Paris: In a historic vote, nations unanimously approved on Friday a ground-breaking overhaul of the international system of measurements, uniting together behind new scientific definitions for the kilogram and other units. The redefinition of the kilogram, the globally approved unit of mass, was the mostly hotly anticipated change. For more than a century, the kilogram has been defined as the mass of a cylinder of platinum-iridium alloy kept in a high-security vault in France. That artefact, nicknamed “Le Grand K,” has been the world’s sole true kilogram since 1889.
But now, with the vote, the kilogram and all of the other main measurement units will be defined using numerical values that fit handily onto a wallet card. Those numbers were read to the national delegates before they voted. Scientists at the meeting were giddy with excitement: for them, the update represents decades of work. They clapped, cheered and even wept as the 50-plus nations one by one said “yes” or “oui” to the update.
Some even sported tattoos on their forearms that celebrated the science. Nobel Prize winner William Phillips called it “the greatest revolution in measurement since the French revolution,” which ushered in the metric system of meters and kilograms. The change will have no discernible impact for most people. Their bathroom scales won’t get kinder and kilos and grams won’t change in supermarkets.But it will mean redundancy for the Grand K and its six official copies. The new formula-based definition of the kilogram will have multiple advantages over the precision-crafted metal lump that has set the standard for more than a century.
Unlike a physical object, the formula cannot pick up particles of dust, decay with time or be dropped and damaged. It is also expected to be more accurate when measuring very, very small or very, very large masses.Even in retirement, the “Grand K” and its six official copies — collectively known as “the heir and the spares” — will still be kept in the high-security vault on the outskirts of Paris where they are stored. That’s because scientists want to keep on studying them, to see whether their masses gradually change over time.
Only exceedingly rarely have they seen the light of day since 1889, when they were taken out on a very few occasions to check whether other master kilograms that nations around the world use were still accurately calibrated, give or take the mass of a dust particle or two.The metal kilo is being replaced by a definition based on Planck’s constant, which is part of one of the most celebrated equations in physics but also devilishly difficult to explain.Suffice to say that the updated definition will, in time, spare nations the need to occasionally send their kilos back to France for calibration against the “Grand K.” Scientists instead should be able to accurately calculate an exact kilo without having to measure one lump of metal against another.