in to collide,

small black holes orbiting
round supermassive black holes,

tracing out the dents in space-time
of those supermassive objects.

A long-term goal
is to probe back further

towards what we think of
as the Big Bang,

the earliest moments
that we understand

of the universe as we know it.

If you think about it, time and time
again in the history of science,

unlocking the mysteries of gravity

have led to a deeper
understanding of the universe.

Galileo and his ramp,
Newton and his apple,

Einstein and the falling man
in the lift.

Each of these characters challenged
the scientific consensus of the day.

And even today, understanding the
true nature of gravity

remains one of the biggest
challenges in science.

Which brings me back
to the smartphone app.

And it's at this point that our
story, for me, at least,

takes a completely unexpected turn.

Unfortunately, it's all gone
a bit pear-shaped.

OK, so, here's what's happened.
A couple of months ago,

we launched the app and it was all
going really well.

Thousands of people downloaded it

and have been sending us their
results.

We've been collecting the data to
create this nationwide map

to show how time flows at different
rates for different people

around the country.

Unfortunately, I've just realised
there's a big problem.

You see, I was going over the
scientific literature

and I came across this
subtle point about relativity

which basically made me sit
bolt upright.

There was this horrible dawning
realisation

that I'd made a mistake in the
equations that get fed into the app.

What this means is all the results
we've been gathering are wrong.

The issue lies in the strange
and subtle effects

of Einstein's theories of
relativity,

and it's fundamental to
the way time flows

across the surface of the globe.

Now, what if I use my smartphone
app where I live here,

on the south coast of England

and then go and spend a few days
down near the equator?

So, here on the West Coast of
Africa.

Now, we know from the road trip that
gravity is weaker by the equator.

So, that means
time ticks faster there.

But there's another important factor
we have to take into account -

movement.

You see, when I'm here,
near the equator,

I'm moving more quickly

than I was back in Britain because
of the rotation of the Earth.

Einstein says movement
slows down time

so clocks will tick
slower at the equator.

This is where the error crept in.

You see, I had taken into account
these two effects,

but I'd missed a crucial point.

They cancel each other out exactly.

In fact, the Earth bulges out

exactly the right amount
for its rotational speed

to make sure they cancel out,

so all clocks on the surface
of the Earth, at sea level, tick

at exactly the same rate.

So, now I'm having to go right back
to square one

and completely rewrite
the equations for the app.

And, to test if it's working,

I'm going to use it over the course
of a normal working week.

This is where I live,
this is Portsmouth,

which means I'm very close
to sea level,

and this is how I start
most mornings,

catching the train to work.

The app records my speed
as I'm on the train

and calculates how this slows down
my personal clock.

I think the train journey

should have slowed my time down
by a tiny...

A few trillionths of second.

I'm heading for the BBC's
headquarters in Central London,

and gravity should be
a bit weaker here.

I'm a few metres above sea level,
I guess, here.

And so there will be a speed-up
of my time because of altitude.

The app compares
the way my time flows

with a stationary clock
at sea level.

So, what's my result?

On an average day, my movement makes
me age slower by a third

of a nanosecond. That's a third
of a billionth of a second.

But the weaker gravity I'm in

means I age faster - overall,

half a nanosecond faster.

I've also given the app to some
other volunteers

to compare how they age
over an average day.

Nick flies cargo planes.

He flies from Chicago to Germany.

Tomorrow morning,

we have to leave to go first to
Milan and then on to Tokyo.

His travel slows down his ageing,

but much weaker gravity
at high altitude

speeds his clock up
by just a bit more.

Overall, he's ageing five
nanoseconds faster

than a stationary clock
at sea level.

Vanessa runs a pub
in the Yorkshire Dales.

I'm going to take you outside to
see the weather conditions here.

So, here we are,
outside the Tan Hill Inn.

We live right in the middle of the
National Park on the moor.

The Tan Hill Inn is famous as
Britain's highest altitude pub

at over 500 metres above sea level.

We don't have any neighbours,
we just have sheep.

Her altitude means she ages faster
every day

by around four nanoseconds

compared to someone at sea level.

There's Kevin, a mountaineer
in the Highlands.

I'm on a mountain in Glencoe
called Sgor na h-Ulaidh.

I've been at an altitude generally
of between 2,000-3,000 feet

for a lot of the day.
Throughout the day,

I've just been logging on to the
phone, logging on to the app,

and just checking it out
and having a look,

and I've been watching it get bigger

and watching the value get bigger
and bigger.

So, it's been quite a lot of fun.

On an average day of climbing,

Kevin's personal clock
goes faster by one nanosecond.

Gary works for a Scottish
water retailer.

My job takes me all over the UK,

dealing with energy consultants
and energy brokers,

as far up north as Inverness,
as far down south as London.

I approximately do about
1,000 miles a week, sometimes more,

depending on the number
of meetings I have.

Gary's car journeys
do slow his time down a bit,

but being above sea level

means he still ages faster
by three quarters of a nanosecond.

Our final volunteer is Walter.

He lives close to sea level
at the iconic John O'Groats.

I run the tourism business
and I started about 50 years ago,

so when people come here,
they can actually speak to someone

who's been born in John O'Groats
and, if they ask questions,

I can tell them all sorts
of useless information

because I'm full of useless
information.

So our final results show that,
if you want to age more slowly,

try to live near sea level,
like Walter.

Or there is another way to do it -

get a job on the
International Space Station.

Its 17,000-mile-an-hour orbit
will give you a boost.

We did the maths for the astronauts.

Every month, you are about one
millisecond younger,

so one thousandth of a second.

So, after six months,

you're that much younger
than people on Earth.

So, I'm younger than I should be.

I hope I look it.

Of course, for us on Earth,

time dilation
is so utterly minuscule,

a few billionths of a second
between us,

you might think it's too frivolous
to even bother about.

And yet, in the long and difficult
process of designing the app,

I've come to an
extraordinary conclusion.

The different ways that time flows

may not be some quirky by-product
of gravity.

It may actually BE gravity.

It may be the CAUSE of gravity...

the reason why objects fall.

One of the colleagues I've been
consulting is Kip Thorne.

He's one of the world's leading
theoretical physicists

and a driving force
behind the creation of LIGO.

While I was going back over some of
the basic physics behind the app,

I came across an intriguing
idea of his.

It's a very interesting
and different way

of describing gravity.

This is what Kip says.

"Everything likes to live
where it'll age the most slowly,"

"and gravity pulls it there."

Kip's based at Caltech in California

and is one of the most respected
theoretical physicists in the world.

Firstly, Kip, a serious thank you

for helping out
with the debacle over the app!

Well, I sympathise.

I've made so many errors of my own
over the years

that I am totally sympathetic.

One of the things that struck me,

thinking about this,
is something you wrote, Kip.

You said, "Everything likes to live
where it'll age the most slowly",

"and gravity pulls it there."

Was this a way of explaining
something

that you felt was a neat explanation

or is there something
deeply profound about that?

I think there is something deeply
profound, in some sense,

but it's a lovely description

of Einstein's first major
insight about gravity.

In 1912, he realised that gravity

that we feel on Earth is due to a
slowing of time on Earth.

So, time comes before gravity,
in that sense?

On the Earth's surface,
time runs more slowly

and that accounts for why gravity
wants to keep us there?

Well, I think, in a very deep sense,
this is true.

Objects WANT to fall.

The flow of time, or the rate
of flow of the time,

is the thing that produces
the gravity,

it is the thing that is ultimately
responsible for the fall.

So, somehow, it's in the nature
of all objects

to move towards a region
where time runs slower.

Kip's formulation works anywhere in
the universe

where the gravitational field is
such as on Earth.

The difference in the rate of flow
of time is tiny.

At high altitude
and on the surface of the Earth,

the difference in the rate of flow
of time is one second in 100 years.

That's not very much!

But that is enough that it's
precisely the right amount

to produce the gravitational pull
that we feel

and produce the accelerations
we're talking about.

Wow, OK. I need to go
and write this one down!

So, my investigation deep into the
weird ways of gravity

has finally left me face-to-face

with one of the greatest mysteries
in all of physics,

the nature of time itself.

It sounds like such a simple
question.

Why does the apple fall?

And yet, hundreds of years of
scientific enquiry

investigating this single action

have led us to completely redefine

the way we think about the very
nature of space and time.

And now I've been presented with
this extraordinary proposition,

that somehow, in some profound way,

the apple falls because it's seeking
out the place

where time runs the slowest.

So, does gravity dictate the flow
of time?

Or does time itself define gravity?

Could this hint to fundamental new
laws of physics,

as yet undiscovered?

I think I'm going to have to think
about this a bit more.