New Definition of the SI Unit Kilogram – Spherical Interferometry as the Limiting Factor

Abstract

Metrology is the science of measurement and its application. Whenever one performs a measurement one compares the measurand to an appropriate unit. The number of these units has changed since the metre convention in 1875 to 1960 when the 11th CGPM (Conference Generale des Poids et Mesures) decided the actual SI (Systeme International d’Unites). Currently the SI is based on seven units – kilogram for mass, second for time, metre for length, ampere for electric current, Kelvin for thermodynamic temperature, candela for luminous intensity and – since 1971 – mol for amount of substance. Their definition also sometimes changes, due to demands on the unit or the advent of new methods for a possible better realisation. The metre, for instance, was incipiently defined as a part of a meridian of the earth. This definition was impractical to realise, so for the realisation the length between the ends of a platinum bar was used. To overcome the difficulties of the handling and measurement of this length standard, the definition was again revised to the distance between two marks on a new, x-shaped Pt-Ir rod. Although the measurement capabilities grew and even though interferometry was discovered early, the next redefinition took two ages. Eventually, in 1960 the metre was defined by a multiple of the wavelength of the radiation of 86krypton. And since 1983, for the present finally, the metre is defined by a time-of-flight definition: one metre is the distance which light travels in 1/299792458 seconds where this special number is given by the fixed value of the speed of light.