Text 2. The century-old artifact that defines the kilogram, the fundamental unit of mass, is to be replaced by a more accurate standards based on an invariant property of nature

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Weighty Matters. By: Robinson, Ian, Scientific American, 00368733, Dec2006, Vol. 295, Issue 6

In an age when technologies typically grow obsolete in a few years, it is ironic that almost all the world's measurements of mass (and related phenomena such as energy) depend on a 117-year-old object stored in the vaults of a small laboratory outside Paris, the International Bureau of Weights and Measures. According to the International System of Units (SI), often referred to as the metric system, the kilogram is equal to the mass of this “international prototype of the kilogram” (or IPK) – a precision-fabricated cylinder of platinum-iridium alloy that stands 39 millimeters high and is the same in diameter.

The SI is administered by the General Conference on Weights and Measures and the International Committee for Weights and Measures. During the past several decades the conference has redefined other base SI units (those set by convention and from which all other quantities are derived) to vastly improve their accuracy and thus keep them in step with the advancement of scientific and technological understanding. The standards for the meter and the second, for example, are now founded on natural phenomena. The meter is tied to the speed of light, whereas the second has been related to the frequency of microwaves emitted by a specific element during a certain transition between energy states.

Today the kilogram is the last remaining SI unit still based on a unique man-made object. Reliance on such an artifact poses problems for science as measurement techniques become more precise. Metrologists (specialists in measurement) are therefore striving to define mass using techniques depending only on unchanging properties of nature. Two approaches seem most promising – one based on the concept underlying the Avogadro constant, the number of atoms in 12 grams of carbon 12, and the other involving Planck's constant, the fundamental value physicists use, for example, to calculate a photon's energy from its frequency. Because scientists measure constants in SI units (including the kilogram), any drift in the IPK's real mass will give rise to a drift in the value of a measured constant – a seeming paradox for what is commonly considered an immutable phenomenon. In the process of more accurately redefining the kilogram independently of the IPK, however, scientists will choose a best estimate of the constant's value and thus “fix” it.

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