The unbearable lightness of … weight

While most units of measurement have been converted into reproducable formulae — a meter is the distance light travels in a vacuum in 1/299,792,458 of a second, a second is the time it takes a cesium atom to vibrate 9,192,631,770 times, etc. — the venerable kilogram is still measured based on a cast platinum-iridium cylinder, dating back to 1889 (that being more reliable than the previous measure, the mass of a liter of water).

Problem is — that benchmark measure is changing. The reference kilo is now 50 micrograms lighter than it used to be. Nobody’s sure why, but it’s upsetting a lot of scientists, who are rushing to come up with new, high-tech ways of defining the weight.

The final recommendation will be made by the International Committee on Weights and Measures, a body created by international treaty in 1875.
The agency guards the international reference kilogram and keeps it in a heavily guarded safe in a chateau outside Paris. It is visited once a year, under heavy security, by the only three people to have keys to the safe. The weight change has been noted on the occasions it has been removed for measurement.
“It’s part ceremony and part obligation,” said Dr. Richard Davis, head of the mass section at the research arm of the international committee. “You’d have to amend the treaty if you didn’t do it this way.”

Current competing plans for the new measurement include creating a spherical silicon crystal of a certain number of atoms (Germany, Australia, Italy and Japan), and measuring the electromagnetic force needed to balance a reference kilogram under a particular gravity (US, UK, France, Switzerland).

One interesting side note in all of this is the history of those reference kilogram weights.

A total of 80 copies of the reference kilogram have been created and distributed to signatories of the metric treaty. The sometimes colorful history of these small metal cylinders underscores how long the world has used the same definition of the kilogram.
Some of the metal plugs were issued to countries that later vanished, including Serbia and the Dutch East Indies. The Japanese had to surrender theirs after World War II. Germany has acquired several weights, including the one issued to Bavaria in 1889 and the one that belonged to East Germany.

And in case you’re an American patting himself on the back that we’re not subject to such odd whims of mass … be aware that the official definition of the pound is 0.45359237 kilograms (the relation to the kilo dates back to 1893).