Smithsonian.com

The Denmark-owned replica of the kilogram standard (via wikipedia)

Here’s an easy question: What is a kilogram?

A. 1000 grams
B. a standard unit of mass (often ignored in the United States)
C. a platinum-iridium cylinder kept in a vault in Sèvres, France
D. all of the above

The answer is D, of course. And that’s a problem for the scientists in charge of the science of measurement. You see, that little cylinder hidden in its French vault keeps changing. When scientists compare the mass of the 130-year-old International Prototype Kilogram (as it is officially known) with that of replicas made from the same material and stored in similar conditions around the world, they don’t match up. Those little cylinders are likely undergoing material decay, but since the IPK is officially a kilogram no matter how it compares to its replicas, there’s no way to tell what is going on. And no way to know what a kilogram really is.

It’s a conundrum scientists need to solve because the kilogram is one of a few base units (like the second and the meter) that are used in the definitions of other, more complex units, such as those used to measure temperature, electricity or density. If the kilogram is off, even a little, critical measurements in science, engineering and commerce get messed up.

The kilogram is the only standard that has this problem; years ago scientists replaced other standards based on physical objects, like the meter, with definitions based on fundamental constants in nature, such as the speed of light. And there have been plenty of potential replacements proposed for the kilogram over the years (Wikipedia has a good list; my favorite is the shiny sphere of pure silicon).

The U.S. National Institute of Standards and Technology is backing a definition based on the Planck Constant, a number from quantum mechanics. It’s a more complex definition than a simple cylinder, but it’s unlikely to change over the next century.

NIST submitted its definition last month to the International Committee for Weights and Measures, one of the advisory bodies that oversees such things, and it approved the change. The matter will next be taken up in a resolution by the General Conference on Weights and Measures, the international group that has the power to replace the kilogram, at its next meeting, in October 2011. If the resolution passes, the definition could change by the end of the decade.

“This is actually an advance in the science of measurement,” NIST spokesman Ben Stein [told CBS News]. “This is the next phase. There have been discoveries in measuring the constants of nature, such as the speed of light, the amount of energy in a quantum of energy….we know them pretty well now. These numbers are not changing, so why not define the units in terms of these constants? They’re from the natural world.”

(HT: Geeks Are Sexy)