The international kilogram conundrum

From the LA Times - Read the Full Story by clicking on the Link, Article by Jia-Rui Chong, Los Angeles Times Staff Writer April 17, 2008

GAITHERSBURG, MD. -- 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.

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International Bureau of Weights and Measures

A copy of the international prototype kilogram under its protective casing. The real prototype, known as Le Grand K, is rarely removed from its temperature- and humidity-controlled vault to avoid damage.