There are about a dozen quantities in atomic and nuclear physics that are fundamental to our current understanding of science. One is the speed of light and another is the fine-structure constant, which governs the spectra of atoms. A third is the mass of the lightest atomic particle – the electron.
The atomic energy scale is related to our everyday world through the equivalence of the electron’s mass and energy, E = mc2. However, while the speed of light, c, is a fixed constant by definition – it has a value of exactly 299 792 458 m s1 – the mass of the electron is not. The mass standard in the atomic world is set by one atom of the element carbon, which is defined to be 12 atomic mass units (about 2 x 1026 kg). All other masses in the micro-world are related to this mass standard by sophisticated experiments.
In the June issue of Physics World, Wolfgang Quint, of the GSI laboratory in Dramstdt describes how theorists are leading experimentalists in a friendly race to pin down the mass of the electron.
