World / Science & Health

Mass movement: Scientists adopt new definitions of the kilogram, amp, kelvin — and mole


How we measure the world underwent a quantum leap Monday as scientists adopted new hyperaccurate definitions for units of weight, electricity and temperature derived from the universal laws of nature.

The change, on the occasion of World Metrology Day, sees one of science’s most influential objects — a metal cylinder used to define what a kilogram is — retired in favor of an infinitely more precise measurement taken from a quantum ratio.

“Le Grand K,” as the platinum-iridium object is known, stood as the world’s benchmark of the kilogram for nearly 130 years before a decision was taken in November to bring several units of measurement into the 21st century.

“The kilogram is the last unit of measurement based on a physical object,” Thomas Grenon, director of France’s National Laboratory of Metrology and Testing, said after the decision last year.

“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.”

Instead of relating to the mass of a singular physical object, the kilogram will now be defined in terms of the Planck constant — the ratio of a frequency of light, on the one hand, to the quantum energy of that frequency, on the other.

Energy is intrinsically linked to mass, as Einstein demonstrated with his equation E = mc2.

The Planck constant, combined with two quantum phenomena that allow for the creation of electrical power, can be used to calculate mass based on the equivalent mechanical power needed to displace it.