By the kilo

By the kilo

The last arbitrarily defined SI unit is being redefined to echo the music of the spheres

The kilogram is to be redefined tomorrow, but there is no cause for alarm. You will not have to fight with your grocer for being diddled. Tinpot dictators nursing their kiloton-yield nukes will not suddenly feel like lightweights. All it means is that science and technology will be unburdened of the Big K, the reference kilogram weight, a platinum-iridium cylinder stored at St Cloud, against which weights the world over are measured.

The Système International or SI system of units, which includes the Big K, is a milestone in human cooperation as important as — and possibly more useful than — the UN Charter. Everyone who needs to measure anything accurately has agreed to use it, without nationalist histrionics, whether to measure the mass of an atom or the volume of a bottle of cola. Earlier, there had been dissonance about fundamental units like the second, the metre and the kilogram, which were arbitrarily defined, and did not echo the music of the spheres. The institution of SI in 1960 brought the whole system on key. The second, derived from astronomy, was redefined in terms of the oscillations of the caesium-133 atom. The metre, originally defined as a geographical distance and measured by a metal rod kept in Paris, was redefined in terms of the speed of light, an absolute constant.

Now, the kilogram, the last poorly defined unit left, will be redefined in terms of Planck’s constant, a central feature of quantum mechanics. This will make calculations across the sciences easier and remove dependence on the Big K, the Old Dreadful of weights and measures. Let us not go into the intricacies of this matter. Let us, rather, find satisfaction in the certainty that with a better definition, we can still have biryani by the kilo, but without fear of ever going wrong.

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