|| © CAMBRIDGE UNIVERSITY PRESS 1998
In particle physics it is usual to simplify the appearance of equations by using units in which = 1 and c = 1. In electromagnetism we set 0 = 1 (so that the force between charges q1 and q2 is q1q2 / 4r2), and µ0 = 1, to give c2 = (µ00)-1 = 1. We shall occasionally reinsert factors of and c where it may be reassuring or illuminating, or for the purposes of calculation. It is useful to remember that
|c 197 MeV fm,||e2 / 4 1.44 MeV fm,|
|= e2 / 4 c (1/137),||c 3 x 1023 fm s-1.|
Energies, masses, and momenta are usually quoted in MeV or GeV, and we shall follow this convention.