Change for a Unit Humanity Uses to Measure Its World

The kilogram is getting an update. No, your bathroom scales won't suddenly become kinder and a kilo of fruit will still weigh a kilo. But the way scientists define the exact mass of a kilogram is about to change. Until now, its mass has been defined by the granddaddy of all kilos: a golf ball-sized metal cylinder locked in a vault in France. For more than a century, it has been the one true kilogram upon which all others were based. No longer. Gathering in Versailles, west of Paris, governments are expected on Friday to approve plans to instead use a scientific formulation to define the exact mass of a kilo. The change is expected to have practical applications in industries and sciences that require ultra-precise measurements of mass. An in-depth explainer from the AP:

• The move will mean redundancy for the so-called Grand K, the kilo that has towered above them all since 1889. Made of a corrosion-resistant alloy of 90% platinum and 10% iridium, the international prototype kilo has rarely seen the light of day. Three different keys, kept in separate locations, are required to unlock the vault where the Grand K and six official copies—collectively known as "the heir and the spares"—are entombed together under glass bell-jars at the International Bureau of Weights and Measures, in Sevres on the western outskirts of Paris.

• Founded by 17 nations in 1875 and known by its French initials, the BIPM is the guardian of the seven main units humanity uses to measure its world: the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance, and the candela for luminous intensity.
• Of the seven, the kilo is the last still based on a physical artifact, the Grand K. The meter, for example, used to be a meter-long metal bar but is now defined as the length that light travels in a vacuum in 1/299,792,458th of a second.
• "This, if you like, is a moment of celebration because it's like the last standard remaining from 1875 that will finally be replaced by new innovation," Martin Milton, the BIPM director, said in an AP interview. "That's a tribute to what was done in 1875, that it's lasted this long."
• Although many Americans commonly think of weight in pounds and ounces, the United States is officially a kilo country, too: It was one of the original 17 founders of the BIPM in 1875. The United States' primary kilo is called K20 and was assigned to the country in 1889 by the BIPM, along with another, K4. One kilo is equivalent to 2.2 pounds.
• To verify their mass, K20 and other kilos from around three dozen other countries were measured in Sevres against the BIPM's master kilos in a painstaking calibration exercise from 1988 to 1992. K20 was most recently then measured again at the BIPM in 2014.
• 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 update should, in time, spare nations the need to occasionally send their kilos back to Sevres for calibration against the Grand K. Scientists instead should be able to accurately calculate an exact kilo without having to measure one precious lump of metal against another.

(A 5th-grade exam question recently left the internet stumped.)