Before today, a kilogram equaled about 2.2 pounds. And now, it equals … well, about 2.2 pounds. But today marks a scientific milestone for the metric mass—from now on, it will be measured differently, and more precisely.
As indicated in the video below, 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 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. Replicas of the platinum-iridium alloy exist around the world 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.
- 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. “The Planck constant is a concept in quantum mechanics 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 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. For the record, similar changes will be applied to the ampere, kelvin, mole, and candela which will be defined by physical and atomic properties, not actual objects.