Weighing up the New Kilogram

by David Bradley

The kilogram is the SI unit of mass, explained Dr Richard Davis of the Bureau International des Poids et Mesures. Its definition dates from 1889 and is based on the "international prototype" standard for the kilogram made from an alloy of platinum and iridium deemed to weigh exactly one kilogram. The problem with this object is that its mass can change when referred to an invariant of nature such as the mass of a carbon atom, due to wear and tear, surface chemistry (adsorption of oxygen on the surface), and other physico-chemical effects.

This is an extraordinary situation in which to find science as the SI values certain fundamental physical constants (the Planck constant, the Newtonian constant of gravitation, the mass of the electron, the mass of an atom of carbon-12, etc) are traceable to this manufactured artefact of the nineteenth century which pre-dates the whole of "modern" physics, added Davis. Having said that, Davis did emphasise that the best mass measurements have a precision of one part in a billion with the international prototype but this great precision reveals that, over the course of a century, the international prototype may have been getting lighter! With respect to what? asked Davis, but lighter in the sense that there is a small drift away from the average mass of the copies of the international prototype. What we really need to know is what is happening to the kilogram with respect to the fundamental constants.

Actually, scientists have for many years been searching for a new definition that would be referenced to some true experimentally derived invariant which could ultimately be determined with far greater precision, rather than having to ensure the validity of the international prototype or one of its official copies. At present, however, these experiments are unable tell us anything useful about the possible drift of the international prototype with respect to physical constants. As Davis pointed out there has been resistance to change as a re-definition for this reason. A redefinition of the kilogram based on fundamental physical constants will ultimately provide modern physics with a modern definition.

Indeed, it is only possible with this modern understanding of matter that such a definition of the kilogram is possible, after all nineteenth century scientists were not yet sure of the nature of matter and even the existence of atoms. There are many advantages to a new definition of the kilogram on the basis of reference to one of two fundamental constants; either the Planck constant which relates the energy of a photon to its frequency or the Avogadro constant which defines the number of particles in a mole of any substance.

One of the biggest advantages is that it will lead, by definition, to a significant reduction in the uncertainty of the SI values of many of the other fundamental constants, such as the electron's charge and mass. However, the Planck and Avogadro constants are related to each other by the laws of physics and the present experimental determinations of these two constants disagree by about 1000 parts per billion. This begs the question of whether moving to a new definition before the present discrepancy is resolved will create new problems. Davis, in his optimistic moments, sees a resolution of the issues in a few years.

Read on... G Force