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.
In the 18th century, hundreds of thousands of different weights and measures were in use around the world. The French alone employed about 250,000 different units of measure.
The Enlightenment and the French Revolution spurred the idea of standardization, said Ken Alder, a historian at Northwestern University in Evanston, Ill. People could only be free if they could calculate for themselves the weight and cost of things they bought, philosophers reasoned.
The French government created the kilogram in 1795, defining it as the mass of a liter of distilled water at the temperature of melting ice. A century later, the Treaty of the Meter established the kilogram as an international standard.
The foundation of the standard was a cylindrical ingot of 90% platinum and 10% iridium created in 1878 that became known as Le Grand K, or more officially the International Prototype. Forty copies were made and distributed a decade later to governments around the world. Another 50 were made later.
These 90 copies serve as national standards, used to calibrate working weights in science and industry.
About every 50 years, the national prototypes are returned to the headquarters of the International Bureau of Weights and Measures in Sevres, France, to be compared with the International Prototype. During the first major comparison about 1950, scientists noticed discrepancies between the average masses of Le Grand K and its copies. They were concerned but could not discern a trend.
Science was already grappling with inconsistencies in other units and was trying to replace the pieces of metal and other artifacts that delineated the old world. The meter, for example, was changed in 1960 from two scratches on a platinum-iridium bar to a certain number of wavelengths of light emitted from a particular kind of krypton. In 1983, it was changed again to the distance traveled by light in a specific fraction of a second.
At the last major kilogram comparison done in and around 1990, some copies had gained as much as 132 micrograms. A few had lost up to 665 micrograms. The United States' No. 20 was 18 micrograms heavier.
There was no way to tell which was changing: Le Grand K, its copies or both.
Perhaps the platinum in the cylinders was sopping up mercury from the atmosphere. Maybe dissolved gas was escaping from the cylinders. One idea was that cleaning the cylinders with distilled water and ether had altered their weights.
"Nobody has a really good idea why," said Davis of the International Bureau of Weights and Measures. "It's all speculation."