(gentle music) - [Narrator] This is the story of an artifact.

No one knows quite how many artifacts have been created in the history of mankind, but needless to say that there have been very, very, many.

From little Tommy Price's ideal sling-shot and Mrs. Mirandez's decisively demonstrative damask doily to young Papina Peshlakai's perfect pre-teen pinup.

Each is an ideal representation, and definitive example, of something truly important to its creator.

An object against which all others can be measured.

However, there are only a handful of artifacts that have been used as references by everyone.

Standards so essential that their very existence underpins the functioning of our world.

Some were used to define lengths, or volume, or time.

Most have come and gone.

But the one that defines what things weigh, their mass, still remains.

Secured in a subterranean vault, locked by three separate keys, hidden from view, an unchanging constant in an ever-changing world... until now.

- [Narrator] History is about to be made.

Representatives of over 100 nations, from Indonesia to Egypt, are here to solve a crisis.

They've put in millions of hours of effort, accomplishing engineering feats more complex than the moon landing.

These humble men and women aren't businessmen or politicians.

But their actions here will fundamentally change every aspect of our lives, from economics to medicine.

Today, they will literally redefine the world as we know it.

- We're trying to get rid of a Kilogram.

- [Narrator] But to understand what the kilogram actually is, and why they are so keen to get rid of it, we should really start this story at the beginning.

From the moment of our conception, through every aspect of our lives, from the inconsequential to the life determining, the high points, and the low, right up to the very end, everything about our existence is weighed and measured.

We are a species obsessed with quantifying things, because objective measurements ensure everyone is dealt a fair hand in life.

But, that's a lot easier said than done.

- [Narrator] Luckily for us, there are hundreds of people in different countries, spread out across the globe working out the best ways to weigh and measure everything.

They are normal people with hopes, dreams, hobbies, and desires.

Just like you or me, but with one minor difference.

These men and women are the tellers of time, the masters of mass, the safe-guarders of standards.

They are... metrologists.

- [Dr. Lulu Zhang] Metrology is really very, very important for industry and society.

- If there were no metrologists, we would have no measurement system.

- And we do crazy stuff, we measure glossiness of cats, we measure the acoustical crunches of biscuits.

- [Dr. Arnold Nicolaus] We measure the fastest speed, and we measure the smallest distance, and we measure the largest volume.

- Metrology is the science or the art of measurement.

- It means measuring things, as in normal life, but pushing these measurements to the limits of technology.

- [Narrator] For centuries, metrologists have been working away, around the clock, to ensure that there are, in fact, accurate clocks to work around.

And consistent rulers to measure with.

And equitable masses to weigh.

- [Dr. Michael de Podesta] I think our motto should be, we worry about it, so you don't have to.

- [Narrator] But, there is a problem that metrologists are worried about.

The weight of everything on the planet has been changing.

The change is so small few would even notice, less than the weight of a grain of sand.

But they have.

- [Dr. Patrick Abbott] No one really knows why it was happening, so that means no one can stop it, really.

That's alarming.

- [Perdi Williams] It doesn't sound important, but with pharmaceuticals, that could be the difference between helping someone or hurting them.

- [Narrator] So now, it is up to the world's finest metrologists to shoulder the heavy burden.

Together they need to redefine the weight of the world.

Fix it for all time.

But time is not on their side.

(gentle music) - [Narrator] The clock is ticking.

In a little over a year, hundreds of delegates from around the world will travel to the General Conference on Weights and Measures in Versailles, expecting a solution to this weighty problem.

- [Dr. Arnold Nicolaus] It's not enough to work day and night.

- [Dr. Stephan Schlamminger] It's a scramble.

It will be tight for us hitting the deadline.

- [Narrator] To succeed they need to complete one of the most difficult experiments in the world, ranked second only to the search for the infamous God Particle.

- [Dr. Alan Steele] I think it's fair to say that the number of people that are involved it's ridiculously hard and it's really great work over many, many years.

- [Narrator] But to solve this problem they'll need to shift the foundation of our entire measurement system.

To understand why requires a trip back to Paris.

More specifically, to an intergovernmental organization known as BIPM or The International Bureau of Weights and Measures.

- BIPM.

- [Dr. Martin Milton] We are one of the oldest technical international organizations in the world.

We're here in France, but we're not French.

- [Narrator] It is led by Director Martin Milton.

Assisted by a team of staff including Dr. Estefanía de Mirandés and Dr. Richard Davis.

The BIPM uses its impartial status to act as an international guardian of metrology, ensuring measurements are consistent, accurate, and fair for all.

And they are the curators of a truly extraordinary achievement of mankind.

A list, or rather a system, of definitions.

Le Systeme Internationale or the SI for short.

It's a simple list, distilling all the essential properties of our world into just seven definitions of base standard units.

- [Dr. Michael de Podesta] It's because we share this system of units that it gets its power.

It means we can compare things in all the countries on earth easily and transparently.

- [Narrator] The meter.

The second.

The mole.

These are the essential standards to which all our rulers, clocks, thermometers, and scales can be calibrated.

- [Dr. Martin Milton] That provides the basis for all the types of measurement that one needs for science and society and for quality of life.

- [Dr. Michael de Podesta] I love the SI.

I really love it, and the reason I love it is because it has humanity stamped all over it.

- Do you know how many languages are spoken on our planet?

Do you have a feeling of that?

- [Interviewer] 300.

- 300.

No, 3,000.

3,000 different languages.

And it is very complicated for one group of people to communicate with another group of people and so to have a universal language in measurement, that's a global story and that's very, very beautiful.

- [Narrator] But there is a problem with the SI.

One hidden in plain sight.

The kilogram, a fundamental unit of the SI, is simply defined as: - [Narrator 2] The mass of a kilogram is equal to the mass of the international prototype of the kilogram.

- Mass is still tied to a single, a single artifact in the world.

To a hunk of metal in Paris.

- [Narrator] The International Prototype of the Kilogram or IPK, le Grande K, The Last Artifact.

The 'hunk of metal' has many names.

Yet it is utterly unique and absolutely irreplaceable.

It is an object that literally defines the weight of everything else in the world.

Few will ever have the chance to see something this priceless.

Except on a day like today.

A small platinum iridium cylinder standing only 39 millimeters tall, it is not a particularly impressive object to look at.

- [Dr. Estefanía de Mirandés] You have all these bell jars which surround it, so you see this little, little thing, just in the safe.

From one side, you are excited.

On the other side, you have just a piece of metal.

- [Dr. Andrew Wallard] I mean, it's a strange thing.

How do you get excited about a small piece of metal in that way?

But it's what it stands for.

- [Dr. Patrick Abbott] Anything that you weigh, from an apple in a grocery store, to drugs at a pharmacy, eventually goes back to this prototype in Paris.

And it's an elaborate system of traceability that we have to that enables us to get accurate weights for everything that we use and buy and sell in the world.

- [Narrator] Held in vaults across the world are hundreds of copies of the kilogram.

None are exactly identical.

But, all are calibrated precisely against the original.

- [Perdi Williams] So, for example, if it will give you a weight, and you need to know if that weight is the right weight.

So, that scale has been calibrated which in turn has been calibrated against another scale and it runs all the way back, and the top scales will be calibrated against a kilogram.

And that kilogram, in turn, is calibrated and measured against the IPK, kept at BIPM, which is THE kilogram, which is the standard that all standards are compared to.

- [Narrator] And this, is the source of our weight-related problems.

- [Dr. William Phillips] This is what the kilogram, THE Kilogram, the International Prototype of the Kilogram looks like.

Now, I'm handling it with my hands.

By definition, the mass of the International Prototype Kilogram is the definition of the kilogram.

That means if you put a fingerprint on it, it's still a kilogram and everything else in the universe has less mass.

That's a ridiculous situation to be in.

- [Narrator] And that's not the only issue.

- [Dr. Estefanía de Mirandeés] It's a manmade object, an artifact.

- [Dr. Michael de Podesta] Everything that humans make as a standard decays.

Everything, as soon as it's made, it's beginning to decay and change and evolve.

Everything is decaying.

- [Narrator] Since it was forged, the IPK and its replicas have deviated from one another by about 50 micrograms, or the mass of a single eyelash, shifting the world's weight with them.

- [Dr. Takashi Usuda] We do not know whether the prototype, the artifact, is losing its weight or it's accumulating its weight.

- [Dr. Michael de Podesta] There's a fundamental problem with having an artifact that defines the standard against which everything else in the world is compared.

You want it to be isolated and kept separate.

But as the source of all the weighings in the world, you want it to be used practically.

- [Narrator] In order to solve the problem of the world's shifting weight, there is only one solution.

- [Perdi Williams] So, we're dumping the lump.

We're changing the system.

- [Narrator] But, that's not as simple as it sounds.

The kilogram is a fundamental, interlinked, part of the SI units.

Which means to redefine it, you have to redefine the SI.

You have to change the way the world measures almost everything.

- [Dr. Jon Pratt] I always feel like if humanity's going to change something, go big.

- [Narrator] To understand how we got into this situation in the first place and how we can fix it, we should really start this story at the beginning.

No, the beginning of civilization.

- [Dr. Podesta] Measuring is a really simple idea.

and humans have been doing it since time immemorial.

(gentle music) - [Dr. Zeina Kubarych] If we go back to antiquity, thousands of years ago, people measured things with stones and grains to make standards.

- [Narrator] These consistent measuring references revolutionized fair trade.

And naturally, people gravitated towards standards everyone could access that were also close at hand, like a hand, or an eye, or arm.

But, these seemingly democratic standards had their own potential for short-changing.

- [Dr. Barry Wood] If you measure a horse, you can measure it in hands.

If you measure a horse and I measure a horse, we're going to get a different size horse.

- [Narrator] So, many ancient rulers created empire-wide, stable standards of measures.

- [Dr. William Phillips] So, for example, in ancient Egypt, the cubit was the length of measure.

And that was the distance between the elbow and the tip or sometimes between the elbow and the tip of the finger plus a hand.

But those guys realized this wasn't going to let them build the pyramids to the kind of accuracy that they needed to do, so they made a standard.

- [Narrator] Standardized artifact measures, like cubits, allowed those in power to better build, govern, and tax their empires.

- [Dr. Michael de Podesta] Where you can compare something measured in one place with something measured at another time in another place, and then measurement becomes really, really powerful.

- [Narrator] But over the centuries, these measurement standards only multiplied, and massively muddled metrology through the creation of more and more units.

- Pounds, ounces.

- A pile of Charlemagne.

- A yard.

- Avoisdupois.

- Shaku.

- Pints.

- Ells.

- Miles.

- Feet.

- Kan. - A sheckle.

- The inch.

- [Narrator] By the 18th Century, the list was so lengthy that some traders were dealing with 800 different units of measurement and a quarter of a million definitions for them, many of which weren't comparable.

And these inequities stirred up some serious discontent amongst the masses.

We all know the role that tea and taxes played in the American Revolution.

But it could be equally said that baguettes and bakers played a role in the French Revolution just a few years later.

- [Dr. Martin Milton] One of the very worst things that the revolutionaries were concerned about in revolutionary France, they were concerned about the lack of fairness with measurements.

- [Narrator] Bread was the main component of the French working-class diet.

The average 18th-century worker spent half their hard-earned dough on it.

It was so important the King controlled all aspects of bread production, including the price of loaves.

With France in the middle of an historic famine and different standard units of measure in each village, weights could be fiddled.

So, people were often paying the same for less.

And the poor had no way to prove this was happening.

The peasants were hangry, and revolting.

- [Dr. Richard Davis] France was in turmoil.

It began with the execution of the king and ended with the accession of Napoleon.

- [Narrator] Obviously, the causes of the revolution were far more complicated than the price of bread, but it's fair to say it rather brought things to a head.

- [Dr. Martin Milton] When the revolutionaries took over, it was recognized that one of their priorities would be to put in place a measurement system that was gonna be fair.

- [Narrator] On the 7th of April, 1795, true to their word: "A tous les temps, a tous les peuples."

- [Narrator] At all times, to all people, the metric system was formally enshrined in French law.

- [Dr. William Phillips] This idea of a democratic approach to measurement, this wasn't tied to the length of the king's foot or something like that.

- [Narrator] Instead, they turned back to constants found in nature, but in a much more scientific and global way.

- [Dr. Jon Pratt] They were going to use the earth itself.

- [Dr. Martin Milton] Anybody could access the circumference of the earth and nobody could change the circumference of the earth.

- [Narrator] For example, a meter would be a fraction of the planet's circumference.

But at the time, this proved too difficult to measure precisely and consistently.

So, they decided to create physical points of reference for these new units.

They made artifacts.

The original artifacts of the metric revolution are now locked away in the Archives Nationales.

- [Dr. Pierre Fournié] One of the first decisions of the National Assembly was to create first, the meter, and then, the kilogram.

- [Dr. William Phillips] They defined a new unit of mass that was the mass of a liter of water.

And they called that the kilogram.

- [Dr. Pierre Fournié] And this is the first prototype of what we call le kilogram des archives.

It was made at the end of the 18th century, fabricated in platinum.

- [Dr. William Phillips] Among the things that people were thinking about was liberté, egalité, fraternité, and part of this was that the system of measurements should be something that should be accessible to everybody.

- And that has been the mass system that we have maintained for years and it has served us well.

- [Narrator] But if it's not broken, why fix it?

Well apart from the possibilities of damage to, or theft of, an irreplaceable artifact, metrologists are thinking about the future.

- [Dr. Willie May] Redefinition allows us many things, two that come to mind.

One is, we freeze any potential drift in the mass of this artifact.

Then it allows us to go from the quantum level to the cosmic level, without changing scales or units.

- [Dr. Alan Steele] Most science and a lot of technology is developed when we solve a measurement challenge.

Having better access to better definitions and better measurement techniques actually propels the rest of society and its scientific progress forward.

- [Dr. Richard Davis] And that brings us to the next step, which is to replace the present definition of the kilogram with something that's not an object.

- [Dr. Zeina Kubarych] Something that doesn't change with time, that doesn't get destroyed, something that you can reproduce everywhere, anywhere in the world.

- [Dr. William Phillips] We have faced this kind of situation repeatedly in metrology.

It used to be that a second was one over 24 times 60 times 60 of a day.

The trouble is that by the early 20th century, it was clear that a day was not a constant thing.

And that realization meant that we had to change the definition of time.

- [Narrator] And they did this by using what metrologists call fundamental constants.

- [Dr. William Phillips] Today, the definition of time is based on atomic standards, it's based on the unchanging frequencies of atoms.

- [Narrator] And, like the second, fundamental constants have been used to redefine other units of the SI.

- [Perdi Williams] Originally for the meter, it used to be a meter stick.

Now it's based on the speed of light in a vacuum.

- [Dr. Michael de Podesta] Which sounds like it isn't a profound change but it actually is an incredibly profound change.

- [Narrator] This moves the reference for objective reality off of a manmade artifact and onto an unchanging constant of the universe.

- [Perdi Williams] In the case of the kilogram, we're moving from an artifact to basically an equation.

- [Narrator] But unlike the speed of light, there is no obvious fundamental constant to redefine mass.

Cue two obscure but promising candidates.

One from chemistry, the other from physics.

The teams representing each are tackling the same kilogram-sized problem, but a little friendly competition never hurts.

- [Dr. Estefanía de Mirandés] It's very challenging and then it's competitive, too.

- [Dr. Jon Pratt] There is national pride at stake.

- [Perdi Williams] Everyone wants to be the lab that changes the face of the kilogram.

- [Narrator] One group set out to define the kilogram using the Avogadro Constant.

- [Dr. Peter Becker] Of course, there is competition, of course, that is the power who brings us further.

- [Narrator] The other, using the Planck constant.

- [Dr. Richard Green] One of the things that makes metrology unique is we rely on other people repeating our measurements.

- [Dr. Ian Robinson] Rivalry in metrology is extremely useful.

People will go out of their way to think of new ways of doing things, and some of them will survive, some of them won't.

- [Narrator] Both routes would push the teams to the limits of what's possible.

The first candidate is a fundamental constant from chemistry, the Avogadro Constant.

Named after the early 19th-century Italian scientist Amedeo Avogadro, the Avogadro Constant is currently defined by the SI as the number of atoms in one mole.

No, not that kind of mole.

The mole is a unit that describes the amount of substance that is equal to the number atoms in 12 grams of Carbon-12.

This is currently calculated to be 602 sextillion 214 quintillion 85 quadrillion 774 trillion atoms.

To put that into perspective, that's 80,000 times more atoms than there are grains of sand in all the beaches, deserts, and sandpits of the entire world.

To further complicate things, those 12 grams are still defined by the IPK.

So, why then, posited scientists, can't One of the teams tackling this, is the Physikalisch-Technische Bundesanstalt in Germany.

- [Edyta Beyer] The PTB is a very accurate, organized little machine.

Not that little.

- [Narrator] It's home to team members including PhD student Edyta Beyer, Physicist and fossil hunter, Ingo Busch and physicist, engineer, and precision marksman, Arnold Nicolaus.

They're working with the National Metrology Institute of Japan to build cutting-edge technology to crack this Avogadro endeavor.

Here, team members including Prime Senior Researcher and pianist, Kenichi Fujii, Surface and Nanoanalysis Group physicist and flower arranger, Lulu Zhang, Mass Standards Group Leader and weightlifter, Naoki Kuramoto, are led by Director General and avid motorcyclist, Takashi Usuda.

- [Narrator] Both teams are aiming to redefine the kilogram as an equation by measuring the value of Avogadro's Constant more precisely and more accurately than ever before.

And they custom-make the perfect tools for the job.

- [Edyta Beyer] I'm working with the roundest object on the earth, which is made from the purest silicon on the earth.

- [Narrator] And machining them is no walk in the park.

Mechanical engineer Rudolf Meess has spent years perfecting the process.

- [Dr. Rudolf Meess] In the first step, we remove several grams per hour.

But in the end, the removal rate is just one nanometer per minute, and it takes three months to produce a sphere.

- [Narrator] The result is an almost perfect sphere, of almost perfect purity and a price tag of several million dollars per sphere.

Luckily, for the Avogadro team, these spheres aren't irreplaceable artifacts like the IPK.

Rather, they are just astoundingly accurate, if expensive, tools.

- [Dr. Jens Simon] The silicon sphere is much more than just a mass standard.

It's a counting machine for atoms.

- [Narrator] New spheres can be made, but their perfection is priceless.

It allows them to be measured to previously unfathomable precision and determine the exact number of atoms inside each one.

But there are so many atoms inside, that if you could count them at a speed of 10 million atoms per second, it would take you 68 billion years, or about five times longer than the universe has existed.

So, they use their spheres' remarkable shape and structure to calculate Avogadro's constant instead.

- [Edyta Beyer] Imagine we have a truck full of oranges.

So, we measure the truck, that's the volume of the sphere.

We measure the volume of an orange, that's the volume of the atom.

And together with the information about the inside organization of the oranges, we know exactly how many oranges are inside and we can determine our Avogadro Constant.

- [Narrator] With their perfect spheres and meticulous measurement, the Avogadro project aren't pulling their punches.

But there is another heavyweight contender competing in the kilogram redefinition ring.

The Planck Constant.

Named after the German physicist who discovered it, Max Karl Ernst Ludwig Planck, it's a fundamental constant that even top metrologists have trouble summing-up.

- How can I do this?

I gotta think about this a bit more 'cause that's a hard question to explain what Planck's Constant is.

- Oh, God, jeez.

- [Interviewer] Do we want to try and describe what Planck's Constant is?

- No.

- [Narrator] The actual constant may be hard to explain, but the concept is easy enough.

In the late 1800s, physics was facing a crisis.

Physicists were trying to model atomic vibrations, but they kept getting it wrong.

They had assumed that atomic vibrations were continuous, that is, they could vibrate at any frequency.

- [Dr. William Phillips] Planck came up with a kind of a crazy idea.

He said, it's not a continuous process.

It's always in little packets of energy that today we call photons.

- [Narrator] Think of it like your morning coffee.

When you order a cup of Joe, you can add sugar in two different ways, as a continuous stream, or as cubes.

Planck postulated that you could only change energy in minimal increments like sugar cubes and the field of Quantum Mechanics was born.

- [Dr. Alan Steele] It was a surprise to everyone that energy was quantized.

And it changed our understanding of physics entirely.

- [Narrator] And because Planck's Constant relates to energy, you can link it to an object's mass via Einstein's relativity equation, energy is equal to mass times the square of the speed of light using Planck's Constant to redefine the kilogram.

But first, metrologists have to measure it.

Doing the heavy-lifting for the Planck Constant are labs like the National Research Council in Canada.

- [Dr. Richard Green] We spend a lot of hours, a lot of long, long hours.

Physics runs on coffee for the most part.

- [Narrator] Team members including Research Officers in Electrical Measurements, Carlos Sanchez and Barry Wood, and dedicated Dad and Team Leader in Mass Metrology, Richard Green, are led by Canada's Chief Metrologist, Alan Steele.

- [Dr. Alan Steele] Intense.

Dedicated.

You know, no fear of taking things apart and putting them back together over and over and over again until you get it right.

- [Narrator] The NRC are joined by yet another set of Planck Pioneers from the National Institute of Standards and Technology in the United States of America.

Team members include Chief of the Quantum Measurements Division and string strummer, Jon Pratt.

- [Dr. Jon Pratt] It's certainly a common stereotype, but I feel like it's a little bit of truth, the U.S. tends to be a little bit more freewheeling on these types of things.

It's a great new idea, let's go do that.

- [Narrator] Leader of the Mass and Force Group and coffee aficionado, Zeina Kubarych, Mass and Force Group physicist and technical-tinkerer, Patrick Abbott, and Fundamental Electrical Measurements Group physicist and should-be prize-winning pretzel-maker, Stephan Schlamminger.

- [Dr. Stephan Schlamminger] I think we have a very strong team.

I think there's almost no technical problem that we can't solve.

- [Narrator] To derive the most precise and accurate possible value of Planck's Constant, both teams are measuring a readily available supply of calibrated kilograms in an electromagnetic device called a Watt Balance.

- [Dr. Carlos Sanchez] The Watt Balance is, simply, a clever device that can weigh a mass using only electrical measurements.

- [Dr. Zeina Kubarych] The Watt Balance's basic principle is, you're comparing a gravitational force to electromagnetic force.

- [Narrator] Think of it like a junkyard electromagnet.

Larger cars have a greater gravitational force pulling on them, making them heavier.

So, more electrical current is required to overcome gravity and lift a heavier car.

Except instead of pulling an object upwards, the Watt Balance electromagnet pushes it up against the force of gravity until the electrical force equals the mass of the now balanced object, without the need to compare it to anything on the other side of the scale.

This gives the Watt Balance the ability to link electrical energy, and its base unit, Planck's Constant, to the mass of an object.

But devising a device to measure the quantity of this force exactly required the genius of Dr. Brian Kibble, who cracked the engineering of the world's first Watt Balance at the National Physical Laboratory in the United Kingdom.

- [Dr. Ian Robinson] This laboratory was where the Watt Balance was born.

Brian Kibble and I built the first one here.

- [Narrator] Today, the balance is also called a Kibble balance, in honor of its creator.

His legacy lives on through the work of the lab's many contributors including electrical measurement specialist, Ian Robinson, and Research Scientist, Perdi Williams.

- [Perdi Williams] I don't think you could get a more British place than this crazy British lab.

We have tea parties in the office, and there's a grand piano.

I'd describe the basement lab as Ian's brain has just thrown-up in a lab.

But some of the most groundbreaking science is done in a messy, unorganized shed in the basement of a massive stately home.

- [Narrator] Whilst NPL are no longer in the business of weighing kilos, they are leading the world in building affordable, miniaturized Kibble balances for the masses.

- [Dr. Ian Robinson] We're now in the lucky position of being able to build a generation of balance that should take the balance out and make it egalitarian.

Everybody in the world should be able to access this technology, and contribute to the world mass scale.

- [Narrator] And they're lending their expertise in Watt Balance building to a devilishly difficult task, for good reason.

- [Dr. Ian Robinson] Measuring the Planck constant to the level that we need is extremely difficult.

- [Dr. Stephan Schlamminger] Working with the Watt Balance is, I would say an emotional roller coaster.

Bring it down, okay let's have a look.

And building this thing was an exhausting and kind of, oh my God, is it gonna work, you know?

- [Dr. Darine Haddad] Can you see it?

- [Dr. Stephan Schlamminger] Stop, stop it there.

- [Dr. Alan Steele] The Watt Balance is a really, really hard experiment.

- [Dr. Jon Pratt] These instruments tend to have a certain character to them that only the people who built them can really make them work the way that they're supposed to work.

- [Dr. Carlos Sanchez] We need to consider about a hundred different effects that affect the measurement.

- [Perdi Williams] From noise, temperature, magnetic fields, we need to measure all those things and to a level that we haven't been able to measure before.

- [Dr. Jon Pratt] This is a big change we're proposing.

We'd like to get it right.

- [Narrator] After years of painstaking trial and error, a decision has to be made about which fundamental constant would redefine the kilogram.

In 2011, the metrological community decrees Planck, and its ability to link all electrical units in the SI, as their constant of choice.

But there's a twist which means the work of the Avogadro teams can still contribute to the redefinition effort.

- [Dr. Jens Simon] The Avogadro Constant and Planck's Constant are very close related to each other.

If you have one of these two constants, you will get the second constant for free.

And that's the way Planck's Constant occurs in the silicon spheres.

That's it.

- [Narrator] The challenge then becomes that the Planck values from both experiments must match.

- [Dr. Peter Becker] If both projects result in the same numbers, then I think there is no doubt to redefine the kilogram.

- [Narrator] If they don't, then something has gone very wrong.

Today, is one of the most important meetings of the Consultative Committee for Mass.

Today, is the deadline for the teams to get their results in.

- [Dr. Alan Steele] Everybody's coming up with their answers at the last minute.

Canada's was just accepted about a week and a half ago.

The German Avogadro number came about three or four days ago.

The NMIJ one came out about two days ago.

Just yesterday, I think it was, from the United States.

- [Narrator] Today, they decide if the data is good enough to be put to the vote.

- [Prof. Joachim Ullrich] The CCM put up some conditions, which should be fulfilled in order to go for redefinition.

- [Dr. Richard Green] We want to all have agreement on what the value for Planck's Constant is, and a new value has come online which is controversial and maybe possibly a little bit out.

- [Dr. Stephan Schlamminger] Some countries think that the criteria have been met, and others think they have not been met.

- [Prof. Joachim Ullrich] This is now an intense discussion, how we should proceed.

- [Narrator] If they cannot reach an agreement, the whole redefinition could be called off.

- [Dr. Richard Green] For us, it would mean four more years of digging for uncertainties.

And I don't think we can.

I don't want to do that.

I want to move on to other science.

- [Narrator] As the debate becomes more heated, it continues behind closed doors.

- [Dr. Estefanía de Mirandés] One year ago, I would have said, everything's set for the redefinition.

Now, in the recent times, I'm not certain if the current status of knowledge will be considered to be enough by the general conference.

- [Narrator] But in the always exciting world of metrology, nothing is ever straightforward.

The teams have been given just two months to publish values in closer alignment for Planck's constant, or risk putting next year's redefinition vote in jeopardy.

- [Dr. Stephan Schlamminger] Right now, we should be writing.

We're still not writing our article, instead we're still doing measurements.

So, I think it will be a close call, I think we'll have a whole bunch of all-nighters to get it done.

- [Narrator] Once the papers are in, the Avogadro and Kibble balance teams will average their numbers to obtain the lowest possible uncertainty for the final fixed value of Planck's constant.

- [Dr. Richard Green] Uncertainty is measurement of how confident you are in that number.

The worst thing a metrologist can do is have a number that turns out to be outside their uncertainties.

That's our idea of hell, I think.

- [Edyta Beyer] We still have some problems to solve and I think the pressure is too big for the redefinition, for the redesigning of the SI.

- [Dr. Jon Pratt] I'm sympathetic to the conservatives.

But I'm also a U.S. guy and I'm just like, let's change.

- [Narrator] The final agreed numbers for Planck's constant roll in, fixing its value in the kilogram equation.

There's nothing more the teams can do but wait.

It's now up to the metrological community of the world to vote on whether their consensus value for Planck is good enough to redefine the kilogram.

- [Perdi Williams] We're going to Versailles to go and watch the vote.

We'd better redefine the kilogram, we've done too much work not to do it.

- [Narrator] Today's vote will determine whether the kilogram will become an equation using Planck's constant, or if it will remain an artifact.

- [Perdi Williams] The fact we've got multiple experiments, multiple labs, means that we're pretty sure we've got the right answer.

We're not pretty sure.

We've got the right answer.

- [Dr. Carlos Sanchez] Now, we're really getting close to a win-win-win situation, so it's nobody loses.

So, why would anybody want to, to delay?

It's way past time to redefine.

- [Dr. Richard Green] You don't want science to ever be limited by one particular artifact or one particular laboratory or one particular experiment and now we're ready.

I think it's time.

- [Dr. Martin Milton] Ladies and gentlemen, you have heard that the proposal for the revision of the SI is that it should be based on constants, on fundamental constants.

Mr. President, I think we're ready to propose a vote.

- [Dr. Sébastian Candel] Thank you.

Each country will be called and you will say yes or no.

- [Dr. James McLaren] Canada.

(speaking foreign language) - [Dr. Alan Steele] Oui.

- [Dr. James McLaren] Oui.

Tres bien.

Kenya.

- [Scientist] Yes.

- [Dr. James McLaren] Inde.

India.

- [Scientist] Yes.

- [Dr. James McLaren] Japon.

Japan.

- [Dr. Takashi Usuda] Yes.

- [Dr. James McLaren] United States of America.

- [Dr. Walter Copan] Yes.

- [Dr. James McLaren] Korea.

- [Scientist] Yes.

- [Dr. James McLaren] Iran.

- [Scientist] Yes.

- [Dr. James McLaren] China.

- [Scientist] Yes.

- [Scientist] Yes.

- [Scientist] Yes.

- [Scientist] Yes.

- [Scientist] Yes.

- [Scientist] Yes.

- [Scientist] Yes.

- [Perdi Williams] We've redefined the kilogram.

- [Dr. Sébastian Candel] The vote was unanimous.

Thank you so much.

(crowd applauding) - [Dr. Estefanía de Mirandés] To witness a historic moment like that and be a part of it.

I know that this is something I will remember forever.

- [Dr. Stephan Schlamminger] It was the ride of my life.

It was a lot of fun.

- [Dr. Jon Pratt] We measure Planck's Constant, but what people really want to see are the tatts.

- [Perdi Williams] My tattoo in no way resembles my job role, but I have keeper of the kilogram tattooed on my back.

- [Dr. Alan Steele] The idea of the French that it would be a system of units for all people, for all times.

I think we're really a lot closer to that now.

- [Dr. Jon Pratt] I'm just overwhelmed with the community that is built, and come to consensus, and make progress, real progress, for mankind.

- [Narrator] In a monumental metrological moment, a democratic equation anyone can access, at any time, takes its place as the definition of the kilogram.

While metrologists get back to work, the rest of the world continues unaware.

- [Dr. William Phillips] Okay, so what's going to change when we change the kilogram?

How is that going to affect the average person?

- [Edyta Beyer] When the kilogram is redefined, when we do a good job, no one will notice it.

But that's good.

- [Dr. Alan Steele] When the person on the street will start seeing differences is when new technologies start getting built into better science or better products.

I think that might happen in individualized medicine or cellular medicine or pharmaceuticals.

The better we get at measuring something, the bigger the world of opportunity for scientific exploration becomes.

- [Dr. Ingo Busch] There's a new undiscovered land.

We have wiped out all the maps and say, okay, something completely new.

- [Dr. Michael de Podesta] Some people think metrology is dull.

They think it's some kind of accountancy.

That completely misses the point.

Metrology is looking closer and closer at things, seeing their form more perfectly, seeing them more clearly in themselves.

And just appreciating the full wonder and beauty of the world we have around us.

(bright music) (gentle music) - [Dr. Willie May] So, Le Grand K isn't really going away.

It's gonna be in that vault, but it's still going to be with us, because its mass, at the time of redefinition is essentially built into the Planck's Constant.

It's immortalized, if you will.

- [Perdi Williams] I feel quite, weirdly protective over a lump of metal.

And I mean, if they do want to get rid of it one way, I'm happy to take it home and look after it.

(gentle music)