Redefining SI units: What is the new standard for kilogramme?

School textbooks to be rewritten as world adopts new definition for kilogramme

A replica of the International Prototype Kilogram at the International Bureau of Weights and Measures (BIPM) in Sevres near Paris | Reuters A replica of the International Prototype Kilogram at the International Bureau of Weights and Measures (BIPM) in Sevres near Paris | Reuters

School textbooks had once told us that there is a cylinder of platinum-iridium alloy kept under control conditions in a laboratory in Paris which weighs one kilo. All the metric system weights in the world are weighed against this prototype. Well, school textbooks will be rewritten, and with haste, as the world has adopted a new definition for the kilogramme. 

On May 20, 2019, the world unanimously adopted the new definition of the kilogramme, which is based on a physical constant, the Planck's constant. The Planck's constant is a fixed figure, a quantum of electromagnetic action, and relates the energy carried by a photon to its frequency. 

All this does sound confusing. But then, the business of metrology is a complicated one, and scientists across the globe have been deliberating on the new definitions of its base units for years.

As science got more precise, the calibrations became finer. And scientists began noting that there were divergences in the quantum of the units from the prototypes. A microgramme difference between two weighing scales might not make any difference when we are weighing potatoes. However, when it comes to really small measurements, like say mixing molecules for a particular drug, a “faulty'' measuring system may result in the product being quite different from what it was intended to be. 

“We were therefore looking for constants of nature to define the base units, instead of using artefacts like the platinum cylinder for weight or the platinum-iridium metre rod for distance,'' said D. Aswal, director of the National Physical Laboratory, the national custodian of the seven base units—distance (metre), mass (kilogramme), time (second) , temperature (kelvin), electrical current  (ampere), luminescence (candela) and amount of substance (mole). 

The standards of metrology were laid down on May 20, 1875 in Paris, where the member nations agreed to a common International System of units, called the SI (systeme internationale). This was to rule out the confusion of local systems, since the world was getting more connected, demanding a uniform metrology. The definitions of what comprises a kilogramme, or a second have been constantly revised, based on newer science, and more accurate ways of measuring. The aim was to do away with all physical artefacts as the base or prototype, to reduce the errors. Physical constants, thus, were constantly being looked for. For time, the definition of the second has evolved over the years. It was initially fixed as 1/86,400 of a mean solar day. In today's science, this is at best a crude calculation, and over years, the second has been redefined. Today, it reads as duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the unperturbed ground state of the 133Cs atom. 

Last November, the  Bureau Internationale de Poids et Measures (international bureau of weights and measures) had a historical meeting, where the members agreed to redefine four of the seven base units—kilogramme, kelvin, mole and ampere in terms of physical constants. The other units had already been defined earlier in these terms, though they were redefined to synchronise all the units, which were previously kept independent of each other. The conventional artefacts will continue to be maintained, though they will now be the secondary calibrations. The seven redefining constants now are:

the speed of light in vacuum c;

the Planck constant h;

the elementary charge e;

the Boltzmann constant k;

the Avogadro constant NA; and

the luminous efficacy of a defined visible radiation Kcd

The NPL will now have to get into action. It has already spoken to the NCERT about changing text book definitions. It will also need to calibrate units in the country as per the new standards. “These new standards should hold good for another 200 years at least. After that, maybe there will be new science and therefore newer definitions,'' Aswal said.