Kilogram question weighs on world

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.

"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.

The kilogram is the last of seven base units in the International System of Units that is still based on a physical object, a remnant of the era before relativity and quantum mechanics transformed our understanding of the universe.

"The real problem is, people in other areas of science don't want to measure, say, fundamental constants with respect to this artifact made in the 19th century," said Richard Davis, head of the mass section at the International Bureau of Weights and Measures in France.

Besides, Jabbour added: "You could drop it."

In fact, at least six copies of the kilogram have been lost or damaged over the years from war, clumsiness or other reasons.

Two ideas have emerged as the leading contenders to redefine the kilogram. One involves counting the trillion trillion atoms in the most perfect silicon sphere ever made. The other attempts to measure the electrical current necessary to balance a one kilogram weight against Earth's gravity.

Serious complications ensnare both approaches.

"We're running into the wall of measurement," said Richard Steiner, the physicist heading NIST's effort to define a new kilogram.