A small cylinder called Le Grand K has defined the kilogram for more than a hundred years.
Le Grand K was forged in 1879 and is held in a locked vault outside Paris—revered and kept under lock and key because its mass, a little over 2 pounds, is the official definition of the kilogram.
If Le Grand K gets heavier or lighter—or absorbs atoms of something from the air—the definition of the kilogram literally changes.
Bill Phillips, a Nobel laureate from the National Institute of Standards and Technology in Maryland, told the assembled delegates at the international General Conference on Weights and Measures that basing the kilogram's official definition on a hunk of metal held in a vault was "a situation that is clearly intolerable." His colleagues clearly agreed, as no country voted to keep the old definition.
Going forward, the world's system of mass measurement will not be based on some special hunk of metal, but rather on unalterable features of the universe—such as the speed of light, time and Planck's constant, a number that helps scientists figure out the energy of a photon of light, given its wavelength.
The new definition of the kilogram will go into effect in May 2019. Meanwhile, Le Grand K will remain in its lonely vault.