It’s official: We’re redefining the kilogram

Soon, our system of measurement will rely only on fundamental constants

Le Grand K

SPECIAL K  Carefully secured under bell jars, a metal cylinder known as Le Grand K defines the kilogram. Now, scientists are doing away with that benchmark, instead relying on a fundamental constant, the Planck constant, to define the kilogram.  

Courtesy of BIPM

Out with the old — kilogram, that is.

Scientists will soon ditch a specialized hunk of metal that defines the mass of a kilogram. Oddly enough, every measurement of mass made anywhere on Earth is tied back to this one cylindrical object. Known as “Le Grand K,” the cylinder, cast in 1879, is kept carefully sequestered in a secure, controlled environment outside Paris.

On November 16, at a session of the 26th General Conference on Weights and Measures in Versailles, France, representatives of countries from around the world voted to kick that convoluted system to the curb, enacting a plan to redefine several units of measurement, in addition to the kilogram (SN: 11/12/16, p. 24).

For a small cadre of scientists called metrologists — those who specialize in the science of measurement — it’s a big day. “It’s about as excited as you’re going to see metrologists get,” says David Newell of the National Institute of Standards and Technology in Gaithersburg, Md. He has spent much of his career working toward the change. “I can’t believe we’re finally getting it done.”

On May 20, 2019, Le Grand K will lose its special status, and the mass of a kilogram will be defined by a fundamental constant of nature known as the Planck constant. At the same time, other mainstays of the metric system will also be revamped: the ampere (the unit of electric current), the kelvin (the unit of temperature) and the mole (the unit for amount of substance).

Now, instead of being based on arbitrary quantities or physical artifacts that might change over time, “all the definitions will be based on what we call the fundamental constants of nature,” says metrologist Estefanía de Mirandés of the International Bureau of Weights and Measures in Sèvres, France.

Those unchanging numbers — which include the speed of light and the charge of the electron — are the same everywhere in the universe, making them useful pegs upon which to hang the metric system’s hat. “It’s a very big change of paradigm, and now it’s complete,” de Mirandés says.

Most people won’t notice the switcheroo: A kilogram of ground beef will still make the same number of burgers. But metrologists say the change will put precision measurements on a firmer foundation. For example, it will be easier to measure masses that are much smaller than a kilogram, a feature that could be useful for tasks like doling out tiny quantities of pharmaceuticals.

In preparation for the kilogram’s update, several teams of scientists carefully measured the Planck constant, quantifying it to an accuracy of around 10 parts per billion. After May 20, the value of the Planck constant will be fixed at exactly 6.62607015 × 10−34 kilograms times meters squared per second.

As a result, Le Grand K will no longer be a perfect kilogram — its mass will have a fudge factor of plus or minus 10 micrograms. Despite Le Grand K’s loss of stature, metrologists will keep studying the object to understand how stable its mass is over time. Scratches or gunk on the surface of the object may cause its mass to change slightly, for example.

The kilogram’s history can be traced back to 1795, when France adopted a standardized system of units — the metric system. The kilogram was originally designed to be equal to the mass of a liter of water. Soon, the mass came to be represented by a cylinder, and other countries adopted the units.

A key idea behind the development of the metric system — known formally as the International System of Units — was that the units should be accessible to everyone, and should last forever. “When they defined the kilogram, they fell short of this,” says physicist Stephan Schlamminger, also of NIST. Only a select few people have access to Le Grand K, and instead countries rely on imperfect copies of the official kilogram. Soon, however, anyone with the right expertise will be able to use the fixed value of the Planck constant to measure mass, using a device known as a Kibble balance.

In celebration of the new, more accessible kilogram, Schlamminger and Newell had the Planck constant tattooed on their arms, along with the French phrase, “A tous les temps, à tous les peuples” — for all times and for all people — an ideal that the new kilogram will now meet.

three scientists showing their forearm tattoos, of the Planck constant
IT’S PERMANENT Three scientists had the Planck constant tattooed on their arms. Stephan Schlamminger (left), Jon Pratt (middle), and David Newell (right), all of NIST, measured the constant using a Kibble balance (shown in background). Jason Stoughton/NIST

Physics writer Emily Conover has a Ph.D. in physics from the University of Chicago. She is a two-time winner of the D.C. Science Writers’ Association Newsbrief award.

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