0:00:01 > 0:00:05In 1852, clockmaker Edward Dent set out
0:00:05 > 0:00:10to construct the largest and most accurate public clock in the world.
0:00:11 > 0:00:13It took seven years to build.
0:00:13 > 0:00:16CLOCK STRIKES
0:00:16 > 0:00:18A testament to a very human need.
0:00:20 > 0:00:23Our modern day lives are completely driven by precise measurement.
0:00:23 > 0:00:27Take Big Ben. For over 150 years it's been ringing out
0:00:27 > 0:00:30the correct time to the people of London.
0:00:30 > 0:00:33When built, it was an engineering marvel
0:00:33 > 0:00:36accurate to an incredible one second an hour.
0:00:36 > 0:00:38But times have changed.
0:00:41 > 0:00:48Today we can build clocks which lose one second in 138 million years.
0:00:49 > 0:00:53And now there are plans for a clock accurate to within one second
0:00:53 > 0:00:57over the lifetime of the universe.
0:00:57 > 0:01:02What is it that drives us to such extremes of ever greater precision?
0:01:02 > 0:01:05Why do we feel the need to quantify and measure,
0:01:05 > 0:01:09to impose order on the world around us.
0:01:09 > 0:01:14Since our ancestors first began to count the passing of the seasons,
0:01:14 > 0:01:16successive civilisations have used measurement
0:01:16 > 0:01:20to help master the world around them.
0:01:20 > 0:01:24It's taken us to the moon and split the atom.
0:01:24 > 0:01:27And it fascinates me.
0:01:27 > 0:01:30Ever since I was young, I've been obsessed with measuring things,
0:01:30 > 0:01:34trying to make sense of the world around me.
0:01:34 > 0:01:36But where did these measurements come from?
0:01:36 > 0:01:42I mean, who decided a kilo was a kilo, and a second a second?
0:01:42 > 0:01:44What we measure, how we measure it,
0:01:44 > 0:01:50and how accurately we can measure it are surprisingly complex questions.
0:01:50 > 0:01:53Questions which have obsessed generations of great minds,
0:01:53 > 0:01:58and created a system that describes everything in our world with
0:01:58 > 0:02:02just seven fundamental units of measurement.
0:02:02 > 0:02:05And the quest to define those seven units with ever greater precision
0:02:05 > 0:02:08has changed our world.
0:02:22 > 0:02:26In this series, I want to explore why we measure.
0:02:26 > 0:02:28What drives us to try and reduce the chaos
0:02:28 > 0:02:33and complexity of the world to just a handful of elementary units.
0:02:33 > 0:02:36In this first programme, I'm going to be looking at two of the most
0:02:36 > 0:02:41fundamental measurements, namely the metre and the second.
0:02:42 > 0:02:44It's likely that time and distance
0:02:44 > 0:02:46were the first things people ever tried to measure.
0:02:48 > 0:02:51They seem closely linked in our minds.
0:02:51 > 0:02:54We even talk about length of time.
0:02:57 > 0:02:59And, as we'll see, time and distance
0:02:59 > 0:03:03are inextricably connected by modern science.
0:03:04 > 0:03:07Being able to measure time actually means spotting patterns and
0:03:07 > 0:03:10that's actually a very mathematical way of looking at the world.
0:03:10 > 0:03:15In fact, measuring time is an incredibly sophisticated act.
0:03:15 > 0:03:19So where did it all begin?
0:03:19 > 0:03:21Our ancestors would have first picked up
0:03:21 > 0:03:23on the patterns of the seasons.
0:03:26 > 0:03:30Marking time as the leaves turned brown, or the days got shorter,
0:03:30 > 0:03:33when rivers flooded, or berries ripened.
0:03:33 > 0:03:37These very practical observations would have helped them
0:03:37 > 0:03:39in their daily struggle to survive.
0:03:42 > 0:03:45One of the first examples of humans' attempts to measure
0:03:45 > 0:03:49was discovered here in Southern France by four teenagers
0:03:49 > 0:03:50and their dog called Robot.
0:03:51 > 0:03:56It was 1940 and the 18-year-old Marcel Ravidat
0:03:56 > 0:03:57was exploring these woods
0:03:57 > 0:04:02when he came across a hole where a tree had been uprooted by a storm.
0:04:02 > 0:04:05He needed some tools to make the hole bigger
0:04:05 > 0:04:08so he came back four days later with his three friends,
0:04:08 > 0:04:14and they uncovered the entrance to a huge system of unexplored caves.
0:04:14 > 0:04:17But what they discovered inside was even more exciting.
0:04:30 > 0:04:32Wow!
0:04:33 > 0:04:37The boys must have been absolutely staggered to have come here
0:04:37 > 0:04:40and see these images painted on the wall.
0:04:40 > 0:04:42I mean, these are some of the oldest cave paintings.
0:04:42 > 0:04:44Oh, look at this!
0:04:44 > 0:04:46All over the wall!
0:04:54 > 0:04:57Marcel and his friends had discovered
0:04:57 > 0:04:59some of the earliest cave paintings ever found.
0:04:59 > 0:05:04These date back 17,000 years and were painted by Cro-Magnon man.
0:05:06 > 0:05:09It's beautiful!
0:05:09 > 0:05:12You can really feel the energy of these animals
0:05:12 > 0:05:14rushing across the walls.
0:05:20 > 0:05:22This cave is a replica of the original
0:05:22 > 0:05:27which is a few hundred metres from here and is now carefully preserved.
0:05:33 > 0:05:36Dr Michael Rappenglueck believes that these paintings
0:05:36 > 0:05:41are evidence of man's first attempt to measure time.
0:05:41 > 0:05:44This one is very, very beautiful.
0:05:45 > 0:05:48To him, this is a giant calendar.
0:05:49 > 0:05:53The clues lie in these strange patterns of dots.
0:05:53 > 0:05:55Each dot represents a week.
0:05:55 > 0:05:5813 dots represent one quarter of the year.
0:06:00 > 0:06:04His theory is that each seven day phase of the moon,
0:06:04 > 0:06:06what today we'd call a week,
0:06:06 > 0:06:10is marked with a dot on the wall to chart the passing of time.
0:06:17 > 0:06:20It was a distinctively-shaped cluster of dots
0:06:20 > 0:06:22that eventually allowed him to unlock
0:06:22 > 0:06:24the full meaning of the paintings.
0:06:26 > 0:06:30Look up to the ceiling. You see six dots.
0:06:30 > 0:06:33It reminds a little dipper,
0:06:33 > 0:06:36and I think this is the star pattern of the Pleiades.
0:06:36 > 0:06:38Oh, so these dots are not representing weeks any more,
0:06:38 > 0:06:40these are stars up there?
0:06:40 > 0:06:45Yes. These are stars, and they serve to start the year.
0:06:48 > 0:06:53When our ancestors saw the stars form this same alignment in the sky,
0:06:53 > 0:06:56it would mark the start of their year.
0:06:57 > 0:07:01Dr Rappenglueck believes the animals have meaning too.
0:07:01 > 0:07:05The stag represents autumn equinox
0:07:05 > 0:07:08and it's starting a time cycle to the horse.
0:07:08 > 0:07:11The horse represents spring time
0:07:11 > 0:07:15and you see the horse is pregnant, highly pregnant,
0:07:15 > 0:07:18so three-quarters of the year are represented on the wall.
0:07:18 > 0:07:20So, it's the star calendar
0:07:20 > 0:07:23followed by the calendar marking the weeks that allows them to know
0:07:23 > 0:07:27when the stags are rutting, or pregnant animals...
0:07:27 > 0:07:30Yes, they synchronised biological rhythms of animals
0:07:30 > 0:07:32with astronomical rhythms.
0:07:32 > 0:07:36- It's an extraordinarily sophisticated system...- Yes, it is.
0:07:36 > 0:07:37- ..for 17,000 years ago.- It is.
0:07:37 > 0:07:39It's amazing!
0:07:44 > 0:07:47With the aid of this basic calendar, for the first time,
0:07:47 > 0:07:52our ancestors could start to predict what would happen, and when.
0:07:52 > 0:07:56They could prepare to hunt when animals migrated close by
0:07:56 > 0:08:00or, as agriculture developed, determine the best time to plant crops.
0:08:03 > 0:08:05Measurement was making life easier.
0:08:12 > 0:08:17But as communities grew, so did the need for more precise timekeeping
0:08:17 > 0:08:22beyond the cycles of the moon, the stars and the seasons.
0:08:22 > 0:08:2613,000 years after our ancestors painted the caves in Lascaux,
0:08:26 > 0:08:29first the Mesopotamians and then the Egyptians
0:08:29 > 0:08:32started to tackle the problem of dividing up the day.
0:08:32 > 0:08:35And they took their inspiration from the sun.
0:08:42 > 0:08:45By observing how the length of a shadow changed through the day,
0:08:45 > 0:08:49they found an easy way to measure time.
0:08:49 > 0:08:51And they used a device just like this.
0:08:54 > 0:08:58This is a replica of an Ancient Egyptian sundial.
0:08:58 > 0:09:02It's one of the first instruments ever created to measure time.
0:09:02 > 0:09:06Now at midday, this stone here would have cast no shadow.
0:09:06 > 0:09:10But, as the day went on, the shadow would get longer and longer,
0:09:10 > 0:09:15so the Ancient Egyptians decided to divide the day up into 12 units.
0:09:15 > 0:09:19You can see the lines here - we've got one, two, three...
0:09:19 > 0:09:22We've got six lines for the afternoon, and six for the morning.
0:09:22 > 0:09:25It's just coming up to three o'clock.
0:09:26 > 0:09:30By linking time and distance, they could reliably measure
0:09:30 > 0:09:32shorter periods of time.
0:09:32 > 0:09:35Telling the time, by measuring the length of a shadow.
0:09:46 > 0:09:48Although the sundial was a brilliant invention,
0:09:48 > 0:09:50it was fundamentally flawed.
0:09:53 > 0:09:55It didn't work at night.
0:09:59 > 0:10:03Like the cavemen of Lascaux, who used stars to mark the seasons,
0:10:03 > 0:10:05the Egyptians went one step further.
0:10:07 > 0:10:10They used them to divide up the hours of darkness.
0:10:10 > 0:10:12But on a cloudy night, just as on a cloudy day,
0:10:12 > 0:10:15they still had no way of telling the time,
0:10:15 > 0:10:19and this is where they made a conceptual leap.
0:10:23 > 0:10:25This is a water clock.
0:10:28 > 0:10:30It's a very simple idea.
0:10:30 > 0:10:32Basically, what they did was to take a bucket
0:10:32 > 0:10:35and make a hole in the bottom.
0:10:35 > 0:10:38Then as night fell, they would fill the bucket with water.
0:10:45 > 0:10:47Now, as the water drips out,
0:10:47 > 0:10:50they can use lines marked on the side of the bucket
0:10:50 > 0:10:53to tell how much time has passed through the night.
0:10:58 > 0:11:00They could measure 12 hours
0:11:00 > 0:11:02independently of the sun or the stars.
0:11:04 > 0:11:05But why count 12 hours at all?
0:11:11 > 0:11:15The answer lies in how business was done thousands of years ago.
0:11:15 > 0:11:17Throughout the Middle East,
0:11:17 > 0:11:21the number 12 and the number 60 were important in commerce.
0:11:21 > 0:11:27They're numbers that were familiar to traders in markets just like this.
0:11:27 > 0:11:30And the reason they use them is all to do with arithmetic.
0:11:32 > 0:11:35As a mathematician, I love the answer
0:11:35 > 0:11:38because it's about the mathematical properties of these two numbers.
0:11:38 > 0:11:40They're highly divisible.
0:11:48 > 0:11:50Take the number 60.
0:11:50 > 0:11:54I can divide 60 beans in to 6 groups of 10 beans,
0:11:54 > 0:11:585 groups of 12 beans...
0:11:59 > 0:12:03..4 groups of 15 beans...
0:12:03 > 0:12:053 groups of 20 beans.
0:12:05 > 0:12:07Five, there.
0:12:07 > 0:12:092 groups of 30 beans...
0:12:10 > 0:12:13..or 1 group of 60 beans.
0:12:15 > 0:12:18But take 100 beans, how can I divide that?
0:12:18 > 0:12:21I can divide it into 2 groups of 50
0:12:21 > 0:12:23but divide by three and I've got to start cutting a bean!
0:12:25 > 0:12:28Because the numbers 12 and 60 were so familiar to the Egyptians,
0:12:28 > 0:12:31it was perhaps no great conceptual leap
0:12:31 > 0:12:34for them to come up with a 12-hour night and day.
0:12:34 > 0:12:36So the idea stuck.
0:12:43 > 0:12:47It wasn't just the measurement of time that the Egyptians needed to tackle.
0:12:49 > 0:12:52They also needed to find better ways to measure distance.
0:12:52 > 0:12:55Every year the Nile would flood,
0:12:55 > 0:12:57bringing great fertility to the land.
0:12:59 > 0:13:01But with each flood,
0:13:01 > 0:13:05the borders of the farmers' land would be washed away.
0:13:05 > 0:13:08So when the waters receded, an accurate way of measuring
0:13:08 > 0:13:12field size and re-establishing boundaries was critical.
0:13:12 > 0:13:17They needed a reliable and uniform measure of length.
0:13:17 > 0:13:20And their solution was this.
0:13:20 > 0:13:25It's a cubit rod and it's the Egyptian equivalent of a ruler.
0:13:25 > 0:13:28Its length was the distance of the pharaoh's cubit,
0:13:28 > 0:13:32which was the length from his elbow to the tip of his middle finger.
0:13:32 > 0:13:36So actually, my cubit is slightly shorter than the pharaoh's.
0:13:36 > 0:13:42But this led to the Egyptians creating some of the most remarkable buildings the world has ever seen.
0:13:46 > 0:13:49This is the great pyramid of Cheops,
0:13:49 > 0:13:54built over 4,500 years ago for the fourth dynasty pharaoh, Khufu.
0:13:57 > 0:14:00It is said 20,000 men took 20 years to build it,
0:14:00 > 0:14:04using over two million limestone blocks,
0:14:04 > 0:14:09all meticulously aligned and measured with the cubit rod.
0:14:09 > 0:14:11This is a miraculous building.
0:14:11 > 0:14:16The length of the side is 440 cubits exactly.
0:14:16 > 0:14:18- Exactly?- Exactly.
0:14:18 > 0:14:20And the height is 280 cubits exactly.
0:14:20 > 0:14:22Also it is very square.
0:14:22 > 0:14:25It has perfection in every part of it.
0:14:25 > 0:14:27Absolutely, and with so many people working on it,
0:14:27 > 0:14:32spread over, I guess, a large area and a large amount of time,
0:14:32 > 0:14:34I mean, actually having a standard unit of measurement
0:14:34 > 0:14:36must have been absolutely essential.
0:14:36 > 0:14:40Exactly. They had a rope which is 100 times this
0:14:40 > 0:14:45that has knots in it every one cubit or every ten cubits,
0:14:45 > 0:14:46which is called khet.
0:14:46 > 0:14:50OK. We want to measure 440 so we need to take the corner stone
0:14:50 > 0:14:55- as our starting point, so if you start measuring.- Yes.
0:14:57 > 0:15:00The original cornerstones are no longer visible
0:15:00 > 0:15:04but the foundations are still here for all to see.
0:15:04 > 0:15:05I think I chose the easy job.
0:15:16 > 0:15:18430...
0:15:18 > 0:15:20Wow!
0:15:20 > 0:15:24- 440 cubits, pretty much on the knot! - Exactly!
0:15:36 > 0:15:39What's so remarkable about the Egyptian system is that they
0:15:39 > 0:15:42were one of the first to standardise length measurement.
0:15:42 > 0:15:44It's said that every full moon,
0:15:44 > 0:15:46the surveyors across the land would gather
0:15:46 > 0:15:51and compare their wooden cubit rod against the royal master cubit.
0:15:51 > 0:15:55Made of granite, this was held by the royal surveyor.
0:15:55 > 0:15:59Failure to maintain an accurate cubit was punishable by death.
0:15:59 > 0:16:02It was a very simple and efficient way
0:16:02 > 0:16:05to standardise length measurement across the land.
0:16:05 > 0:16:09And it enabled the Egyptians to measure things with phenomenal accuracy.
0:16:15 > 0:16:18Mastering and standardising time and length measurement
0:16:18 > 0:16:22was really key to the success of the ancient Egyptian empire.
0:16:31 > 0:16:35The power of measurement is that it created order out of chaos
0:16:35 > 0:16:37and allowed civilisation to flourish.
0:16:53 > 0:16:57The standardisation of measurement which began here in Egypt
0:16:57 > 0:17:00several millennia ago is now central to all our lives.
0:17:00 > 0:17:05Nearly every country in the world has a national measurement body
0:17:05 > 0:17:08whose master lengths and weights are calibrated
0:17:08 > 0:17:10by one international body,
0:17:10 > 0:17:13a little bit like the modern day pharaohs,
0:17:13 > 0:17:17trying to bring standardisation of measurement across the globe.
0:17:17 > 0:17:22But despite the obvious logic of having one international system,
0:17:22 > 0:17:24it hasn't been completely embraced.
0:17:24 > 0:17:26Take me, for example.
0:17:26 > 0:17:29I'm going to the airport in this cab which measures its speed
0:17:29 > 0:17:32in kilometres per hour and miles per hour.
0:17:32 > 0:17:37When I'm up in the air, they'll be measuring their altitude in feet.
0:17:37 > 0:17:40My clothes are measured in inches and my shoes are measured in...
0:17:41 > 0:17:44..well, frankly I've never quite understood
0:17:44 > 0:17:47what the unit of measurement for shoe size is!
0:17:49 > 0:17:51Shoe sizes aside,
0:17:51 > 0:17:55standardisation of measurement underpins all modern science.
0:17:55 > 0:17:59Though the route to standardisation has not been an easy one.
0:18:13 > 0:18:15Throughout history,
0:18:15 > 0:18:18rulers had a nasty habit of ripping up measurement systems
0:18:18 > 0:18:23and demanding that they be replaced by lengths based on their own body parts.
0:18:30 > 0:18:31In 12th century England
0:18:31 > 0:18:35the yard was defined as the length from the tip of the King's nose
0:18:35 > 0:18:38to the top of his outstretched thumb.
0:18:38 > 0:18:41But as each new reign came in, so things changed.
0:18:41 > 0:18:44Henry VII, he defined a yard as the length of his arm.
0:18:47 > 0:18:50Elizabeth I, not to be outdone by her male predecessors,
0:18:50 > 0:18:53added a few more inches.
0:18:53 > 0:18:56And so the chaos continued.
0:19:08 > 0:19:12Lack of standardisation was a problem on the Continent, too.
0:19:14 > 0:19:16If you thought the British had it bad,
0:19:16 > 0:19:18then spare a thought for the French.
0:19:18 > 0:19:20On the eve of the French Revolution,
0:19:20 > 0:19:23the Ancien Regime had over 250,000 different weights and measures,
0:19:23 > 0:19:26including several thousand for length.
0:19:36 > 0:19:37By the end of the 18th century,
0:19:37 > 0:19:40people realised that something needed to be done.
0:19:40 > 0:19:44Trade was impossible and open to fraud, navigation was treacherous
0:19:44 > 0:19:46and building plans made by an architect in one city
0:19:46 > 0:19:48couldn't be reproduced in the other
0:19:48 > 0:19:50cos they didn't have the same measurements.
0:20:00 > 0:20:05The mess was finally sorted out by the French Academy of Sciences.
0:20:05 > 0:20:07It was the last few days of the French monarchy,
0:20:07 > 0:20:09and buoyed by the revolutionary spirit of the time,
0:20:09 > 0:20:12a sense of egalite and rationalism,
0:20:12 > 0:20:14France's best scientists decided to form
0:20:14 > 0:20:18a ground-breaking and revolutionary plan of their own.
0:20:18 > 0:20:21No longer would measurement be based on the human body,
0:20:21 > 0:20:24or the vanity of kings and queens.
0:20:24 > 0:20:26They decided that it should be based
0:20:26 > 0:20:30on something permanent and unchanging. They chose the Earth.
0:20:50 > 0:20:52It's really exciting to be here.
0:20:52 > 0:20:57This is really one of the great scientific centres in the whole of the world.
0:20:57 > 0:21:01And this is where the modern story of measurement really began.
0:21:01 > 0:21:04Where a new standardised unit of length was introduced.
0:21:04 > 0:21:07One that is familiar to us all today.
0:21:09 > 0:21:13On the 26th March 1791, the Academy here decided to call
0:21:13 > 0:21:15this new length measurement the metre.
0:21:15 > 0:21:19Named after the Greek word "metron", meaning measure,
0:21:19 > 0:21:22they decided it should be one ten millionth of the distance
0:21:22 > 0:21:25between the North Pole and the equator.
0:21:27 > 0:21:28It was very clever.
0:21:28 > 0:21:31The Academy knew that a French colloquial measure
0:21:31 > 0:21:34would never be accepted by the rest of the world.
0:21:35 > 0:21:38By basing the metre on the planet itself,
0:21:38 > 0:21:42no-one country could argue for their own measure.
0:21:42 > 0:21:46They had transcended the politics of nations.
0:21:47 > 0:21:50"This is a system for all people for all time",
0:21:50 > 0:21:53announced the Revolutionary government.
0:21:53 > 0:21:55There was one problem, though.
0:21:55 > 0:22:00Nobody knew accurately what the distance between the North Pole and the equator actually was.
0:22:03 > 0:22:07Getting an accurate figure would mean embarking on the most
0:22:07 > 0:22:11ambitious and complex large-scale measurement project ever attempted.
0:22:14 > 0:22:19Two scientists were tasked with turning the theory into reality.
0:22:19 > 0:22:23They were Pierre Mechain and Jean Baptiste Delambre.
0:22:25 > 0:22:31Their task was to measure the distance between two points on a meridian, or line of longitude.
0:22:32 > 0:22:35Then using fairly simple mathematics,
0:22:35 > 0:22:37and knowing the latitude of each point,
0:22:37 > 0:22:40they could extrapolate and calculate the distance.
0:22:40 > 0:22:42from the Pole to the equator.
0:22:48 > 0:22:52This experiment would be difficult enough under normal conditions
0:22:52 > 0:22:55but France was in the middle of a revolution.
0:22:58 > 0:23:02It was a dangerous time to have big ideas that were not
0:23:02 > 0:23:04necessarily easy for the new order to understand.
0:23:08 > 0:23:12Nevertheless, undaunted, the scientists pushed ahead.
0:23:16 > 0:23:21It was here in 1793, from this bell tower in Dunkirk,
0:23:21 > 0:23:24that Jean Baptiste Delambre started the northernmost part
0:23:24 > 0:23:26of his epic quest to measure the Earth.
0:23:33 > 0:23:35While 800 miles to the south,
0:23:35 > 0:23:38Barcelona was chosen for Pierre Mechain.
0:23:41 > 0:23:44Their plan was to work towards each other
0:23:44 > 0:23:46and meet in Rodez in southern France.
0:23:51 > 0:23:54You can imagine Delambre's excitement as he stood up here
0:23:54 > 0:23:57200 years ago, ready to start his journey.
0:23:57 > 0:24:00A journey that would take him seven years to complete.
0:24:04 > 0:24:07And the rather splendid piece of equipment they used was this,
0:24:07 > 0:24:10a repeating circle.
0:24:10 > 0:24:13A device that measure angles extremely accurately
0:24:13 > 0:24:17and as good today as the day it was made.
0:24:17 > 0:24:20Now, obviously, Delambre wouldn't measure every distance from here to Barcelona
0:24:20 > 0:24:23but what he can do is use a method called triangulation.
0:24:23 > 0:24:27So, the first point of the triangle is the top of this belfry.
0:24:27 > 0:24:31Then Delambre would have looked across the countryside, trying to find two high points.
0:24:31 > 0:24:35And he would use this piece of equipment to line up the telescopes
0:24:35 > 0:24:37on those two other points.
0:24:37 > 0:24:41Then all he had to do was measure the angle between the two points
0:24:41 > 0:24:43and measure the distance to the closest one.
0:24:45 > 0:24:47By then moving to the next high point
0:24:47 > 0:24:50and measuring the angles again,
0:24:50 > 0:24:53simple geometry gave him the distances between all three.
0:24:57 > 0:25:01So it's an amazing principle because just one measurement of distance
0:25:01 > 0:25:03and then it's triangles all the way to Barcelona.
0:25:11 > 0:25:14Delambre had a number of close scrapes along the way.
0:25:15 > 0:25:19He was arrested several times, accused of being a spy.
0:25:21 > 0:25:25Why else would he be scaling towers carrying strange equipment?
0:25:26 > 0:25:29He tried to explain that he was measuring the size
0:25:29 > 0:25:32of the Earth for the Academy of Sciences
0:25:32 > 0:25:35but a drunk militiaman interrupted,
0:25:35 > 0:25:40"There is no more Academy. We are all equal now. You'll come with us."
0:25:47 > 0:25:50But in general, they were literally above it all.
0:25:50 > 0:25:56On roof tops, towers and church spires they carried out their quest.
0:25:57 > 0:25:59It was an extraordinary feat.
0:25:59 > 0:26:04Seven long years later, the two men had measured the exact distance
0:26:04 > 0:26:07between Dunkirk and Barcelona.
0:26:08 > 0:26:11Now the metre was just a simple calculation.
0:26:27 > 0:26:32The result of all Mechain and Delambre's hard labour, the prototype metre bar,
0:26:32 > 0:26:36is held here at the French National Archives in Paris.
0:26:38 > 0:26:41Made in 1799 of pure platinum, it's meant to represent
0:26:41 > 0:26:44one ten millionth of the distance
0:26:44 > 0:26:46between the North Pole and the equator.
0:26:46 > 0:26:49In fact, due to errors that Mechain made early on
0:26:49 > 0:26:51in his survey, it's fractionally wrong.
0:26:58 > 0:27:01The errors that Mechain made were pretty much irrelevant
0:27:01 > 0:27:02because for the first time,
0:27:02 > 0:27:05the world had a unit of length
0:27:05 > 0:27:11that was based on something they believed was permanent and unchanging, the Earth.
0:27:14 > 0:27:16There it is. The metre.
0:27:20 > 0:27:22A thing of beauty.
0:27:22 > 0:27:26Not so much the object but the idea it represents.
0:27:29 > 0:27:32This metre bar ushered in the era of metrification.
0:27:32 > 0:27:34And the achievement is immense.
0:27:34 > 0:27:36Even Napoleon, in a moment of humility, admitted that,
0:27:36 > 0:27:39"Conquests come and go, but this work will endure."
0:27:39 > 0:27:41And he was right,
0:27:41 > 0:27:45this lump of metal really represents a change in our thinking.
0:27:45 > 0:27:47For the first time,
0:27:47 > 0:27:51we had measurement based on something fundamental and universal.
0:28:03 > 0:28:05The concept was brilliant,
0:28:05 > 0:28:07but the metre's triumphant arrival
0:28:07 > 0:28:11was not embraced with universal enthusiasm.
0:28:11 > 0:28:14In fact, it took several decades before the metre was finally
0:28:14 > 0:28:18accepted as a standard international unit of measurement.
0:28:23 > 0:28:28It was on a spring day in 1875 that it all became official.
0:28:29 > 0:28:32The historic Metre Convention was signed
0:28:32 > 0:28:35and metre clones sent out around the world.
0:28:37 > 0:28:41It was the beginning of our global system of precision and accuracy.
0:28:41 > 0:28:4617 countries signed the convention to form the BIPM,
0:28:46 > 0:28:49the Bureau International des Poids et Mesures.
0:28:50 > 0:28:55The custodians of international weight and measurement.
0:28:55 > 0:28:58It's a role they still perform today.
0:28:58 > 0:29:02Metrication was to be the basis for a new system of measurement,
0:29:02 > 0:29:06the System Internationale or SI.
0:29:07 > 0:29:10It even led to a new science, metrology,
0:29:10 > 0:29:13the study and refinement of measurement.
0:29:13 > 0:29:18The metre had united the world. At least, in theory.
0:29:31 > 0:29:36Alongside the metre, seismic changes had happened in how we measured time.
0:29:38 > 0:29:40For more than 3,000 years,
0:29:40 > 0:29:44the sundial was the timekeeper of choice across the world.
0:29:45 > 0:29:47But it was not without its problems.
0:29:49 > 0:29:52And the reason is it's just not possible to fix the exact
0:29:52 > 0:29:56length of an hour because the shadow cast on the dial
0:29:56 > 0:29:58alters daily throughout the seasons.
0:30:04 > 0:30:08The Greek astronomer Hipparchus was the first to notice
0:30:08 > 0:30:13the equal length of day and night at the spring and autumn equinoxes
0:30:13 > 0:30:17and that this could give us a standard for setting a fixed length of hour.
0:30:26 > 0:30:31But up until the 14th century, we had no practical way of doing this.
0:30:33 > 0:30:38It took the invention of the mechanical clock to change everything.
0:31:00 > 0:31:04This is the Salisbury Cathedral clock.
0:31:04 > 0:31:11It dates back to 1386 and it's believed to be the oldest surviving mechanical clock in the world.
0:31:14 > 0:31:17For me this is an absolutely staggering achievement,
0:31:17 > 0:31:19I mean, this is the 14th century,
0:31:19 > 0:31:21the medieval time,
0:31:21 > 0:31:25and here a blacksmith and a stonemason have created something
0:31:25 > 0:31:29that is able to regulate time.
0:31:29 > 0:31:31Now, it isn't driven by a pendulum,
0:31:31 > 0:31:34those sort of clocks wouldn't be invented until the 17th century.
0:31:34 > 0:31:37Instead it's these weights at the back
0:31:37 > 0:31:38which are controlling the clock.
0:31:38 > 0:31:43And as the weights fall they unwind the ropes around these barrels.
0:31:47 > 0:31:50It's gravity that drives the clock.
0:31:50 > 0:31:55and all you need to power it is some muscle to raise the weights.
0:32:05 > 0:32:10The intriguing thing is there isn't any clock face on this clock.
0:32:10 > 0:32:12It was already quite an achievement in that time
0:32:12 > 0:32:15just to get that bell to bong every hour.
0:32:23 > 0:32:26BELL RINGS
0:32:32 > 0:32:36By the end of the 14th century many cathedrals across Europe
0:32:36 > 0:32:40had built clock towers, towering up to the heavens, glorifying God,
0:32:40 > 0:32:42but perhaps more importantly,
0:32:42 > 0:32:45controlling the lives of us mere mortals down below.
0:32:45 > 0:32:47The clocks weren't terribly accurate,
0:32:47 > 0:32:50probably the best ones lost 15 minutes a day,
0:32:50 > 0:32:54but they began to irrevocably change people's lives.
0:32:54 > 0:32:56No longer dependent on the sun,
0:32:56 > 0:32:58we were tied to the chimes of man-made clocks.
0:33:03 > 0:33:07In the 15th and 16th centuries, as the mechanisms became more accurate,
0:33:07 > 0:33:12the clock face itself appeared, something we now take for granted.
0:33:13 > 0:33:16It then became possible to break down our day
0:33:16 > 0:33:18into even smaller units.
0:33:19 > 0:33:22For the first time, the hour could be divided
0:33:22 > 0:33:24into minutes and seconds.
0:33:27 > 0:33:30The idea came from the Greek mathematician Ptolemy
0:33:30 > 0:33:36who divided a circle into 360 equal parts called degrees.
0:33:36 > 0:33:40He then split each degree into 60 minutes
0:33:40 > 0:33:43and each minute into 60 second minutes...
0:33:50 > 0:33:53..which gave us the words we use today.
0:33:56 > 0:34:00The relationship between time and length was getting closer.
0:34:01 > 0:34:04We now measured the passage of time by the distance the hand
0:34:04 > 0:34:07travelled around the clock face.
0:34:11 > 0:34:14Mechanical clocks gave us a fixed hour.
0:34:14 > 0:34:18But actually setting them to the right time was still a problem.
0:34:20 > 0:34:23We still looked to the sun and set our clocks and watches to noon
0:34:23 > 0:34:26when the sun was directly overhead.
0:34:26 > 0:34:30But that meant that each town had its own different time.
0:34:30 > 0:34:32For example, here in Salisbury,
0:34:32 > 0:34:36the clocks were over seven minutes later than the clocks in London.
0:34:36 > 0:34:38The reason?
0:34:38 > 0:34:41Well, we're further west here, so the sun arrives overhead later.
0:34:52 > 0:34:56But with the development of steam power in the early 19th century,
0:34:56 > 0:35:00things had to change because it was impossible
0:35:00 > 0:35:02to set busy train timetables
0:35:02 > 0:35:05if every town had its own different time.
0:35:08 > 0:35:11A single national time was urgently needed.
0:35:11 > 0:35:15Under the unswerving leadership of Sir George Airy,
0:35:15 > 0:35:17the Astronomer Royal at the Greenwich Observatory,
0:35:17 > 0:35:20Greenwich time became THE time for Great Britain.
0:35:28 > 0:35:32The railways were the first to switch their entire timetable
0:35:32 > 0:35:34to this new time.
0:35:36 > 0:35:39And they did it by sending the correct time to virtually
0:35:39 > 0:35:43every station in the country by the new telegraph lines
0:35:43 > 0:35:45which often ran alongside the railways.
0:35:50 > 0:35:53Gradually, national and international time
0:35:53 > 0:35:57became essential for business and in 1884,
0:35:57 > 0:36:00Greenwich time was universally adopted as the basis
0:36:00 > 0:36:02for a new system of international time zones.
0:36:06 > 0:36:10The reason for its enthusiastic adoption was because
0:36:10 > 0:36:13the Greenwich Observatory produced the most accurate
0:36:13 > 0:36:17nautical almanacs used by mariners throughout the world.
0:36:18 > 0:36:22And as these almanacs were all set with Greenwich lying on zero degrees of longitude,
0:36:22 > 0:36:24the prime meridian, at a stroke,
0:36:24 > 0:36:28Great Britain became the centre of the world.
0:36:31 > 0:36:33Time was no longer calibrated locally
0:36:33 > 0:36:35by when the sun was at its highest,
0:36:35 > 0:36:39it was set astronomically at Greenwich.
0:36:48 > 0:36:50But while Greenwich time had gone international,
0:36:50 > 0:36:54for most people, actually getting your hands on the correct time
0:36:54 > 0:36:56was still a challenge.
0:36:56 > 0:36:59And for businesses, this was fast becoming a problem.
0:37:01 > 0:37:05And one family realised a cunning way to exploit this need.
0:37:05 > 0:37:08Every week, John Henry Belville would come up the hill
0:37:08 > 0:37:12here to Greenwich and set his chronometer to the correct time.
0:37:15 > 0:37:17And then he'd go back down to London.
0:37:17 > 0:37:20to sell the right time to watchmakers and businesses.
0:37:24 > 0:37:28By the 1940s, thanks to the radio and cheap clocks and watches...
0:37:30 > 0:37:32..we could all run on time.
0:37:35 > 0:37:37Time was money.
0:37:37 > 0:37:40International trade, business and travel were all thriving.
0:37:42 > 0:37:44As the world embraced Greenwich time,
0:37:44 > 0:37:47our journey towards globalisation started.
0:37:55 > 0:37:58While universal time was transforming our world
0:37:58 > 0:38:01the same could not be said for the metre.
0:38:02 > 0:38:0617 countries had enthusiastically signed up to the historic
0:38:06 > 0:38:11metre convention but, in practice, few had enforced it.
0:38:11 > 0:38:14And the muddle of different measurements continued,
0:38:14 > 0:38:18with standards and gauges differing from town to town,
0:38:18 > 0:38:20and even factory to factory,
0:38:20 > 0:38:24which was to have dire consequences, here in the United States.
0:38:28 > 0:38:33When a huge fire ripped through the American city of Baltimore in 1904,
0:38:33 > 0:38:36a disaster of epic proportions was unfolding.
0:38:40 > 0:38:44As fire crews from the nearby cities of Washington
0:38:44 > 0:38:47and New York rushed to the scene, all they could do was sit
0:38:47 > 0:38:49and watch the inferno engulf the city.
0:38:50 > 0:38:55None of their fire hoses would fit Baltimore's fire hydrants.
0:38:55 > 0:38:59Despite being less than 200 miles apart,
0:38:59 > 0:39:02all the fire crews were using different sized equipment.
0:39:02 > 0:39:08The fire raged out of control for two days, destroying 1,500 homes.
0:39:16 > 0:39:20Length measurement needed to be standardised and fast.
0:39:20 > 0:39:22NIST, America's measurement body,
0:39:22 > 0:39:24started campaigning for better standards.
0:39:34 > 0:39:37Spurred on by the NIST campaign,
0:39:37 > 0:39:39American industrialists soon realised
0:39:39 > 0:39:43that they could capitalise on improvements in accuracy.
0:39:44 > 0:39:48Henry Ford started commissioning increasingly accurate gauges and measures.
0:39:54 > 0:39:57Precise and standardised measurement
0:39:57 > 0:39:59meant that mass production was possible.
0:39:59 > 0:40:04At the same time, strict patterns of shift work tied their workforces to the clock.
0:40:10 > 0:40:12It was the dawn of the modern age.
0:40:12 > 0:40:15For the first time, millions of identical parts
0:40:15 > 0:40:18could be produced at rapid speed and minimal cost.
0:40:18 > 0:40:20The American boom was under way.
0:40:20 > 0:40:24And when you see inspectors checking parts for accuracy
0:40:24 > 0:40:27to dimensions measured in 10,000th of an inch,
0:40:27 > 0:40:31you see where quantity production of quality products
0:40:31 > 0:40:35actually begins because parts must fit together perfectly.
0:40:35 > 0:40:39It would provide a profound lesson to the world.
0:40:39 > 0:40:44Precise measurement had the power to change the fortunes of a nation.
0:40:51 > 0:40:55But the problem with any technological breakthrough
0:40:55 > 0:40:57is no-one quite knows where it will lead.
0:40:57 > 0:41:02It took the paranoia of the Cold War and the resulting arms race
0:41:02 > 0:41:06to trigger the next big leap in length measurement.
0:41:06 > 0:41:09And it led us further than we ever thought possible.
0:41:11 > 0:41:16But history and our own conscience will judge us harshly,
0:41:16 > 0:41:21if we do not now make every effort to test our hopes by action.
0:41:22 > 0:41:25The stakes were rising but our level of accuracy
0:41:25 > 0:41:28was failing to keep up with our aspirations.
0:41:30 > 0:41:32Up to the 1960s,
0:41:32 > 0:41:36we could measure with an accuracy of one ten millionth of a metre.
0:41:36 > 0:41:41But an error of this magnitude in the components of a rocket navigation system
0:41:41 > 0:41:45would mean missing the moon by 4,000 miles.
0:41:47 > 0:41:50Now the challenge was to improve the accuracy a hundredfold.
0:41:52 > 0:41:57We choose to go to the moon in this decade and do the other things,
0:41:57 > 0:42:01not because they are easy but because they are hard.
0:42:01 > 0:42:03Because that goal will serve to organise
0:42:03 > 0:42:08and measure the best of our energies and skills.
0:42:09 > 0:42:12The metre bar was no longer accurate enough.
0:42:12 > 0:42:17A new and more precise way of measuring length was needed.
0:42:17 > 0:42:21The answer lay in the fundamental properties of the universe.
0:42:21 > 0:42:24It was the dawn of the quantum age.
0:42:52 > 0:42:54Since the 1870s,
0:42:54 > 0:42:57there had been a growing desire to take measurement away from earthly
0:42:57 > 0:43:01constants like circumference of the globe or the length of the day...
0:43:03 > 0:43:08..and to tie measurement to the fundamental and unchanging laws of nature.
0:43:08 > 0:43:12Things like the speed of light or the charge on a single electron.
0:43:15 > 0:43:18It was a Scottish genius, James Clerk Maxwell,
0:43:18 > 0:43:22who first suggested that these universal constants
0:43:22 > 0:43:25could hold the key to more precise measurement.
0:43:27 > 0:43:31Considered by many to be the 19th century's most influential physicist,
0:43:31 > 0:43:36Maxwell's theories would change the course of measurement history.
0:43:36 > 0:43:38He said at the time,
0:43:38 > 0:43:42"If then we wish to obtain standards which shall be absolutely permanent,
0:43:42 > 0:43:46"we must seek them not in the dimensions or motion of our planet,
0:43:46 > 0:43:49"but in the wavelength, the period of vibration
0:43:49 > 0:43:53"and the absolute mass of these imperishable and unalterable
0:43:53 > 0:43:56"and perfectly similar molecules."
0:43:58 > 0:44:02Maxwell's idea was as revolutionary as the decision a century earlier
0:44:02 > 0:44:07to take length measurement away from the human body and base it on the Earth.
0:44:07 > 0:44:11Maxwell changed the direction of the science of measurement.
0:44:13 > 0:44:18Maxwell, it's hard to overestimate the influence he had
0:44:18 > 0:44:20on scientific thought in the 19th century.
0:44:20 > 0:44:24It was a very influential idea he had and he said,
0:44:24 > 0:44:26"We should be measuring length
0:44:26 > 0:44:29"in terms of the wavelength of a colour of light."
0:44:29 > 0:44:33But even he couldn't figure out how to really do it
0:44:33 > 0:44:38to the accuracy that would be required to replace the, sort of,
0:44:38 > 0:44:40metre definition.
0:44:41 > 0:44:44Maxwell was never able to turn his dream of using
0:44:44 > 0:44:48the wavelength of light to measure distance into reality
0:44:48 > 0:44:51because the technology to achieve it simply didn't exist.
0:44:53 > 0:44:56But his ideas were revolutionary.
0:45:02 > 0:45:06It wasn't until decades later, a scientist at the BIPM,
0:45:06 > 0:45:09the same place where the world's master metre bar is held,
0:45:09 > 0:45:12would start to bring Maxwell's vision to life.
0:45:16 > 0:45:18Albert Michaelson began to design
0:45:18 > 0:45:21and build machines called interferometers
0:45:21 > 0:45:24that could actually measure the wavelength of different light sources.
0:45:28 > 0:45:32So this is one of Michaelson's original interferometers.
0:45:32 > 0:45:35What was he using it for and how did he use it?
0:45:35 > 0:45:39Well, he wanted to demonstrate that it would be possible
0:45:39 > 0:45:44to measure a wavelength of light, because light travels in waves,
0:45:44 > 0:45:46and then in a future time,
0:45:46 > 0:45:51define the metre in terms of this wavelength of light.
0:45:51 > 0:45:54Wavelengths of light are invisible to the human eye.
0:45:54 > 0:45:58Michaelson's genius was realising that when light is split
0:45:58 > 0:46:03and then recombined, it forms a unique pattern called interference
0:46:03 > 0:46:06that can be used to count wavelengths.
0:46:06 > 0:46:10So by counting how many, going from light to dark, light to dark,
0:46:10 > 0:46:12take a metre, divide by the number of those,
0:46:12 > 0:46:14you'll get the wavelength of light,
0:46:14 > 0:46:17- something you can't see with your naked eye.- Right.
0:46:17 > 0:46:21What he had to do was build up from a wavelength of light to a metre.
0:46:21 > 0:46:23And in a half a millimetre,
0:46:23 > 0:46:28- there are about more than 1,000 wavelengths.- Extraordinary.
0:46:32 > 0:46:36It was the breakthrough that was to change the destiny of the metre.
0:46:40 > 0:46:43After over half a century of laborious research,
0:46:43 > 0:46:45scientists were ready.
0:46:45 > 0:46:48Maxwell's dream was about to become a reality.
0:46:48 > 0:46:53On Friday the 14th of October 1960, delegates from across the globe,
0:46:53 > 0:46:57from Russian and America, gathered here in the grounds of the BIPM.
0:46:57 > 0:47:00The fate of the metre was in the balance.
0:47:03 > 0:47:06At six o'clock that evening, to much applause,
0:47:06 > 0:47:10the metre was redefined in terms of the number
0:47:10 > 0:47:13of wavelengths of light emitted by a special krypton lamp.
0:47:14 > 0:47:17Finally, the metre bar was consigned to history.
0:47:17 > 0:47:20But I don't think those French Revolutionaries
0:47:20 > 0:47:25who first came up with the idea of the metre would be too disappointed
0:47:25 > 0:47:27because it was really realising their dream of tying
0:47:27 > 0:47:31the metre to something unchanging and universal.
0:47:37 > 0:47:39Distance could be measured accurately
0:47:39 > 0:47:43using a universal constant, the wavelength of light.
0:47:44 > 0:47:47But how could we put this new science into practice?
0:47:59 > 0:48:03That would need the help of a project codenamed Laser.
0:48:07 > 0:48:11It was the brainchild of Californian Theodore Maiman.
0:48:13 > 0:48:19Well, this device happens to be the original laser.
0:48:20 > 0:48:23The beauty of the laser, is that it is light of a precise,
0:48:23 > 0:48:26fixed wavelength.
0:48:26 > 0:48:28By bouncing this beam off an object,
0:48:28 > 0:48:32and precisely measuring the time it takes to bounce back,
0:48:32 > 0:48:36suddenly we could measure distances with incredible precision.
0:48:36 > 0:48:39Within years, the laser was helping us
0:48:39 > 0:48:42to measure our world in ways we never thought possible.
0:48:42 > 0:48:45And there was no better illustration of this
0:48:45 > 0:48:47than the Apollo 11 lunar landings.
0:49:06 > 0:49:11One small step for man, one giant leap for mankind.
0:49:13 > 0:49:16When Neil Armstrong and Buzz Aldrin
0:49:16 > 0:49:19landed on the Sea of Tranquillity more than 40 years ago
0:49:19 > 0:49:22on the 21st July 1969,
0:49:22 > 0:49:24they left a mirror on the moon's surface.
0:49:30 > 0:49:33When astronomers later fired a laser pulse at it,
0:49:33 > 0:49:36Maiman's invention was also about to make history.
0:49:38 > 0:49:42The beam took 2.5 seconds to reflect back to Earth.
0:49:44 > 0:49:48For the first time, scientists could calculate the distance
0:49:48 > 0:49:50to the moon at any phase of its orbit
0:49:50 > 0:49:52to an accuracy of three centimetres.
0:49:58 > 0:50:01Lasers changed everything.
0:50:01 > 0:50:04They made scientists rethink what was possible.
0:50:04 > 0:50:08We could measure distance with extraordinary precision.
0:50:26 > 0:50:30Distance was tied to a universal, unchanging constant
0:50:30 > 0:50:32but time was not.
0:50:33 > 0:50:36The second was still based on the rotation of the Earth,
0:50:36 > 0:50:39which is actually rather variable.
0:50:46 > 0:50:48Finding a better way of defining time
0:50:48 > 0:50:51was to come from an unexpected quarter.
0:50:56 > 0:51:00Just a few years before that landmark 1960 meeting in Paris,
0:51:00 > 0:51:03an English scientist called Louis Essen
0:51:03 > 0:51:07was working here at the UK's National Physical Laboratory.
0:51:07 > 0:51:10His passion was precision timekeeping,
0:51:10 > 0:51:13and he was beginning work on a new generation of clock,
0:51:13 > 0:51:16the atomic clock.
0:51:17 > 0:51:21We set our quartz clocks to keep time with the rotation of the Earth.
0:51:21 > 0:51:24But for some of our modern problems, this is not quite accurate enough,
0:51:24 > 0:51:28and now we're setting our quartz to keep time
0:51:28 > 0:51:31with the vibrations of the atom.
0:51:33 > 0:51:35The theory was to define time
0:51:35 > 0:51:38through the vibration of individual atoms.
0:51:38 > 0:51:42Across the Atlantic, the Americans, at their national laboratory,
0:51:42 > 0:51:46were already pushing forward with a well-funded programme.
0:51:46 > 0:51:50Back in Britain, Essen was struggling.
0:51:50 > 0:51:53There was little enthusiasm for his clock project
0:51:53 > 0:51:55and funding was always a problem.
0:51:55 > 0:52:00His first experiment imploded, destroying much of his equipment.
0:52:01 > 0:52:04But in a classic story of the underdog winning through,
0:52:04 > 0:52:10Essen eventually created the world's first working atomic clock.
0:52:10 > 0:52:12It was called the Caesium One.
0:52:12 > 0:52:17And it was accurate to 1 second in 300 years.
0:52:17 > 0:52:21The second was no longer based on the movement of our planet.
0:52:21 > 0:52:26Time was now locked to the beating heart of a caesium atom.
0:52:26 > 0:52:31A movement that was unchanging and fundamental across the universe.
0:52:37 > 0:52:43In Britain, the latest incarnation of Essen's atomic clock is the CSF2.
0:52:45 > 0:52:49It's one of a global network of atomic clocks that sets our time.
0:52:53 > 0:52:55To most people this doesn't look like a clock at all,
0:52:55 > 0:52:57so how does it actually measure time?
0:52:57 > 0:53:00Well, what we're doing here is using lasers
0:53:00 > 0:53:02to slow down the caesium atoms.
0:53:02 > 0:53:04We form a cloud of very slowly moving caesium atoms
0:53:04 > 0:53:07and we use the lasers to throw that cloud upwards through
0:53:07 > 0:53:09an enclosure containing microwaves.
0:53:09 > 0:53:12Then they fall back through it a second time under gravity.
0:53:12 > 0:53:15When the atoms change from one energy level to another,
0:53:15 > 0:53:18they emit or absorb one very precise frequency,
0:53:18 > 0:53:21and we can use that frequency to keep track of time.
0:53:21 > 0:53:23We simply count up the oscillations.
0:53:26 > 0:53:30So it's the number of oscillations that will define the length of a second.
0:53:30 > 0:53:33Those oscillations are a particular property of that caesium atom.
0:53:33 > 0:53:35That's right, yes.
0:53:35 > 0:53:38So, any caesium atom always has the same number
0:53:38 > 0:53:40of oscillations per second.
0:53:42 > 0:53:47The oscillations of these caesium atoms are the ticking of the clock.
0:53:47 > 0:53:54and they give the CSF2 accuracy to 1 second in 138 million years.
0:53:57 > 0:54:01It's a degree of precision our ancestors could never have imagined.
0:54:03 > 0:54:06The genius of Maxwell, Michaelson
0:54:06 > 0:54:09and Essen now touch every part of our lives.
0:54:16 > 0:54:19They could never have guessed their work would one day
0:54:19 > 0:54:22be at the centre of everything from our banking systems to phones,
0:54:22 > 0:54:25GPS and the internet.
0:54:27 > 0:54:31These only exist because of the accuracy of atomic clocks
0:54:31 > 0:54:35and their ability to synchronise time across the planet.
0:54:36 > 0:54:40Measurement has taken us in directions we could never have dreamt possible.
0:54:50 > 0:54:53But the story doesn't end there.
0:54:53 > 0:54:57In one last twist, scientists looked at the metre again...
0:54:58 > 0:55:02..and realised that they could now redefine length
0:55:02 > 0:55:04using the new accuracy of the second.
0:55:09 > 0:55:14It was 1983 and in a collaboration between different measurement labs
0:55:14 > 0:55:18across the world, atomic clocks measured the speed of light
0:55:18 > 0:55:20with incredible precision.
0:55:22 > 0:55:25The metre could finally be defined by how far light
0:55:25 > 0:55:29travels in a tiny fraction of a second.
0:55:29 > 0:55:32Time and length were intimately intertwined.
0:55:41 > 0:55:43We've come a long way since the days of the pharaohs,
0:55:43 > 0:55:46when time was defined by the length of a shadow.
0:55:46 > 0:55:52After 3,000 years, time and distance are once again linked,
0:55:52 > 0:55:55joined together by one of the most fundamental
0:55:55 > 0:55:59and universal constants of nature, the speed of light.
0:56:25 > 0:56:28Despite all the great advances in time and length measurement,
0:56:28 > 0:56:31the quest is still on.
0:56:31 > 0:56:34Scientists are trying to create ever more accurate clocks.
0:56:34 > 0:56:36Clocks that will only lose one second
0:56:36 > 0:56:38in the lifetime of the universe.
0:56:40 > 0:56:44And once they're deployed we can only begin to imagine how it's going to change our world.
0:56:44 > 0:56:48Instant communication, quantum computers,
0:56:48 > 0:56:50planes that can land themselves.
0:56:50 > 0:56:53Science fiction will become a reality.
0:56:53 > 0:56:55And that's the beauty of measurement.
0:56:55 > 0:56:59Every leap in precision, from the cubit rod to the atomic clock,
0:56:59 > 0:57:01has led to a technological revolution.
0:57:04 > 0:57:08Through history measurement has changed every aspect of our lives...
0:57:10 > 0:57:13..splitting the year into seasons and lunar cycles
0:57:13 > 0:57:16allowed man to plan ahead for the first time
0:57:16 > 0:57:19and gain advantage over the rest of nature.
0:57:22 > 0:57:25Dividing the day still further into 24 hours
0:57:25 > 0:57:28was the bedrock for civilisation.
0:57:28 > 0:57:31The fixed hour controlled the working day.
0:57:35 > 0:57:37And uniform national and international time
0:57:37 > 0:57:40allowed the globalisation of industry.
0:57:41 > 0:57:43The world would never be the same.
0:57:47 > 0:57:51The story of measurement has shaped and changed our history.
0:57:51 > 0:57:54And will continue to do so as we delve deeper in to the atomic
0:57:54 > 0:57:58fabric of the universe in search of greater precision.
0:58:03 > 0:58:09Next time, I meet the biggest problem in measurement, the kilogram.
0:58:09 > 0:58:14This 19th-century artefact is the world's master kilo,
0:58:14 > 0:58:16and it's losing weight.
0:58:17 > 0:58:20Now, a head-to-head race is on to replace it...
0:58:22 > 0:58:25..as the best minds in measurement science fight it out,
0:58:25 > 0:58:27there can only be one winner.