After 129 years, the world’s model kilogram retires

The humble kilogram got an unprecedented makeover as a global gathering of scientists in Versailles redefined the unit of mass.For 129 years, a cylinder made of platinum and iridium, about 39mm high and 39mm across, has served as the internationally recognised standard kilogram.The International Prototype Kilogram – its official title – has been held in the offices of the International Bureau of Weights and Measures (BIPM) in the Paris suburb of Sevres since 1889.It is jealously guarded: Three officials who hold keys must all be present to open the safe in which it is held.It will finally retire.Scientists meeting at the General Conference of Weights and Measures in Versailles voted on a proposal redefining the kilogram in terms of a physical constant called the Planck constant.Representatives of more than 50 countries at the General Conference on Weights and Measures unanimously approved new definitions for the International System of Units, known as SI from its French acronym.With effect from May 20 next year, the kilogram will be defined on the basis of a calculation from Planck’s constant, a fundamental constant of quantum physics that expresses a relationship between energy and the frequency of subatomic particles such as photons.The new definition brings the kilogram in line with the metre, which up until 1960 was based on a prototype metal bar, but is now based on the speed of light.Three of the other seven base units in the SI system – the mole, which measures amount of substance; the kelvin, which measures temperature; and the ampere, which measures electric current – are also redefined based on fundamental physical constants.The candela, measuring light intensity, and the second, the base unit of time, were already based on constants.“Today will mark a turning point in the definition of the units of the international system,” Sebastien Candel, president of the French Academie des Sciences, said as he opened the session that approved the change.Scientists say the new standards will be more stable as well as enabling more precise measurements in domains ranging from pharmaceutics to climate change research.