The Kilogram Will Never Be What It Was

Before Monday, a kilogram equaled about 2.2 pounds. After Monday, it equals ... well, about 2.2 pounds. But Monday also marks a scientific milestone for the metric mass—from now on, it will be measured differently, more precisely. No longer will be it based on an actual physical object, one that has been kept in a climate-controlled vault for more than a century. Instead, it will be based on a constant of physics. The details:

• The old: Since 1889, the kilogram has been defined by Le Grand K, or the International Prototype Kilogram, which is literally a piece of metal kept in a vault at the International Bureau of Weights and Measures in Sevres, France, reports Vox. Replicas of the platinum-iridium alloy exist around the world, all used to calibrate scales and such and to make sure that a kilogram is, in fact, a kilogram.
• The problem: Physical objects change, no matter how carefully handled. Even air particles can affect things. Scientists began detecting minute differences between the mass of the prototype and the replicas—a difference approximately equivalent to the weight of an eyelash, explains CNN.

• A big deal: "This is fine when it comes to measuring a bag of sugar, but is becoming unacceptable for more sophisticated science, such as when measuring doses in pharmaceuticals," says a statement from the National Physical Laboratory, which houses a replica in Britain.
• The new way: From now on, the kilogram will be based on the Planck constant, ensuring that it never changes. You might need to bust out a science textbook for a fuller understanding. Here is Vox: "The Planck constant is a concept in quantum mechanics (i.e. the study of how the tiniest components of the universe works), which describes how the tiniest bits of matter release energy in discrete steps or chunks (called quanta). Basically, you can think of the Planck constant as the smallest action an electron can take." And here is Vice: "Defined as 6.626 x 10−34 joule-seconds, the constant fixes the kilogram to the speed of light and a temporal unit of measurement—the second."
• Not moving: So what happens to Le Grand K now? It will remain in its vault, under two bell jars, for posterity, per Science Alert. For the record, similar changes will be applied to the ampere, kelvin, mole, and candela. That is, they will be defined by physical and atomic properties, not actual objects.

(More kilogram stories.)