In a basement vault near Paris, under a series of nested bell jars, sits a one kilogram platinum-iridium cylinder. Even as it wears away or collects dust particles, it is still exactly one kilogram. That’s because it is the kilogram: “Le Grand K.” For more than 100 years, it has been the international standard for the unit of mass, even though its mass has changed by 50 micrograms in that time. This has caused headaches for certain industries, like manufacturing, that rely on precise mass measurements. When units are defined based on physical objects, it’s much harder to be certain of the value.
On November 16, representatives from 57 countries voted to make the kilogram a less fickle standard. Going forward, it will be defined based on Planck’s constant, a fixed value relating the amount of energy in a photon of light to the light’s wavelength (a physical property of light that we perceive as its color). Planck’s constant won’t change—and now neither will the kilogram. We won’t see the effects of this new definition on our bathroom scales, but it means that precision-based industries like pharmaceuticals and manufacturing can breathe a sigh of relief. This is not the first time a unit has been redefined. For example, the second used to be defined based on the Earth’s revolutions, but in 1967 it became calibrated against the timescale of energy release by atoms of cesium-133. It’s still hard to measure, but more precise.
This time around, the kilogram isn’t the only standard unit being shaken up. The amp (electrical current), the mole (quantity of particles), and the kelvin (temperature) will also be redefined using universal constants. Being able to redefine units is a result of technological improvement (permitting more precise measurements of these constants), and it will enable more precise technologies in the future. For example, redefining units of time and distance in the ’60s allowed GPS to be better. The new kilogram could push the frontiers of science even further.
Managing Correspondent: Aparna Nathan
Original article: A Turning Point for Humanity: Redefining the World’s Measurement System – National Institute of Standards and Technology
Media coverage: A massive change: Nations redefine the kilogram – The Washington Post
Image credit: Japs_88/Wikimedia Commons