Newswise — Once a year, three officials bearing three separate keys meet at the bottom of a stairwell at the International Bureau of Weights and Measures, in Sevres, France. There they unlock a vault to check that a plum-size cylinder of platinum iridium alloy is exactly where it should be. Then they close the vault and leave the cylinder to sit alone, under three concentric bell jars, as it has for most of the past 125 years.

This lonely cylinder is the International Prototype of the Kilogram (IPK), known colloquially as Le Grand K, and it is the last remaining physical object to define a unit of measure. The mass of the IPK is used to set balances around the world, but there's one problem: its mass is changing.

Now, after decades of work, two challenging strategies that can link mass to fundamental, unchanging properties of nature are finally getting precise enough to replace the IPK. If all goes well, the cylinder will soon begin a well-deserved retirement.

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