To do physical science, one needs uniform references for fundamental quantities: length, duration, mass, and so forth. Over time, as the need for accuracy has increased, attempts have been made to place the fundamental units on ever more precise footing. For example, the reference meter, which was declared in 1791 by the French Academy of Sciences to be one ten-millionth of the distance between the North Pole and the Equator along a meridian running through Paris†, is now taken to be “the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.”
The kilogram, however, has to this day been referred to the mass of the International Prototype Kilogram, a platinum-iridium cylinder that resides in a chateau in Sevres, France. It is the only SI unit that is still defined by reference to an artifact.
In a puzzling turn of events, however, it appears that this master kilogram — the kilogram — is, for some inexplicable reason, losing mass.
Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.
“The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart,” he said.
“We don’t really have a good hypothesis for it,” Davis said in a phone interview Wednesday.
While 50 micrograms isn’t much — it certainly won’t get you far along your way to that Life Membership at Weight Watchers — in the rarefied world of international physical standards, it’s a pretty big deal, especially in light of the fact that nobody knows why it’s happening.
Learn more here.
- † By the way, the measurement of this distance by a seven-year surveying expedition undertaken by Jean Baptiste Joseph Delambre and Pierre François André Méchain is a fascinating story, and is told in the recent book The Measure of All Things, by Ken Alder.