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Mass movement: scientists adopt new kilogram definition

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How we measure the world underwent a quantum leap on Monday as scientists adopted new hyper-accurate 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 favour 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.

Put another way, it is 6.626 x 10-34 joule seconds.

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

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.

Proponents of this approach say it will be at least one million times more stable than physical artefacts and will have a range of practical applications, from pharmaceutical measurements to scientific research.

Monday also saw new definitions adopted for the ampere — the rate of electrical current flow; the kelvin — the base unit of temperature; and the mole — the unit measuring the amount of a given substance.

While changing how we define fundamental units might sound like a weighty affair, the BIPM, the Paris-based international custodian of measurement systems, said much care had been taken to ensure the shift will have “no perceptible impact on daily life.”

“Few users outside national metrology laboratories will notice the changes,” it said.


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