PARIS, FRANCE (NOVEMBER 14, 2018) (REUTERS) – 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 on Friday (November 16) 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).
The problem is the prototype doesn’t always weigh the same. Even inside its three glass bell jars it picks up microparticles of dirt and is affected by the atmosphere. Sometimes it needs cleaning, which can affect its mass.
Scientists have been trying for decades to define a constant value for the kilogram that is derived from immutable physics, in the same way they have done for other standard units (SI units) overseen by the BIPM.
For example, a metre isn’t 100 centimetres, it’s actually “the length of the path travelled by light in a vacuum during a time interval of 1/299,792,458 of a second”.
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.
It is arguably the most significant redefinition of an SI unit since the second was recalculated in 1967, a decision that helped ease communication across the world via technologies like GPS and the internet.
The new definitions agreed by the BIPM will come into force on May 20, 2019.