Jia-Rui Chong in the Los Angeles Times:
Forty feet underground, secured in a temperature- and humidity-controlled vault here, lies Kilogram No. 20.
It’s an espresso-shot-sized, platinum-iridium cylinder that is the perfect embodiment of the kilogram — almost perfect.
In the more than a century since No. 20 and dozens of other exact copies were crafted in France to serve as the world’s standards of the kilogram, their masses have been mysteriously drifting apart.
The difference is on average about 50 micrograms — about the weight of a grain of fine salt. But the ramifications have rippled through the world of precision physics, which uses the kilogram as the basis for a host of standard measures, including force of gravity, the ampere and Planck’s constant — the omnipresent figure of quantum mechanics.
In essence, no one really knows today what a kilogram is.
“How do I trust what I have?” asked Zeina Jabbour, the physicist at the National Institute of Standards and Technology, or NIST, in charge of maintaining No. 20, the official U.S. kilogram.
The kilogram crisis has kicked off an international race to redefine the measure. Instead of using an object, scientists are searching for some property of nature or scientific constant, such as the vibrations of a cesium atom now used to define a second.
More here. [Thanks to Winfield J. Abbe.]