In a vault beneath a 17th-century pavilion on the outskirts of Paris sits a platinum cylinder known as Le Grand K. Since 1889 it has been the international prototype for the kilogram, the standard against which all other kilos are measured.So it has changed mass by about 50 parts per billion. That's appreciable in the world I live in, where we routinely measure quantities in parts per trillion and sometimes even parts per quadrillion. Not only that, but the kilogram is used to define a number of other units, the joule, watt, volt, farad, weber and ohm a few of the units derived in part from the kilo. It also changes the estimated mass of the universe by about 10 to the 45th power kilograms. Quite a weight loss program.
But over the years, scientists have noticed a problem: Le Grand K has been losing weight. Weigh-ins at the International Bureau of Weights and Measures show that the bar has shed approximately 50 micrograms—roughly equal to a grain of sand...."It's a scandal that we've got this kilogram hanging around changing its mass and therefore changing the mass of everything else in the universe!" Bill Phillips, a Nobel Prize winning physicist, exclaimed at a scientific summit in London this week. No one knows for sure what went wrong with Le Grand K, but some theorize it lost weight from being cleaned.
How to fix it?
The aim is to tie each to a widely accepted property of nature, rather than to a lump of metal or some other imprecise benchmark. The meter, for instance, was once measured as the distance between two notches on a metal bar. It is now defined as the distance light travels in a vacuum in 1/299,792,458 of a second.The logical choice is Planck's constant which ties mass, time and distance together, and has been measured to eight decimal places (about a tenth of the accumulated error of the kilo).