The State of the Unit: a documentary film about the redefinition of the kilogram

This forthcoming documentary traces the fascinating history of the kilogram, the redefinition, and features interviews with the guardians of the prototypes. Scientists around the world chase the realization of a 200-year-old challenge: to create a mass standard that will never change with time, or space.

Filming to date includes:

  • Paris, France: Historic and scientific sites including the Bureau International des Poids et Mesures (BIPM), home of Le Grand K, 36 rue de Vaugirard (home of one of the meter standards installed in Winter 1796), Musée des Arts et Métiers, Centre Nationale des Arts et Métiers, the Laboratoire national de métrologie et d’essais (LNE) and Montparnasse Cemetery, Montmartre Cemetery, and Père Lachaise Cemetery
  • Braunschweig, Germany: Physikalisch-Technische Bundesanstalt, one of Germany’s metrology institutes and home of the Avogadro Project measurements.
  • Switzerland: home of Mettler-Toledo and METAS, home of one the newer generation watt balances
  • Gaithersburg, Maryland, USA: National Institute of Standards and Technology, the national metrology laboratory and location of the US watt balance.

Status of the film

Principal photography is done and the film is in post-production: editing, stop-motion sequences, and so on.


Background on the kilogram

In 1780, more than 40,000 different measures of length were in use in France for trading wool, corn, wood, all household staples. Every city and village had their own carved stone trade measures, stored at the local church.

The varying measures led to serious economic and trade problems, adding to the atmosphere of unrest in France. So, in 1787 King Louis XVI charged the French Academy of Science with the creation of a uniform set of measures. Scientific giants such as Laplace and Lagrange--their names today deeply engraved in physics equations, principles, and formulas--were part of the committee. Louis XVI did not survive the French Revolution nor see the fruits of his charge. In June 22, 1799 the meter and the kilogram were realized as physical objects; today these are kept secure at the International Bureau of Weights and Measures (BIPM) in Pavillon de Breteuil near Paris. The kilogram is stored under multiple glass lids; a speck of dust changes the kilogram's weight a tiny bit.

All current science and technology could not exist without the kilogram prototype, and the modern world needs a standard that is the everywhere the same. To realize this, the General Conference on Weights and Measures may redefine the kilogram as the calculated value of the Planck constant, the very same constant that revolutionized 20th-century physics in the form of transistors and computers. Today the speed of light is no longer a measured quantity, but rather a fixed value that implicitly defines the meter. Similarly, the Planck constant would define the kilogram and make the physical kilogram obsolete.

Since the retirement of the meter, the kilogram prototype is the only human-made measurement standard. Forever it will be for science what Cervantes did for literature and Goya did for painting: the foundation of modern instances. And one hundred years from today, the kilogram prototype might well become mankind's most treasured artifact.

Director’s statement

My current film is about the kilogram. When I tell people that, some ask "What is there to say about the kilogram?" And others wonder, "How can you fit everything in only one film?"

Well, I had in mind a shorter film about how The Kilogram (a specific platinum-iridium cylinder in a vault in Paris) is losing weight, and what that really means.

But I found there's much more to the story.

It turns out that there is real path of measurements which connects the scale at your supermarket, to local and state inspectors, to scientists at national labs, and finally to The Kilogram in Paris, which by international treaty is defined to always weigh exactly one kilogram.

In addition to traveling across countries, the kilogram's story also travels across time. Many of today's national kilograms were created over a hundred years ago. And the idea for standardized, permanent measures came about just before the French Revolution. At the time, the lack standards caused a lot of problems with trade inside France: buyers and sellers couldn't agree on how much fabric, wine, wheat, or land, they were dealing with, and this problem was one of the reasons for the Revolution. So to improve trade in his own country, King Louis XVI asked his best scientists to come up with standards, which would be distributed around France, and about eighty years later to the rest of the world. And this is where our U.S. kilogram comes from.

These national kilograms are remarkably stable, but over hundreds of years they all still change by micro-amounts. (So that's why saying "the kilogram has lost weight" is not completely accurate. It's more correct to say that it “has changed differently from other kilograms.”) This change in man-made materials is inevitable, so scientists have decided to revise the definition of the kilogram; from the mass of The Kilogram, to be defined through a relationship to a fundamental constant of physics. (One such fundamental constant you know is the speed of light. The meter is defined as the distance light travels in a fraction of a second.) Finalizing this new definition requires many years of incredibly precise work, with a continual awareness of past measures to maintain continuity.

In my film, I aim to bridge measurements from daily life to these efforts to re-define the kilogram at some of the major scientific players including NIST in Gaithersburg, CNAM and LNE in Paris, and PTB in Braunschweig, Germany. With this new definition, they hope to fulfill a 100-year-old dream of some of the greatest physicists of all times. And I aim to show the social and cultural relevance behind our measures.

I hope that you find the history and redefinition of the kilogram as fascinating as I do.

About the Director

Amy Young graduated from CalArts with an MFA in Film Directing. She began work on the The State of the Unit as part of an documentary film exchange program at La Fémis in Paris, France.