Gravity and Me: The Force That Shapes Our Lives

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.

BELLS RING

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,

981124007

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.

THEY LAUGH

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.

SHE MOUTHS, ALL LAUGH

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!

ALL LAUGH

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!

LAUGHTER

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.

LAUGHTER

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.

REPORTERS CLAMOUR

'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?

CRASHING

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.

RHYTHMIC PULSES

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,