2008 Keynote Addrees Dr. Richard Davis, BIPM

Why the kilogram should be redefined

One of these days, perhaps in 2011, the decision will be made to redefine the kilogram. When this happens, the unit of mass will no longer be derived from an artifact maintained and used at the International Bureau of Weights and Measures, putting an end to a remarkable 120-year history. Why has an artifact manufactured in the 19 th century served so long, and why is it no longer deemed adequate by a number of measurement scientists? These are interesting questions that I hope to answer.

The artifact in question is known as the international prototype of the kilogram. It is a cylinder of platinum-iridium alloy. The present definition of the kilogram could not be more straightforward: a kilogram is equal to the mass of the international prototype. The idea is simple and the artifact is handsome. This is a definition based on Newtonian physics and, of course, predates the scientific revolutions of the 20 th century. The very simplicity of the current definition of the kilogram makes it the paradigm for an SI unit definition that everyone immediately understands. As metrologists, we also readily understand why the international prototype (or indeed any artifact) can never be a perfect standard. The general public is often less discerning, leading to waves of sensationalized stories circulating in cyberspace and the print media.