The Planck constant, the ratio of energy to frequency in quantum mechanics, is one of the fundamental constants of physics, along with the speed of light and the charge on the electron. Many basic units in science - such as the second and the metre - are defined in terms of these fundamental constants, but others are still defined in terms of a physical artefact. The kilogram is the mass of a piece of platinum/iridium alloy stored in a vault in Paris. This may soon change, however, following improved measurements of the Planck constant.
So how can the unit of mass be defined in terms of the Planck constant? The link is made through energy, the most fundamental entity in our present theoretical model of the physical world. Energy and mass are linked by Einstein's equation, E = Mc2, where c is the speed of light, and energy and frequency are linked by E = hn, where h is the Planck constant and n is the frequency. Therefore it seems logical to try to define mass in terms of h, just as the metre is defined in terms of c.
Recently, Edwin Williams and colleagues at the US National Institute of Standards and Technology (NIST) in Gaithersburg have succeeded in carrying out this measurement by making the most precise measurement of Planck's constant yet reported.
Bryan Kibble and Ian Robinson from the National Physical Laboratory in Teddington, UK describe the latest results in the December issue of Physics World magazine (information).