the kilogram goes on a diet

Having trouble keeping the kilograms off? Simple! Just redefine the kilogram!

The 118-year-old cylinder that is the international prototype for the metric mass, kept tightly under lock and key outside Paris, is mysteriously losing weight – if ever so slightly. Physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris, says the reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies.

Somewhere, an evil marketing genius is cackling.

In all seriousness, this is actually a genuine problem, because unlike most of the other units, the kilo is still defined in terms of a physical entity rather than a natural process. The second is defined using the ground state frequency of the Cesium atom, and the meter according to the speed of light. However, the Kg is still defined as the mass of that cylinder in France, so your bathroom scale literally becomes incorrect if that cylinder’s mass should vary appreciably. Its the scale, not the cylinder, that’s wrong. The Wikipedia entry is highly informative about the consequences for all the “downstream” measures like energy, work, etc that are dependent on the Kg.

Of course, there are proposals for alternative definitions of the Kg in the works. The case of the mysteriously dieting cylinder has only underlined the essential need for an artifact-independent definition. One side benefit of this could be to “round off” another fundamental universal constant in SI units. For example, the definition of the meter fixes the speed of light to an integer number of meters per second. Analogously, one redefinition scheme for the kilogram uses the Watt balance method which would relate the Kg to the Planck constant. This would set the value of h equal to h = 6.626 068 96 × 10–34 precisely. There are other proposals as well; it’s kind of fascinating reading.