A KILO OF CRISIS

In the 120 years since No. 20 and several dozen other exact copies were crafted in France to serve as the world’s standards of the kilogram, they have been mysteriously drifting apart.

The difference is on average about 50 micrograms—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, from the force of gravity to the ampere to 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 US 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.

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 ensare both approaches. “We’re running into the wall of measurement,” said Richard Steiner, the physicist heading NIST’s effort to define a new kilogram.

The French government created the kilogram in 1795, defining it as the mass of a litre of distilled water at the temperature of melting ice. The foundation of the standard was a cylindrical ingot of 90 percent platinum and 10 percent iridium created in 1878 that became known as Le Grand K, or more officially the International Prototype. Forty copies were made and distributed to governments around the world. Another 50 were made later.

These 90 copies serve as national standards. 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. .

At the last major kilogram comparison about 1990, some of the copies had gained as much as 132 micrograms. A few had lost up to 665 micrograms. The US’s No. 20 was 18 micrograms heavier. There was no way to tell what was changing: Le Grand K or its copies.

Steiner’s idea is to tie the kilogram standard to a constant in quantum mechanics known as Planck’s constant. If scientists know the current, the strength of the magnetic field and Planck’s constant they can accurately determine the mass of an object.

“We are basically creating a calibrated bathroom scale,” Steiner said.

The problem with Steiner’s watt balance is that it can be finicky. Distant earthquakes, motors from nearby office buildings and tides have shaken up the measurements, he said. There are about 20 potential sources of error, including the buoyancy of air, electrical current leaks and the changes in local gravity.

Peter Becker, a 63-year-old physicist at Germany’s National Metrology Institute, thinks he has a simpler solution—count atoms. The idea is to define the new kilogram as the number of atoms of a specific element. Becker’s hopes ride on two silvery croquet-ball-sized spheres of the purest silicon that cost $3.2 million to make. They are the roundest objects ever made—within 30 nanometers of perfection, about the width of a few atoms, he said.

Since it’s impossible to count atoms individually, Becker’s group, known as the International Avogadro Project, used X-rays to determine the spacing of atoms in an object. Once that was known, and the volume of an object determined using precise laser measurements, they could derive the number of atoms — about 20 trillion trillion for a kilogram of silicon.

After decades of work, both efforts have so far produced stunningly precise measurements — but still too inconsistently to prove the accuracy of their methods. Steiner’s latest calculation of uncertainty is less than the 50 microgram difference between the kilogram copies and Le Grand K. But a few months ago, a British team used a watt balance of their own design to generate a value that was significantly different. Neither lab can explain the gap.

The disagreement has given Becker’s atom-counting effort some hope. “Our goal is to reduce (the uncertainty) by a factor of 10 or better,” Becker said. “We are convinced we can reach it.”
-Jia-Rui Chong (Los Angeles Times)