A weighty matter: The kilogram's value is redefined

PARIS • It may not change how you buy bananas, but scientists have voted to redefine the value of a kilogram, in what they called a landmark decision that will boost the accuracy of scientific measurements.

Since 1889, a kilogram has been defined by a shiny lump of platinum-iridium kept in a special glass case and known as the International Prototype of the Kilogram.

It is housed at the headquarters of the International Bureau of Weights and Measures (whose French acronym is BIPM), just outside Paris.

Members of the BIPM, which groups some 60 nations, agreed last Friday after a week-long meeting at the nearby Palace of Versailles to redefine a kilogram in terms of a tiny but unchanging value called the "Planck constant".

They also voted to update definitions for the ampere (electrical current), the kelvin (thermodynamic temperature) and the mole (amount of a substance).

All modern mass measurements are derived from the kilogram, whether micro-grams of pharmaceutical medicine or gold dust, kilos of fruit or fish, or tonnes of steel.

The problem is that the prototype does not always weigh the same. Even inside its three glass bell jars, it picks up micro-particles of dirt and is affected by the atmosphere. Sometimes it needs cleaning, which can affect its mass.

That can have profound implications. If the prototype were to lose mass, atoms would in theory weigh more since the base kilogram must by definition always weigh a kilogram.

The "Planck constant", which derives from quantum physics, can be used along with a Kibble balance, an exquisitely accurate weighing machine, to calculate the mass of an object using a precisely measured electromagnetic force.

"The SI redefinition is a landmark moment in scientific progress," said Dr Martin Milton, director of the BIPM. "Using the fundamental constants we observe in nature as a foundation for important concepts such as mass and time means that we have a stable foundation from which to advance our scientific understanding, develop new technologies and address some of society's greatest challenges."

It is arguably the most significant redefinition of an SI (International System) unit since the second was recalculated in 1967, a decision that helped ease communication via technologies like GPS and the Internet.

The new definitions agreed by the BIPM will come into force on May 20 next year.


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A version of this article appeared in the print edition of The Sunday Times on November 18, 2018, with the headline A weighty matter: The kilogram's value is redefined. Subscribe