Quantum leap for mass as science redefines the kilogramme

SEVRES: Sealed in a vault beneath a duke’s former pleasure palace among the sycamore-streaked forests west of Paris sits an object the size of an apple that determines the weight of the world.

Forged against a backdrop of scientific and political upheaval following the French Revolution, a single, small cylinder of platinum-iridium alloy has laid largely undisturbed for nearly 130 years as the world’s benchmark for what, precisely, is a kilogramme.

The international prototype of the kilogramme, or “Le Grand K” as it is tenderly known, is one of science’s most hallowed relics, an analogue against which all other weights are compared and a totem of the metric system that accompanied the epoch of liberty, equality and fraternity. It’s so revered, in fact, that it’s only been weighed four times since 1889 and the room housing it in the Pavillion de Breteuil may only be opened when the three living key holders — who for security reasons must be of different nationalities — turn the lock simultaneously. And yet it’s soon to be out of a job. Hundreds of scientists from around the world will gather this week in the opulence of Versailles Palace for the 26th General Conference on Weights and Measures. There, in an act belatedly fulfilling the metric system’s founding promise of “For all ages, for all people”, they will replace the Grand K with a universal formula that defines the kilogramme using the quantum laws of Nature. “The kilogramme is the last unit of measurement based on a physical object,” said Thomas Grenon, director of France’s National Laboratory of Metrology and Testing. “The problem is that it’s had a life, it could fluctuate.

That’s not good enough, given the level of precision we need today.” With the adoption of the metric system, scientists in the late 18th century needed to codify a single structure that expressed distance, time, electrical processes and mass in similar, transferrable, units of measurement.