All language subtitles for Precision.The.Measure.Of.All.Things - 1of3 - Time.And.Distance

In 1852,
clockmaker Edward Dent set out

to construct the largest and most
accurate public clock in the world.

It took seven years to build.

A testament to a very human need.

Our modern day lives are completely
driven by precise measurement.

Take Big Ben. For over 150 years
it's been ringing out

the correct time to the
people of London.

When built,
it was an engineering marvel

accurate to an incredible
one second an hour.

But times have changed.

Today we can build clocks which lose
one second in 138 million years.

And now there are plans for a clock
accurate to within one second

over the lifetime of the universe.

What is it that drives us to such
extremes of ever greater precision?

Why do we feel the need to quantify
and measure,

to impose order
on the world around us?

Since our ancestors first began
to count the passing of the seasons,

successive civilisations have
used measurement

to help master the world
around them.

It's taken us to the moon
and split the atom.

And it fascinates me.

Ever since I was young, I've been
obsessed with measuring things,

trying to make sense
of the world around me.

But where do those
measurements come from?

I mean, who decided a kilo was
a kilo, and a second a second?

What we measure, how we measure it,

and how accurately we can measure it
are surprisingly complex questions.

Questions which have obsessed
generations of great minds,

and created a system that describes
everything in our world with

just seven fundamental
units of measurement.

And the quest to define those seven
units with ever greater precision

has changed our world.

In this series,
I want to explore why we measure.

What drives us to try
and reduce the chaos

and complexity of the world to just
a handful of elementary units.

In this first programme, I'm going
to be looking at two of the most

fundamental measurements,
namely the metre and the second.

It's likely that time and distance

were the first things
people ever tried to measure.

They seem closely
linked in our minds.

We even talk about length of time.

And as we'll see, time and distance

are inextricably
connected by modern science.

Being able to measure time actually
means spotting patterns and

that's actually a very mathematical
way of looking at the world.

In fact, measuring time is
an incredibly sophisticated act.

So where did it all begin?

Our ancestors would have
first picked up

on the patterns of the seasons.

Marking time as the leaves turned
brown, or the days got shorter,

when rivers flooded,
or berries ripened.

These very practical observations
would have helped them

in the daily struggle to survive.

One of the first examples of humans'
attempts to measure

was discovered here in Southern
France by four teenagers

and their dog called Robot.

It was 1940 and the 18-year-old
Marcel Ravidat

was exploring these woods

when he came across a hole where
a tree had been uprooted by a storm.

He needed some tools to make
the hole bigger

so he came back four days later
with his three friends,

and they uncovered the entrance to
a huge system of unexplored caves.

But what they discovered inside
was even more exciting.

Wow!

The boys must have been absolutely
staggered to come in here

and see these images
painted on the wall.

I mean, these are some of the oldest
cave paintings.

Oh, look at this!

All over the wall!

Marcel and his friends
had discovered

some of the earliest cave paintings
ever found.

These date back 17,000 years
and were painted by Cro-Magnon man.

It's beautiful!

You can really feel
the energy of these animals

rushing across the walls.

This cave is a replica
of the original

which is a few hundred metres from
here and is now carefully preserved.

Dr Michael Rappenglueck believes
that these paintings

are evidence of man's first
attempt to measure time.

This one is very, very beautiful.

To him, this is a giant calendar.

The clues lie in these strange
patterns of dots.

Each dot represents a week.

13 dots represent one quarter
of the year.

His theory is that each seven-day
phase of the moon,

what today we'd call a week,

is marked with a dot on the wall
to chart the passing of time.

It was a distinctively-shaped
cluster of dots

that eventually
allowed him to unlock

the full meaning of the paintings.

Look up to the ceiling.
You see six dots.

It reminds a little dipper,

and I think this is the star
pattern of the Pleiades.

Oh, so these dots are not
representing weeks any more,

these are stars up there?

Yes. These are stars,
and they serve to start the year.

When our ancestors saw the stars
form this same alignment in the sky,

it would mark
the start of their year.

Dr Rappenglueck believes
the animals have meaning too.

The stag represents autumn equinox

and it's starting a time
cycle to the horse.

The horse represents spring time

and you see the horse is
pregnant, highly pregnant,

so three-quarters of the year
are represented on the wall.

So, it's the star calendar

followed by the calendar marking
the weeks that allows them to know

when the stags are rutting,
or pregnant animals...

Yes, they synchronised biological
rhythms of animals

with astronomical rhythms.

It's an extraordinarily
sophisticated system... Yes, it is.

..for 17,000 years ago. It is.

It's amazing!

With the aid of this basic calendar,
for the first time,

our ancestors could start to predict
what would happen, and when.

They could prepare to hunt
when animals migrated close by

or, as agriculture developed,
determine the best time to plant
crops.

Measurement was making life easier.

But as communities grew, so did the
need for more precise timekeeping

beyond the cycles of the moon,
the stars and the seasons.

13,000 years after our ancestors
painted the caves in Lascaux,

first the Mesopotamians
and then the Egyptians

started to tackle the problem of
dividing up the day.

And they took their inspiration
from the sun.

By observing how the length
of a shadow changed through the day,

they found an easy way to
measure time.

And they used a device
just like this.

This is a replica of
an Ancient Egyptian sundial.

It's one of the first instruments
ever created to measure time.

Now at midday, this stone
here would have cast no shadow.

But, as the day went on, the shadow
would get longer and longer,

so the Ancient Egyptians decided to
divide the day up into 12 units.

You can see the lines here -
we've got one, two, three...

We've got six lines for the
afternoon, and six for the morning.

It's just coming up to
three o'clock.

By linking time and distance,
they could reliably measure

shorter periods of time.

Telling the time, by measuring
the length of a shadow.

Although the sundial was
a brilliant invention,

it was fundamentally flawed.

It didn't work at night.

Like the cavemen of Lascaux,
who used stars to mark the seasons,

the Egyptians went one step further.

They used them to divide up
the hours of darkness.

But on a cloudy night,
just as on a cloudy day,

they still had no
way of telling the time,

and this is where
they made a conceptual leap.

This is a water clock.

It's a very simple idea.

Basically, what they did
was to take a bucket

and make a hole in the bottom.

Then as night fell, they would
fill the bucket with water.

Now, as the water drips out,

they can use lines marked
on the side of the bucket

to tell how much time
has passed through the night.

They could measure 12 hours

independently
of the sun or the stars.

But why count 12 hours at all?

The answer lies in how business
was done thousands of years ago.

Throughout the Middle East,

the number 12 and the number 60
were important in commerce.

They're numbers that
were familiar to traders
in markets just like this.

And the reason they use them
is all to do with arithmetic.

As a mathematician,
I love the answer

because it's about the mathematical
properties of these two numbers.

They're highly divisible.

Take the number 60.

I can divide 60 beans
into six groups of ten beans,

five groups of 12 beans...

..four groups of 15 beans...

..three groups of 20 beans.

Five, there.

Two groups of 30 beans...

..or one group of 60 beans.

But take 100 beans,
how can I divide that?

I can divide it
into two groups of 50

but divide by three and
I've got to start cutting a bean!

Because the numbers 12 and 60
were so familiar to the Egyptians,

it was perhaps
no great conceptual leap

for them to come up
with a 12-hour night and day.

So the idea stuck.

It wasn't just the measurement
of time that the Egyptians
needed to tackle.

They also needed to find better
ways to measure distance.

Every year the Nile would flood,

bringing great
fertility to the land.

But with each flood,

the borders of the farmers'
land would be washed away.

So when the waters receded,
an accurate way of measuring

field size and re-establishing
boundaries was critical.

They needed a reliable
and uniform measure of length.

And their solution was this.

It's a cubit rod and it's
the Egyptian equivalent of a ruler.

Its length was the distance
of the pharaoh's cubit,

which was the length from his elbow
to the tip of his middle finger.

So actually, my cubit is slightly
shorter than the pharaoh's.

But this led to the Egyptians
creating some of the most remarkable
buildings the world has ever seen.

This is the great
pyramid of Cheops,

built over 4,500 years ago for
the fourth-dynasty pharaoh, Khufu.

It is said 20,000 men
took 20 years to build it,

using over two million
limestone blocks,

all meticulously aligned
and measured with the cubit rod.

This is a miraculous building.

The length of the side
is 440 cubits exactly.

Exactly? Exactly.

And the height
is 280 cubits exactly.

Also it is very square.

It has perfection
in every part of it.

Absolutely, and with
so many people working on it,

spread over, I guess, a large area
and a large amount of time,

I mean, actually having
a standard unit of measurement

must have been absolutely essential.

Exactly. They had a rope
which is 100 times this

that has knots in it every one cubit
or every ten cubits,

which is called khet.

OK. We want to measure 440
so we need to take the corner stone

as our starting point,
so if you start measuring. Yes.

The original cornerstones
are no longer visible

but the foundations
are still here for all to see.

I think I chose the easy job.

430.

Wow!

440 cubits, pretty much on the knot!
Exactly!

What's so remarkable about
the Egyptian system is that they

were one of the first to
standardise length measurement.

It's said that every full moon,

the surveyors across
the land would gather

and compare their wooden cubit
rod against the royal master cubit.

Made of granite,
this was held by the royal surveyor.

Failure to maintain an accurate
cubit was punishable by death.

It was a very simple
and efficient way

to standardise length
measurement across the land.

And it enabled the Egyptians
to measure things
with phenomenal accuracy.

Mastering and standardising
time and length measurement

was really key to the success
of the ancient Egyptian empire.

The power of measurement is that
it created order out of chaos

and allowed civilisation
to flourish.

The standardisation of measurement
which began here in Egypt

several millennia ago is now
central to all our lives.

Nearly every country in the world
has a national measurement body

whose master lengths
and weights are calibrated

by one international body,

a little bit like
the modern day pharaohs,

trying to bring standardisation
of measurement across the globe.

But despite the obvious logic
of having one international system,

it hasn't been completely embraced.

Take me, for example.

I'm going to the airport in this cab
which measures its speed

in kilometres per hour
and miles per hour.

When I'm up in the air, they'll be
measuring their altitude in feet.

My clothes are measured in inches
and my shoes are measured in...

..well, frankly
I've never quite understood

what the unit of
measurement for shoe size is!

Shoe sizes aside,

standardisation of measurement
underpins all modern science.

Though the route to standardisation
has not been an easy one.

Throughout history,

rulers had a nasty habit of ripping
up measurement systems

and demanding that they
be replaced by lengths
based on their own body parts.

In 12th-century England

the yard was defined as the length
from the tip of the King's nose

to the top of his
outstretched thumb.

But as each new reign came in,
so things changed.

Henry VII, he defined a yard
as the length of his arm.

Elizabeth I, not to be outdone
by her male predecessors,

added a few more inches.

And so the chaos continued.

Lack of standardisation was
a problem on the Continent, too.

If you thought the British
had it bad,

then spare a thought for the French.

On the eve of the French Revolution,

the Ancien Regime had over 250,000
different weights and measures,

including several
thousand for length.

By the end of the 18th century,

people realised that
something needed to be done.

Trade was impossible and open to
fraud, navigation was treacherous

and building plans made by an
architect in one city

couldn't be reproduced in the other

because they didn't have
the same measurements.

The mess was finally sorted out
by the French Academy of Sciences.

It was the last few
days of the French monarchy,

and buoyed by the revolutionary
spirit of the time,

a sense of egalite and rationalism,

France's best scientists
decided to form

a ground-breaking and
revolutionary plan of their own.

No longer would measurement
be based on the human body,

or the vanity of kings and queens.

They decided that it should be based

on something permanent and
unchanging. They chose the Earth.

It's really exciting to be here.

This is really one of the
great scientific centres
in the whole of the world.

And this is where the modern
story of measurement really began.

Where a new standardised
unit of length was introduced.

One that is familiar
to us all today.

On the 26th March 1791,
the Academy here decided to call

this new length
measurement the metre.

Named after the Greek word "metron",
meaning measure,

they decided it should be
one ten-millionth of the distance

between the North Pole
and the equator.

It was very clever.

The Academy knew that
a French colloquial measure

would never be accepted
by the rest of the world.

By basing the metre
on the planet itself,

no one country could
argue for their own measure.

They had transcended
the politics of nations.

"This is a system for all
people for all time",

announced the
Revolutionary government.

There was one problem, though.

Nobody knew accurately what the
distance between the North Pole
and the equator actually was.

Getting an accurate figure
would mean embarking on the most

ambitious and complex large-scale
measurement project ever attempted.

Two scientists were tasked with
turning the theory into reality.

They were Pierre M�chain
and Jean Baptiste Delambre.

Their task was to measure the
distance between two points on
a meridian, or line of longitude.

Then using fairly simple
mathematics,

and knowing
the latitude of each point,

they could extrapolate
and calculate the distance

from the Pole to the equator.

This experiment would be difficult
enough under normal conditions

but France was in
the middle of a revolution.

It was a dangerous time
to have big ideas that were not

necessarily easy
for the new order to understand.

Nevertheless, undaunted,
the scientists pushed ahead.

It was here in 1793,
from this bell tower in Dunkirk,

that Jean Baptiste Delambre
started the northernmost part

of his epic quest
to measure the Earth.

While 800 miles to the south,

Barcelona was chosen
for Pierre M�chain.

Their plan was to work
towards each other

and meet in Rodez
in southern France.

You can imagine Delambre's
excitement as he stood up here

200 years ago,
ready to start his journey.

A journey that would take
him seven years to complete.

And the rather splendid piece of
equipment they used was this,

a repeating circle.

A device that measures angles
extremely accurately

and as good today
as the day it was made.

Now, obviously, Delambre
wouldn't measure every distance
from here to Barcelona

but what he can do is use a method
called triangulation.

So, the first point of the triangle
is the top of this belfry.

Then Delambre would have
looked across the countryside,
trying to find two high points.

And he would use this piece of
equipment to line up the telescopes

on those two other points.

Then all he had to do was measure
the angle between the two points

and measure the distance
to the closest one.

By then moving
to the next high point

and measuring the angles again,

simple geometry gave him
the distances between all three.

So it's an amazing principle because
just one measurement of distance

and then it's triangles
all the way to Barcelona.

Delambre had a number of close
scrapes along the way.

He was arrested several times,
accused of being a spy.

Why else would he be scaling towers
carrying strange equipment?

He tried to explain
that he was measuring the size

of the Earth for
the Academy of Sciences

but a drunk militiaman interrupted,

"There is no more Academy. We are
all equal now. You'll come with us."

But in general, they were
literally above it all.

On rooftops, towers and church
spires they carried out their quest.

It was an extraordinary feat.

Seven long years later, the two men
had measured the exact distance

between Dunkirk and Barcelona.

Now the metre
was just a simple calculation.

The result of all M�chain
and Delambre's hard labour,
the prototype metre bar,

is held here at the
French National Archives in Paris.

Made in 1799 of pure platinum,
it's meant to represent

one ten-millionth of the distance

between the North Pole
and the equator.

In fact, due to errors
that M�chain made early on

in his survey,
it's fractionally wrong.

The errors M�chain made
were pretty much irrelevant

because for the first time,

the world had a unit of length

that was based on something
they believed was permanent
and unchanging - the Earth.

There it is. The metre.

A thing of beauty.

Not so much the object
but the idea it represents.

This metre bar ushered in
the era of metrification.

And the achievement is immense.

Even Napoleon, in a moment
of humility, admitted that

"Conquests come and go,
but this work will endure."

And he was right,

this lump of metal really represents
a change in our thinking.

For the first time,

we had measurement based on
something fundamental and universal.

The concept was brilliant,

but the metre's triumphant arrival

was not embraced
with universal enthusiasm.

In fact, it took several
decades before the metre was finally

accepted as a standard
international unit of measurement.

It was on a spring day in 1875
that it all became official.

The historic
Metre Convention was signed

and metre clones sent
out around the world.

It was the beginning of our global
system of precision and accuracy.

17 countries signed the
convention to form the BIPM,

the Bureau International
de Poids et Measures.

The custodians of international
weight and measurement.

It's a role they still
perform today.

Metrication was to be the basis
for a new system of measurement,

the System Internationale or SI.

It even led to a new science,
metrology,

the study and refinement
of measurement.

The metre had united the world.
At least, in theory.

Alongside the metre,
seismic changes had happened
in how we measured time.

For more than 3,000 years,

the sundial was the timekeeper
of choice across the world.

But it was not without its problems.

And the reason is it's just not
possible to fix the exact

length of an hour because
the shadow cast on the dial

alters daily throughout the seasons.

The Greek astronomer Hipparchus
was the first to notice

the equal length of day and night
at the spring and autumn equinoxes

and that this could give us
a standard for setting
a fixed length of hour.

But up until the 14th century, we
had no practical way of doing this.

It took the invention of
the mechanical clock to change
everything.

This is the Salisbury Cathedral
clock.

It dates back to 1386 and it's
believed to be the oldest surviving
mechanical clock in the world.

For me this is an absolutely
staggering achievement,

I mean, this is the 14th century,

the medieval time,

and here a blacksmith and
a stonemason have created something

that is able to regulate time.

Now, it isn't driven by a pendulum,

those sort of clocks wouldn't be
invented until the 17th century.

Instead it's these
weights at the back

which are controlling the clock.

And as the weights fall they unwind
the ropes around these barrels.

It's gravity that drives the clock,

and all you need to power it is
some muscle to raise the weights.

The intriguing thing is there isn't
any clock face on this clock.

It was already quite
an achievement in that time

just to get that bell
to bong every hour.

By the end of the 14th century many
cathedrals across Europe

had built clock towers, towering up
to the heavens, glorifying God,

but perhaps more importantly,

controlling the lives
of us mere mortals down below.

The clocks weren't terribly
accurate,

probably the best ones
lost 15 minutes a day,

but they began to
irrevocably change people's lives.

No longer dependent on the sun,

we were tied to the chimes
of man-made clocks.

In the 15th and 16th centuries, as
the mechanisms became more accurate,

the clock face itself appeared,
something we now take for granted.

It then became possible
to break down our day

into even smaller units.

For the first time,
the hour could be divided

into minutes and seconds.

The idea came from the
Greek mathematician Ptolemy

who divided a circle into 360
equal parts called degrees.

He then split each
degree into 60 minutes

and each minute
into 60 second minutes...

..which gave us
the words we use today.

The relationship between time
and length was getting closer.

We now measured the passage of time
by the distance the hand

travelled around the clock face.

Mechanical clocks
gave us a fixed hour.

But actually setting them to the
right time was still a problem.

We still looked to the sun and set
our clocks and watches to noon

when the sun was directly overhead.

But that meant that each town
had its own different time.

For example, here in Salisbury,

the clocks were over seven minutes
later than the clocks in London.

The reason?

Well, we're further west here,
so the sun arrives overhead later.

But with the development of steam
power in the early 19th century,

things had to change
because it was impossible

to set busy train timetables

if every town
had its own different time.

A single national time
was urgently needed.

Under the unswerving
leadership of Sir George Airy,

the Astronomer Royal
at the Greenwich Observatory,

Greenwich time became
the time for Great Britain.

The railways were the first
to switch their entire timetable

to this new time.

And they did it by sending
the correct time to virtually

every station in the country
by the new telegraph lines

which often ran
alongside the railways.

Gradually, national
and international time

became essential
for business and in 1884,

Greenwich time was universally
adopted as the basis

for a new system of
international time zones.

The reason for its enthusiastic
adoption was because

the Greenwich Observatory
produced the most accurate

nautical almanacs used by mariners
throughout the world.

And as these almanacs were
all set with Greenwich lying
on zero degrees of longitude,

the prime meridian, at a stroke,

Great Britain became
the centre of the world.

Time was no longer
calibrated locally

by when the sun was at its highest,

it was set
astronomically at Greenwich.

But while Greenwich time
had gone international,

for most people, actually getting
your hands on the correct time

was still a challenge.

And for businesses,
this was fast becoming a problem.

And one family realised a cunning
way to exploit this need.

Every week, John Henry Belville
would come up the hill

here to Greenwich and set his
chronometer to the correct time.

And then he'd go
back down to London

to sell the right time
to watchmakers and businesses.

By the 1940s, thanks to the radio
and cheap clocks and watches...

..we could all run on time.

Time was money.

International trade, business
and travel were all thriving.

As the world embraced
Greenwich time,

our journey towards
globalisation started.

While universal time
was transforming our world,

the same could not be
said for the metre.

17 countries had enthusiastically
signed up to the historic

metre convention but, in practice,
few had enforced it.

And the muddle of different
measurements continued,

with standards and gauges
differing from town to town,

and even factory to factory,

which was to have dire consequences,
here in the United States.

When a huge fire ripped through the
American city of Baltimore in 1904,

a disaster of epic
proportions was unfolding.

As fire crews from the
nearby cities of Washington

and New York rushed to the scene,
all they could do was sit

and watch the inferno
engulf the city.

None of their fire hoses
would fit Baltimore's fire hydrants.

Despite being less
than 200 miles apart,

all the fire crews were using
different-sized equipment.

The fire raged out of control for
two days, destroying 1,500 homes.

Length measurement needed
to be standardised and fast.

NIST, America's measurement body,

started campaigning
for better standards.

Spurred on by the NIST campaign,

American industrialists
soon realised

that they could capitalise
on improvements in accuracy.

Henry Ford started commissioning
increasingly accurate gauges
and measures.

Precise and standardised measurement

meant that mass production
was possible.

At the same time,
strict patterns of shift work
tied their workforces to the clock.

It was the dawn of the modern age.

For the first time,
millions of identical parts

could be produced at
rapid speed and minimal cost.

The American boom was underway.

'And when you see inspectors
checking parts for accuracy

'to dimensions measured
in 10,000ths of an inch,

'you see where quantity production
of quality products

'actually begins because parts
must fit together perfectly.'

It would provide a profound
lesson to the world.

Precise measurement had the power
to change the fortunes of a nation.

But the problem with any
technological breakthrough

is no-one quite knows
where it will lead.

It took the paranoia of the
Cold War and the resulting arms race

to trigger the next big
leap in length measurement.

And it led us further
than we ever thought possible.

'But history and our own conscience
will judge us harshly,

'if we do not now make every effort
to test our hopes by action.'

The stakes were rising but our level
of accuracy

was failing to keep up
with our aspirations.

Up to the 1960s,

we could measure with an accuracy
of one ten-millionth of a metre.

But an error of this
magnitude in the components
of a rocket navigation system

would mean missing
the moon by 4,000 miles.

Now the challenge was to improve
the accuracy a hundredfold.

'We choose to go to the moon in this
decade and do the other things,

'not because they are easy
but because they are hard.

'Because that goal will
serve to organise

'and measure the best
of our energies and skills.'

The metre bar was no longer
accurate enough.

A new and more precise
way of measuring length was needed.

The answer lay in the fundamental
properties of the universe.

It was the dawn of the quantum age.

Since the 1870s,

there had been a growing desire to
take measurement away from earthly

constants like circumference of the
globe or the length of the day...

..and to tie measurement
to the fundamental
and unchanging laws of nature.

Things like the speed of light
or the charge on a single electron.

It was a Scottish genius,
James Clerk Maxwell,

who first suggested that
these universal constants

could hold the key
to more precise measurement.

Considered by many
to be the 19th century's
most influential physicist,

Maxwell's theories would change
the course of measurement history.

He said at the time,

"If then we wish to obtain standards
which shall be absolutely permanent,

"we must seek them not in the
dimensions or motion of our planet,

"but in the wavelength,
the period of vibration

"and the absolute mass of these
imperishable and unalterable

"and perfectly similar molecules."

Maxwell's idea was as revolutionary
as the decision a century earlier

to take length measurement away
from the human body
and base it on the Earth.

Maxwell changed the direction
of the science of measurement.

Maxwell, it's hard to
overestimate the influence he had

on scientific thought
in the 19th century.

It was a very influential
idea he had and he said,

"We should be measuring length

"in terms of the wavelength
of a colour of light."

But even he couldn't figure out
how to really do it

to the accuracy that would be
required to replace the, sort of,

metre definition.

Maxwell was never able
to turn his dream of using

the wavelength of light
to measure distance into reality

because the technology to
achieve it simply didn't exist.

But his ideas were revolutionary.

It wasn't until decades later,

a scientist at the BIPM,
the same place where the
world's master metre bar is held,

would start to
bring Maxwell's vision to life.

Albert Michelson began to design

and build machines called
interferometers

that would actually
measure the wavelength
of different light sources.

So this is one of Michelson's
original interferometers.

What was he using it for
and how did he use it?

Well, he wanted to demonstrate
that it would be possible

to measure a wavelength of light,
because light travels in waves,

and then in a future time,

define the metre in terms
of this wavelength of light.

Wavelengths of light
are invisible to the human eye.

Michelson's genius was
realising that when light is split

and then recombined, it forms a
unique pattern called interference

that can be used to
count wavelengths.

So by counting how many, going
from light to dark, light to dark,

take a metre,
divide by the number of those,

you'll get the wavelength
of light,

something you can't see with your
naked eye. Right.

What he had to do was build up from
a wavelength of light to a metre.

And in a half a millimetre,

there are about more than
1,000 wavelengths. Extraordinary.

It was the breakthrough that was
to change the destiny of the metre.

After over half
a century of laborious research,

scientists were ready.

Maxwell's dream
was about to become a reality.

On Friday the 14th of October 1960,
delegates from across the globe,

from Russia and America, gathered
here in the grounds of the BIPM.

The fate of the metre
was in the balance.

At six o'clock that evening,
to much applause,

the metre was redefined in terms
of the number

of wavelengths of light emitted
by a special krypton lamp.

Finally, the metre bar
was consigned to history.

But I don't think
those French Revolutionaries

who first came up with the idea of
the metre would be too disappointed

because it was really
realising their dream of tying

the metre to something
unchanging and universal.

Distance could be
measured accurately

using a universal constant,
the wavelength of light.

But how could we put this
new science into practice?

That would need the help
of a project codenamed Laser.

It was the brainchild of
Californian Theodore Maiman.

Well, this device happens
to be the original laser.

The beauty of the laser,
is that it is light of a precise,

fixed wavelength.

By bouncing this beam off an object,

and precisely measuring the time
it takes to bounce back,

suddenly we could measure distances
with incredible precision.

Within years,
the laser was helping us

to measure our world in ways
we never thought possible.

And there was no better
illustration of this

than the Apollo 11 lunar landings.

'One small step for man,
one giant leap for mankind.'

When Neil Armstrong and Buzz Aldrin

landed on the Sea of Tranquillity
more than 40 years ago

on the 21st July 1969,

they left a mirror
on the moon's surface.

When astronomers later fired
a laser pulse at it,

Maiman's invention was
also about to make history.

The beam took just 2.5 seconds
to reflect back to Earth.

For the first time, scientists
could calculate the distance

to the moon at
any phase of its orbit

to an accuracy of three centimetres.

Lasers changed everything.

They made scientists
rethink what was possible.

We could measure distance
with extraordinary precision.

Distance was tied to a universal,
unchanging constant

but time was not.

The second was still
based on the rotation of the Earth,

which is actually rather variable.

Finding a better
way of defining time

was to come from
an unexpected quarter.

Just a few years before that
landmark 1960 meeting in Paris,

an English scientist
called Louis Essen

was working here at the
UK's National Physical Laboratory.

His passion was
precision timekeeping,

and he was beginning work
on a new generation of clock,

the atomic clock.

We set our quartz clocks to keep
time with the rotation of the Earth.

But for some of our modern problems,
this is not quite accurate enough,

and now we're setting
our quartz to keep time

with the vibrations of the atom.

The theory was to define time

through the vibration
of individual atoms.

Across the Atlantic, the Americans,
at their national laboratory,

were already pushing forward
with a well-funded programme.

Back in Britain,
Essen was struggling.

There was little enthusiasm
for his clock project

and funding was always a problem.

His first experiment imploded,
destroying much of his equipment.

But in a classic story
of the underdog winning through,

Essen eventually created the
world's first working atomic clock.

It was called the Caesium I.

And it was accurate
to one second in 300 years.

The second was no longer
based on the movement of our planet.

Time was now locked to the
beating heart of a caesium atom.

A movement that was unchanging
and fundamental across the universe.

In Britain, the latest incarnation
of Essen's atomic clock is the CsF2.

It's one of a global network
of atomic clocks that sets our time.

To most people this doesn't look
like a clock at all,

so how does it actually
measure time?

Well, what we're doing here
is using lasers

to slow down the caesium atoms.

We form a cloud of very slowly
moving caesium atoms

and we use the lasers to throw
that cloud upwards through

an enclosure containing microwaves.

Then they fall back through
it a second time under gravity.

When the atoms change from one
energy level to another,

they emit or absorb
one very precise frequency,

and we can use that
frequency to keep track of time.

We simply count up the oscillations.

So it's the number of
oscillations that will define
the length of a second.

Those oscillations are a particular
property of that caesium atom.

That's right, yes.

So, any caesium atom
always has the same number

of oscillations per second.

The oscillations of these caesium
atoms are the ticking of the clock.

and they give the CsF2 accuracy
to one second in 138 million years.

It's a degree of precision our
ancestors could never have imagined.

The genius of Maxwell, Michelson

and Essen now touch
every part of our lives.

They could never have
guessed their work would one day

be at the centre of everything
from our banking systems to phones,

GPS and the internet.

These only exist because
of the accuracy of atomic clocks

and their ability to synchronise
time across the planet.

Measurement has taken us
in directions we could
never have dreamt possible.

But the story doesn't end there.

In one last twist, scientists
looked at the metre again...

..and realised that
they could now redefine length

using the new
accuracy of the second.

It was 1983 and in a collaboration
between different measurement labs

across the world, atomic clocks
measured the speed of light

with incredible precision.

The metre could finally
be defined by how far light

travels in a tiny
fraction of a second.

Time and length were
intimately intertwined.

We've come a long way
since the days of the pharaohs,

when time was defined
by the length of a shadow.

After 3,000 years, time and distance
are once again linked,

joined together by
one of the most fundamental

and universal constants of nature,
the speed of light.

Despite all the great advances
in time and length measurement,

the quest is still on.

Scientists are trying to create
ever more accurate clocks.

Clocks that will only
lose one second

in the lifetime of the universe.

And once they're deployed
we can only begin to imagine
how it's going to change our world.

Instant communication,
quantum computers,

planes that can land themselves.

Science fiction
will become a reality.

And that's the beauty
of measurement.

Every leap in precision, from the
cubit rod to the atomic clock,

has led to
a technological revolution.

Through history measurement has
changed every aspect of our lives...

..splitting the year into seasons
and lunar cycles

allowed man to plan ahead
for the first time

and gain advantage
over the rest of nature.

Dividing the day still
further into 24 hours

was the bedrock for civilisation.

The fixed hour controlled
the working day.

And uniform national
and international time

allowed the
globalisation of industry.

The world would never be the same.

The story of measurement
has shaped and changed our history.

And will continue to do so
as we delve deeper into the atomic

fabric of the universe
in search of greater precision.

Next time, I meet the biggest
problem in measurement,
the kilogram.

This 19th-century artefact
is the world's master kilo,

and it's losing weight.

Now, a head-to-head race
is on to replace it...

..as the best minds in
measurement science fight it out,

there can only be one winner.