- 19 hours ago
Category
📺
TVTranscript
00:00In 1852, clockmaker Edward Dent set out to construct the largest and most accurate public clock in the world.
00:11It took seven years to build.
00:15A testament to a very human need.
00:20Our modern day lives are completely driven by precise measurement.
00:24Take Big Ben, for over 150 years it's been ringing out the correct time to the people of London.
00:30When built, it was an engineering marvel, accurate to an incredible one second an hour.
00:37But times have changed.
00:42Today we can build clocks which lose one second in 138 million years.
00:49And now there are plans for a clock accurate to within one second over the lifetime of the universe.
00:57What is it that drives us to such extremes of ever greater precision?
01:02Why do we feel the need to quantify and measure, to impose order on the world around us?
01:09Since our ancestors first began to count the passing of the seasons, successive civilizations have used measurement to help master the world around them.
01:19It's taken us to the moon and split the atom and it fascinates me.
01:26Ever since I was young, I've been obsessed with measuring things, trying to make sense of the world around me.
01:33Where do those measurements come from?
01:37I mean, who decided that a kilo was a kilo, and a second a second?
01:42What we measure, how we measure it, and how accurately we can measure it, are surprisingly complex questions.
01:50Questions which have obsessed generations of great minds, and created a system that describes everything in our world with just seven fundamental units of measurement.
02:02And the quest to define those seven units with ever greater precision has changed our world.
02:22In this series, I want to explore why we measure.
02:26What drives us to try and reduce the chaos and complexity of the world to just a handful of elementary units?
02:34In this first programme, I'm going to be looking at two of the most fundamental measurements, namely the metre and the second.
02:41It's likely that time and distance were the first things people ever tried to measure.
02:48They seem closely linked in our minds.
02:51We even talk about length of time.
02:57And as we'll see, time and distance are inextricably connected by modern science.
03:04Being able to measure time actually means spotting patterns.
03:08And that's actually a very mathematical way of looking at the world.
03:11In fact, measuring time is an incredibly sophisticated act.
03:16So where did it all begin?
03:19Our ancestors would have first picked up on the patterns of the seasons.
03:26Marking time as the leaves turned brown, or the days got shorter,
03:31when rivers flooded, or berries ripened.
03:34These very practical observations would have helped them in the daily struggle to survive.
03:43Another first example of humans' attempts to measure was discovered here in southern France
03:48by four teenagers and their dog called Robot.
03:52It was 1940, and the 18-year-old Marcel Ravida was exploring these woods,
03:58when he came across a hole where a tree had been uprooted by a storm.
04:03He needed some tools to make the hole bigger.
04:05So he came back four days later with his three friends,
04:09and they uncovered the entrance to a huge system of unexplored caves.
04:15But what they discovered inside was even more exciting.
04:17The boys must have been absolutely staggered to come in here and see these images painted on the wall.
04:36I mean, these are some of the oldest cave paintings.
04:43Oh, look at this! It's all over the wall.
04:46Marcel and his friends had discovered some of the earliest cave paintings ever found.
05:00These date back 17,000 years and were painted by Cro-Magnon Mare.
05:07It's beautiful.
05:08You can really feel the energy of these animals kind of rushing across the walls.
05:21This cave is a replica of the original, which is a few hundred metres from here,
05:26and is now carefully preserved.
05:28Dr Michael Rappengluck believes these paintings are evidence of man's first attempt to measure time.
05:41This one is very, very beautiful.
05:45To him, this is a giant calendar.
05:49The clues lie in these strange patterns of dots.
05:53Each dot represents a week.
05:5713 dots represent one quarter of a year.
06:00His theory is that each seven-day phase of the moon,
06:05what today we'd call a week,
06:07is marked with a dot on the wall to chart the passing of time.
06:18It was a distinctively shaped cluster of dots
06:20that eventually allowed him to unlock the full meaning of the paintings.
06:27Look up to the ceiling.
06:29You see six dots.
06:31It reminds a little dipper.
06:34And I think this is the star pattern of the Pleiades.
06:37Right, so these dots are not representing weeks anymore.
06:39These are stars up there.
06:41Yes, these are stars and they serve to start the year.
06:46When our ancestors saw the stars form this same alignment in the sky,
06:54it would mark the start of their year.
06:58Dr Rappengluck believes the animals have meaning too.
07:02The stack represents autumn equinox and it's starting a time cycle to the horse.
07:09The horse represents springtime.
07:12And you see the horse is pregnant, highly pregnant.
07:16So three quarters of a year are represented on the wall.
07:19So it's the star calendar, followed by the calendar marking the weeks,
07:24that allows them to know when the stags are rutting.
07:27Yes. Or the pregnant animals.
07:28They synchronised biological rhythms of animals with astronomical rhythms.
07:33It's an extraordinarily sophisticated system.
07:36Yes.
07:3717,000 years ago.
07:38It is. It is.
07:39Amazing.
07:45With the aid of this basic calendar, for the first time,
07:49our ancestors could start to predict what would happen and when.
07:52They could prepare to hunt when animals migrated close by,
07:57or as agriculture developed, determine the best time to plant crops.
08:02Measurement was making life easier.
08:06But as communities grew, so did the need for more precise time keeping,
08:19beyond the cycles of the moon, the stars and the seasons.
08:2213,000 years after our ancestors painted the caves in Lascaux,
08:28first the Mesopotamians and then the Egyptians started to tackle the problem
08:32of dividing up the day.
08:34And they took their inspiration from the sun.
08:43By observing how the length of a shadow changed through the day,
08:47they found an easy way to measure time.
08:50And they used a device just like this.
08:55This is a replica of an ancient Egyptian sundial,
08:59and it's one of the first instruments ever created to measure time.
09:03Now, at midday, this stone here would have cast no shadow.
09:07But as the day went on, the shadow would get longer and longer.
09:11So the ancient Egyptians decided to divide the day up into 12 units.
09:15And you can see the lines here.
09:18We've got one, two, three, four, five, six lines for the afternoon
09:22and six for the morning.
09:24It's just coming up to three o'clock.
09:27By linking time and distance,
09:30they could reliably measure shorter periods of time,
09:33telling the time by measuring the length of a shadow.
09:36Although the sundial was a brilliant invention, it was fundamentally flawed.
09:51It didn't work at night.
09:55Like the cavemen of Lascaux, who used stars to mark the seasons,
10:04the Egyptians went one step further.
10:07They used them to divide up the hours of darkness.
10:11But on a cloudy night, just as on a cloudy day,
10:14they still had no way of telling the time.
10:16And this is where they made a conceptual leap.
10:24This is a water clock.
10:29It's a very simple idea.
10:31Basically, what they did was to take a bucket and make a hole in the bottom.
10:35Then, as night fell, they would fill the bucket with water.
10:46Now, as the water drips out,
10:49they can use lines marked on the side of the bucket
10:52to tell how much time has passed through the night.
10:58They could measure 12 hours independently of the sun or the stars.
11:03But why count 12 hours at all?
11:11The answer lies in how business was done thousands of years ago.
11:16Throughout the Middle East,
11:18the number 12 and the number 60 were important in commerce.
11:22They're numbers that were familiar to traders in markets just like this.
11:27And the reason they used them is all to do with arithmetic.
11:33As a mathematician, I love the answer
11:36because it's about the mathematical properties of these two numbers.
11:39They're highly divisible.
11:49Take the number 60.
11:51I can divide 60 beans into 6 groups of 10 beans.
11:54Five groups of 12 beans.
11:58Four groups of 15 beans.
12:01Three groups of 20 beans.
12:05Five there.
12:07Two groups of 30 beans.
12:09Or one group of 60 beans.
12:12One group of 60 beans.
12:16But take 100 beans, how can I divide that?
12:19I can divide it into two groups of 50,
12:21but divide by three, I've got to start cutting a bean.
12:24Because the numbers 12 and 60 were so familiar to Egyptians,
12:29it was perhaps no great conceptual leap for them to come up with a 12-hour night and day.
12:33So the idea stuck.
12:34It wasn't just the measurement of time that the Egyptians needed to tackle.
12:48They also needed to find better ways to measure distance.
12:53Every year, the Nile would flood, bringing great fertility to the land.
13:00But with each flood, the borders of the farmer's land would be washed away.
13:05So when the waters receded, an accurate way of measuring field size
13:10and re-establishing boundaries was critical.
13:13They needed a reliable and uniform measure of length.
13:17And their solution was this.
13:19It's a cubit rod and it's the Egyptian equivalent of a ruler.
13:25Its length was the distance of the Pharaoh's cubit,
13:28which was the length from his elbow to the tip of his middle finger.
13:32So actually my cubit is slightly shorter than the Pharaoh's.
13:36But this led to the Egyptians creating some of the most remarkable buildings
13:41the world has ever seen.
13:42This is the Great Pyramid of Cheops, built over 4,500 years ago,
13:52for the fourth dynasty Pharaoh Khufu.
13:57It's said 20,000 men took 20 years to build it,
14:01using over 2 million limestone blocks,
14:04all meticulously aligned and measured with the cubit rod.
14:08This is a miraculous building.
14:11The length of the side is 440 cubit exactly.
14:16Exactly? Wow.
14:18And the height is 280 cubits exactly.
14:21Also, it's very square.
14:23It has perfection in every part of it.
14:25Absolutely. And with so many people working on it,
14:28spread over, I guess, a large area in a large amount of time,
14:31I mean, actually having a standard unit of measurement must have been absolutely essential.
14:36Exactly. They had a rope, which is a hundred times this,
14:40that has knots in it every one cubit or every ten cubits, which is called chet.
14:46OK, we want to measure 440, so we need to take the cornerstone as our starter.
14:51Yes, exactly.
14:53And so, have you start measuring?
14:56Yes.
14:57The original cornerstones are no longer visible,
15:01but the foundations are still here for all to see.
15:04I think I chose the easy job.
15:16430.
15:19Wow.
15:21440 cubits, pretty much on the knot.
15:23Exactly.
15:24What's so remarkable about the Egyptian system is that they were one of the first to standardise length measurement.
15:42It's said that every full moon, the surveyors across the land would gather and compare their wooden cubit rod
15:48against the royal master cubit. Made of granite, this was held by the royal surveyor.
15:55Failure to maintain an accurate cubit was punishable by death.
16:00It was a very simple and efficient way to standardise length measurement across the land,
16:05and it enabled the Egyptians to measure things with phenomenal accuracy.
16:09Mastering and standardising time and length measurement was really key to the success of the ancient Egyptian empire.
16:22The power of measurement is that it created order out of chaos and allowed civilisation to flourish.
16:37The standardisation of measurement which began here in Egypt several millennia ago is now central to all our lives.
16:56Nearly every country in the world has a national measurement body whose master lengths and weights are calibrated by one international body.
17:10A little bit like the modern day pharaohs trying to bring standardisation and measurement across the globe.
17:16But despite the obvious logic of having one international system, it hasn't been completely embraced.
17:25Take me for example, I'm going to the airport in this cab which measures its speed in kilometres per hour and miles per hour.
17:33When I'm up in the air, they'll be measuring their altitude in feet, and my clothes are measured in inches, and my shoes are measured in...
17:42Well, frankly, I've never quite understood what the unit of measurement for shoe size is.
17:46Shoe sizes aside, standardisation of measurement underpins all modern science, though the road to standardisation has not been an easy one.
17:59Shoe sizes aside, standardisation of measurement underpins all modern science, which is an easy one.
18:14Throughout history, rulers had a nasty habit of ripping up measurement systems
18:18and demanding that they be replaced by lengths based on their own body parts.
18:30In 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.
18:38But as each new reign came in, so things changed.
18:42Henry VII, he defined a yard as the length of his arm.
18:45Elizabeth I, well, not to be outdone by her male predecessors, added a few more inches.
18:53And so the chaos continued.
19:09Lack of standardisation was a problem on the continent too.
19:12If you thought the British had it bad, then spare a thought for the French.
19:18On the eve of the French Revolution, the Ancien Regime had over 250,000 different weights and measures, including several thousand for length.
19:26By the end of the 18th century, people realised that something needed to be done.
19:41Trade was impossible and open to fraud.
19:43Navigation was treacherous.
19:45And building plans made by an architect in one city couldn't be reproduced in the other,
19:48because they didn't have the same measurements.
19:49The mess was finally sorted out by the French Academy of Sciences.
20:04It was the last few days of the French monarchy, and buoyed by the revolutionary spirit of the time,
20:10sense of egalite and rationalism, France's best scientists decided to form a groundbreaking revolutionary plan of their own.
20:18No longer would measurement be based on the human body or the vanity of kings and queens.
20:23They decided that it should be based on something permanent and unchanging.
20:29They chose the Earth.
20:30It's really exciting to be here. This is really one of the great scientific centres in the whole of the world.
20:56And this is where the modern story of measurement really began,
21:01where a new standardised unit of length was introduced,
21:05one that is familiar to us all today.
21:09On the 26th of March 1791, the Academy here decided to call this new length measurement the metre.
21:16Named after the Greek word metron, meaning measure,
21:20they decided that it should be one ten millionth of the distance between the North Pole and the equator.
21:26It was very clever. The Academy knew that a French colloquial measure would never be accepted by the rest of the world.
21:35By basing the metre on the planet itself, no one country could argue for their own measure.
21:42They had transcended the politics of nations.
21:45This is a system for all people for all time, announced the revolutionary government.
21:52There was one problem though.
21:55Nobody knew accurately what the distance between the North Pole and the equator actually was.
21:59Getting an accurate figure would mean embarking on the most ambitious and complex large-scale measurement project ever attempted.
22:12Two scientists were tasked with turning the theory into reality.
22:19They were Pierre Méchain and Jean-Baptiste Delambre.
22:23Their task was to measure the distance between two points on a meridian or line of longitude.
22:32Then, using fairly simple mathematics and knowing the latitude of each point,
22:37they could extrapolate and calculate the distance from the pole to the equator.
22:48This experiment would be difficult enough under normal conditions.
22:53But France was in the middle of a revolution.
22:55It was a dangerous time to have big ideas that were not necessarily easy for the new order to understand.
23:08Nevertheless, undaunted, the scientists pushed ahead.
23:16It was here in 1793, from this bell tower in Dunkirk,
23:20Jean-Baptiste Delambre started the northernmost part of his epic quest to measure the earth.
23:33While 800 miles to the south, Barcelona was chosen for Pierre Méchain.
23:41Their plan was to work towards each other and meet in Rodez in southern France.
23:51You can imagine Delambre's excitement as he stood up here 200 years ago,
23:56ready to start his journey.
23:58A journey that would take him seven years to complete.
24:03And the rather splendid piece of equipment they used was this.
24:08A repeating circle.
24:10A device that measures angles extremely accurately,
24:14and as good today as the day it was made.
24:16Now, obviously, Delambre wouldn't measure every distance from here to Barcelona.
24:21But what he can do is use a method called triangulation.
24:24So the first point of the triangle is the top of this belfry.
24:28Then Delambre would have looked across the countryside trying to find two high points.
24:32And he would use this piece of equipment to line up the telescopes on those two other points.
24:37Then all he had to do was measure the angle between the two points
24:40and measure the distance to the closest one.
24:46By then moving to the next high point and measuring the angles again,
24:50simple geometry gave him the distances between all three.
24:53So it's an amazing principle because just one measurement of distance,
25:01and then this triangles all the way to Barcelona.
25:03Delambre had a number of close scrapes along the way.
25:15He was arrested several times, accused of being a spy.
25:20Why else would he be scaling towers carrying strange equipment?
25:24He tried to explain that he was measuring the size of the earth for the Academy of Sciences.
25:33But a drunk militiaman interrupted.
25:36There is no more Academy.
25:38We're all equal now.
25:40You'll come with us.
25:41But in general, they were literally above it all.
25:51On rooftops, towers and church spires, they carried out their quest.
25:56It was an extraordinary feat.
26:00Seven long years later, the two men had measured the exact distance between Dunkerque and Barcelona.
26:07Now the metre was just a simple calculation.
26:28The result of all Méchain and Delambre's hard labour, the prototype metre bar,
26:33is held here at the French National Archives in Paris.
26:38Made in 1799 of pure platinum,
26:41it's meant to represent one ten millionth of the distance between the North Pole and the equator.
26:47In fact, due to errors that Méchain made early on in his survey, it's fractionally wrong.
26:51The errors that Méchain 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.
27:10The earth.
27:11There it is, the metre.
27:12A thing of beauty.
27:13Not so much the object, but the idea it represents.
27:26There it is.
27:29This metre bar ushered in the era of metrification.
27:32The achievement is immense.
27:34Even Napoleon, in a moment of humility, admitted that conquest come and go, but this work will endure.
27:40And he was right.
27:41This lump of metal really represents a change in our thinking.
27:46For the first time we had measurement based on something fundamental and universal.
27:51The concept was brilliant, but the metre's triumphant arrival was not embraced with universal enthusiasm.
28:10In fact, it took several decades before the metre was finally accepted as a standard international unit of measurement.
28:23It was on a spring day in 1875 that it all became official.
28:29The historic metre convention was signed and metre clones sent out around the world.
28:35It was the beginning of our global system of precision and accuracy.
28:4217 countries signed the convention to form the BIPM, the Bureau Internationale des Poids et Mesures.
28:51The custodians of international weight and measurement.
28:55It's a role they still perform today.
28:58Metrication was to be the basis for a new system of measurement.
29:02The System Internationale, or SI.
29:07It even led to a new science, metrology, the study and refinement of measurement.
29:13The metre had united the world, at least in theory.
29:18Alongside the metre, seismic changes had happened in how we measured time.
29:39For more than 3,000 years, the sundial was the timekeeper of choice across the world.
29:44But it was not without its problems.
29:50And the reason is, it's just not possible to fix the exact length of an hour,
29:54because the shadow cast on the dial alters daily throughout the seasons.
29:58The Greek astronomer Hipparchus was the first to notice the equal length of day and night
30:11at the spring and autumn equinoxes,
30:14and that this could give us a standard for setting a fixed length of hour.
30:17But up until the 14th century, we had no practical way of doing this.
30:33It took the invention of the mechanical clock to change everything.
30:37This is the Salisbury Cathedral clock, and it dates back to 1386,
30:55and it's believed to be the oldest surviving mechanical clock in the world.
31:12For me, this is an absolutely staggering achievement.
31:16I mean, this is the 14th century, the medieval time.
31:22And here, a blacksmith and a stonemason have created something that is able to regulate time.
31:29Now, it isn't driven by a pendulum.
31:32Those sort of clocks wouldn't be invented until the 17th century.
31:35Instead, it's these weights at the back which are controlling the clock.
31:38And as the weights fall, they unwind the ropes around these barrels.
31:48It's gravity that drives the clock.
31:51And all you need to power it is some muscle to raise the weights.
31:55The intriguing thing is, there isn't any clock face on this clock.
32:11It was already quite an achievement in that time just to get that bell to bong every hour.
32:25By the end of the 14th century, many cathedrals across Europe had built clock towers, towering up to the heavens, glorifying God,
32:41but perhaps more importantly, controlling the lives of us mere mortals down below.
32:46The clocks weren't terribly accurate.
32:48Probably the best ones lost 15 minutes a day.
32:50But they began to irrevocably change people's lives.
32:54No longer dependent on the sun, we were tied to the chimes of man-made clocks.
33:03In the 15th and 16th centuries, as the mechanisms became more accurate,
33:08the clock face itself appeared, something we now take for granted.
33:13It then became possible to break down our day into even smaller units.
33:18For the first time, the hour could be divided into minutes and seconds.
33:27The idea came from the Greek mathematician Ptolemy,
33:31who divided a circle into 360 equal parts called degrees.
33:36He then split each degree into 60 minutes,
33:40and each minute into 60 second minutes.
33:43Which gave us the words we use today.
33:54The relationship between time and length was getting closer.
34:00We now measured the passage of time by the distance the hand travelled around the clock face.
34:06Mechanical clocks gave us a fixed hour, but actually setting them to the right time was still a problem.
34:20We still looked at the sun and set our clocks and watches to noon when the sun was directly overhead.
34:25But that meant that each town had its own different time.
34:30For example, here in Salisbury, the clocks were over seven minutes later than the clocks in London.
34:36The reason? Well, we're further west here, so the sun arrives overhead later.
34:40But with the development of steam power in the early 19th century, things had to change.
34:58Because it was impossible to set busy train timetables if every town had its own different time.
35:08A single national time was urgently needed.
35:12Under the unswerving leadership of Sir George Eyrie, the Astronomer Royal at the Greenwich Observatory,
35:18Greenwich time became the time for Great Britain.
35:20The railways were the first to switch their entire timetable to this new time.
35:37And they did it by sending the correct time to virtually every station in the country
35:42by the new telegraph lines, which often ran alongside the railways.
35:51Gradually, national and international time became essential for business.
35:56And in 1884, Greenwich time was universally adopted as the basis for a new system of international time zones.
36:07The reason for its enthusiastic adoption was because the Greenwich Observatory
36:12produced the most accurate nautical almanacs used by mariners throughout the world.
36:16And as these almanacs were all set with Greenwich lying on zero degrees of longitude,
36:23the prime meridian, at a stroke, Great Britain became the centre of the world.
36:31Time was no longer calibrated locally by when the sun was at its highest.
36:36It was set astronomically at Greenwich.
36:38But while Greenwich time had gone international, for most people, actually getting your hands on the correct time was still a challenge.
36:56And for businesses, this was fast becoming a problem.
36:59And one family realised a cunning way to exploit this need.
37:05Every week, John Henry Belleville would come up the hill here to Greenwich
37:10and set his chronometer to the correct time.
37:15And then he'd go back down to London to sell the right time to watchmakers and businesses.
37:20By the 1940s, thanks to the radio and cheap clocks and watches, we could all run on time.
37:35Time was money.
37:37International trade, business and travel were all thriving.
37:40As the world embraced Greenwich time, our journey towards globalisation started.
37:55While universal time was transforming our world, the same could not be said for the metre.
38:0217 countries had enthusiastically signed up to the historic metre convention.
38:08But in practice, few had enforced it.
38:12And the muddle of different measurements continued,
38:15with standards and gauges differing from town to town and even factory to factory,
38:20which was to have dire consequences here in the United States.
38:29When a huge fire ripped through the American city of Baltimore in 1904,
38:33a disaster of epic proportions was unfolding.
38:36As fire crews from the nearby cities of Washington and New York rushed to the scene,
38:45all they could do was sit and watch the inferno engulf the city.
38:50None of their fire hoses would fit Baltimore's fire hydrants.
38:54Despite being less than 200 miles apart, all the fire crews were using different sized equipment.
39:02The fire raged out of control for two days, destroying 1,500 homes.
39:07Length measurement needed to be standardized and fast.
39:20NIST, America's measurement body, started campaigning for better standards.
39:24Spurred on by the NIST campaign, American industrialists soon realized that they could capitalize on improvements in accuracy.
39:42Henry Ford started commissioning increasingly accurate gauges and measures.
39:54Precise and standardized measurement meant that mass production was possible.
39:58At the same time, strict patterns of shift work tied their workforces to the clock.
40:04It was the dawn of the modern age. For the first time, millions of identical parts could be produced at rapid speed and minimal cost.
40:18The American boom was underway.
40:21And when you see inspectors checking parts for accuracy to dimensions measured in ten thousandths of an inch,
40:27you see where quantity production of quality products actually begins.
40:32Because parts must fit together perfectly.
40:36It would provide a profound lesson to the world.
40:40Precise measurement had the power to change the fortunes of a nation.
40:48But the problem with any technological breakthrough is no one quite knows where it will lead.
40:58It took the paranoia of the Cold War and the resulting arms race to trigger the next big leap in length measurement.
41:06And it led us further than we ever thought possible.
41:09But history and our own conscience will judge us harshly if we do not now make every effort to test our hopes by action.
41:22The stakes were rising, but our level of accuracy was failing to keep up with our aspirations.
41:28Up to the 1960s, we could measure with an accuracy of one ten millionth of a meter.
41:36But an error of this magnitude in the components of a rocket navigation system would mean missing the moon by four thousand miles.
41:46Now the challenge was to improve the accuracy one hundredfold.
41:50We choose to go to the moon in this decade and do the other things.
41:57Not because they are easy, but because they are hard.
42:01Because that goal will serve to organise and measure the best of our energies and skills.
42:08The metre bar was no longer accurate enough.
42:13A new and more precise way of measuring length was needed.
42:17And the answer lay in the fundamental properties of the universe.
42:22It was the dawn of the quantum page.
42:24A new and more precise age.
42:43Since the 1870s, there have been a growing desire to take measurement away from earthly
42:57constants like the circumference of the globe or the length of the day, and to tie measurement
43:05to the fundamental and unchanging laws of nature, things like the speed of light or
43:11the charge on a single electron.
43:16It was a Scottish genius, James Clerk Maxwell, who first suggested that these universal constants
43:22could hold the key to more precise measurements.
43:27Considered by many to be the 19th century's most influential physicist, Maxwell's theories
43:33would change the course of measurement history.
43:36He said at the time, if then we wish to obtain standards which shall be absolutely permanent,
43:43we must seek them not in the dimensions or the motion of our planet, but in the wavelength,
43:48the period of vibration and the absolute mass of these imperishable and unalterable and perfectly
43:55similar molecules.
43:56Maxwell's idea was as revolutionary as the decision a century earlier to take length measurement
44:04away from the human body and base it on the earth.
44:08Maxwell changed the direction of the science of measurement.
44:12Maxwell, it's hard to overestimate the influence he had on scientific thought in the 19th century.
44:20It was a very influential idea he had and he said, well, we should be measuring length in terms
44:27of the wavelength of a color of light.
44:30But even he couldn't figure out how to really do it to the accuracy that would be required
44:36to replace the sort of old meter definition.
44:40Maxwell was never able to turn his dream of using the wavelength of light to measure distance
44:47into reality because the technology to achieve it simply didn't exist.
44:54But his ideas were revolutionary.
45:02It wasn't until decades later, a scientist at the BIPM, the same place where the world's
45:08master meter bar is held, would start to bring Maxwell's vision to life.
45:17Albert Michelson began to design and build machines called interferometers that would actually
45:22measure the wavelength of different light sources.
45:29So this is one of Michelson's original interferometers.
45:32So what was he using it for and how did he use it?
45:36Well, he wanted to demonstrate that it would be possible to measure a wavelength of light,
45:42like light travels in waves, and then in a future time define the meter in terms of this
45:49wavelength of light.
45:52Wavelengths of light are invisible to the human eye.
45:55Michelson's genius was realizing that when light is split and then recombined, it forms
46:01a unique pattern called interference that can be used to count wavelengths.
46:07So by counting how many, going from light to dark, light to dark, take a meter, divide
46:12by the number of those, you'll get the wavelength of light, something that you can't see with
46:16your naked eye.
46:17Right.
46:18What we do is build up from a wavelength of light to a meter, and in a half a millimetre,
46:23there are about more than a thousand wavelengths of the extraordinary.
46:32It was the breakthrough that was to change the destiny of the meter.
46:41After over half a century of laborious research, scientists were ready.
46:45Maxwell's dream was about to become a reality.
46:48On Friday 14th October 1960, delegates from across the globe, from Russia and America, gathered
46:55here in the grounds of the BIPM.
46:58The fate of the meter was in the balance.
47:04At six o'clock that evening, to much applause, the meter was redefined in terms of the number
47:10of wavelengths of light emitted by a special Krypton lamp.
47:14Finally, the meter bar was consigned to history.
47:19But I don't think those French revolutionaries who first came up with the idea of the meter
47:23would be too disappointed, because it was really realizing their dream of tying the meter
47:28to something unchanging and universal.
47:38Distance could be measured accurately using a universal constant, the wavelength of light.
47:45But how could we put this new science into practice?
47:58That would need the help of a project codenamed laser.
48:08It was the brainchild of Californian Theodore Mayman.
48:14Well, this device happens to be the original laser.
48:21The beauty of the laser is that it is light of a precise, fixed wavelength.
48:26By bouncing this beam off an object, and precisely measuring the time it takes to bounce back,
48:32suddenly we could measure distances with incredible precision.
48:37Within years, the laser was helping us to measure our world in ways we never thought possible.
48:42And there was no better illustration of this than the Apollo 11 lunar landings.
48:47One small step for man, one giant leap for mankind.
49:11When Neil Armstrong and Buzz Aldrin landed on the Sea of Tranquility more than 40 years ago,
49:19on the 21st of July in 1969, they left a mirror on the Moon's surface.
49:26When astronomers later fired a laser pulse at it,
49:33Mayman's invention was also about to make history.
49:38The beam took just 2.5 seconds to reflect back to Earth.
49:45For the first time, scientists could calculate the distance to the Moon at any phase of its
49:50orbit to an accuracy of three centimetres.
49:55Lasers changed everything.
50:02They made scientists rethink what was possible.
50:05We could measure distance with extraordinary precision.
50:08Distance was tied to a universal, unchanging constant, but time was not.
50:33The second was still based on the rotation of the Earth,
50:37which is actually rather variable.
50:46Finding a better way of defining time was to come from an unexpected quarter.
50:56Just a few years before that landmark 1960 meeting in Paris,
51:01an English scientist called Louis Essen was working here at the UK's National Physical Laboratory.
51:07His passion was precision timekeeping,
51:10and he was beginning work on a new generation of clock, the atomic clock.
51:16We've set our quartz clocks to keep time with the rotation of the Earth.
51:20But for some of our modern problems, this is not quite accurate enough.
51:24And now we're setting our quartz to keep time with the vibrations of the atom.
51:32The theory was to define time through the vibration of individual atoms.
51:37Across the Atlantic, the Americans at their national laboratory were already pushing forward with a well-funded program.
51:47Back in Britain, Essen was struggling.
51:50There was little enthusiasm for his clock project, and funding was always a problem.
51:55His first experiment imploded, destroying much of his equipment.
52:00But in a classic story of the underdog winning through,
52:04Essen eventually created the world's first working atomic clock.
52:09It was called the Caesium-1, and it was accurate to one second in 300 years.
52:16The second was no longer based on the movement of our planet.
52:21Time was now locked to the beating heart of a Caesium atom.
52:26A movement that was unchanging and fundamental across the universe.
52:37In Britain, the latest incarnation of Essen's atomic clock is the CSF-2.
52:45It's one of a global network of atomic clocks that sets our time.
52:53To most people, this doesn't look like a clock at all.
52:55So how does it actually measure time?
52:58Well, what we're doing here is using lasers to slow down the Caesium atoms.
53:02We form a cloud of very slowly moving Caesium atoms,
53:04and then we use the lasers to throw that cloud upwards through an enclosure containing microwaves.
53:09And then we fall back down through it a second time under gravity.
53:12When the atoms change from one energy level to another,
53:15the emitor will absorb one very precise frequency.
53:18And we can use that frequency to keep track of time.
53:21We simply count up the oscillations.
53:22So the number of oscillations will define the length of a second.
53:29And those oscillations are a particular property of that Caesium atom.
53:34That's right, yes.
53:34So any Caesium atom will always have the same number of oscillations per second.
53:43The oscillations of these Caesium atoms are the ticking of the clock.
53:47And they give the CSF-2 accuracy to one second in 138 million years.
53:57It's a degree of precision our ancestors could never have imagined.
54:03The genius of Maxwell, Michelson and Essen now touch every part of our lives.
54:16They could never have guessed their work would one day be at the centre of everything,
54:21from our banking systems to phones, GPS and the internet.
54:25These only exist because of the accuracy of atomic clocks,
54:31and their ability to synchronise time across the planet.
54:37Measurement has taken us in directions we could never have dreamt possible.
54:41But the story doesn't end there.
54:53In one last twist, scientists looked at the metre again.
54:59And realised that they could now redefine length using the new accuracy of the second.
55:05It was 1983, and in a collaboration between different measurement labs across the world,
55:15atomic clocks measured the speed of light with incredible precision.
55:22The metre could finally be defined by how far light travels in a tiny fraction of a second.
55:29Time and length were intimately intertwined.
55:35We've come a long way since the days of the pharaohs,
55:44when time was defined by the length of a shadow.
55:47After 3,000 years, time and distance are once again linked.
55:53Joined together by one of the most fundamental and universal constants of nature,
55:57the speed of light.
56:05Despite all the great advances in time and length measurement, the quest is still on.
56:19Scientists are trying to create ever more accurate clocks.
56:23clocks that will only lose one second in the lifetime of the universe.
56:38And once they're deployed, we can only begin to imagine how it's going to change our world.
56:44Instant communication, quantum computers, planes that can land themselves.
56:50Science fiction will become a reality.
56:53And that's the beauty of measurement.
56:55Every leap in precision, from the cubit rod to the atomic clock,
56:59has led to a technological revolution.
57:01Through history, measurement has changed every aspect of our lives.
57:10Splitting the year into seasons and lunar cycles allowed man to plan ahead for the first time
57:16and gain advantage over the rest of nature.
57:22Dividing the day still further into 24 hours was the bedrock for civilisation.
57:28The fixed hour controlled the working day.
57:35And uniform national and international time allowed the globalisation of industry.
57:41The world would never be the same.
57:47The story of measurement has shaped and changed our history
57:51and will continue to do so as we delve deeper into the atomic fabric of the universe
57:56in search of greater precision.
58:03Next time, I meet the biggest problem in measurement.
58:07The kilogram.
58:09This 19th century artefact is the world's master kilo.
58:13And it's losing weight.
58:15Now a head-to-head race is on to replace it.
58:19As the best minds in measurement science fight it out,
58:25there can only be one winner.
58:31Stay with us here on BBC Four.
58:33A heartbreaking tale of abuse and exploitation.
58:36Brand new Storyville, Silence in the House of God, coming up next.
58:41We'll see you next time.
Comments