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1.10 Units

In particle physics it is usual to simplify the appearance of equations by using units in which hbar = 1 and c = 1. In electromagnetism we set epsilon0 = 1 (so that the force between charges q1 and q2 is q1q2 / 4pir2), and µ0 = 1, to give c2 = (µ0epsilon0)-1 = 1. We shall occasionally reinsert factors of hbar and c where it may be reassuring or illuminating, or for the purposes of calculation. It is useful to remember that

hbarc approx 197 MeV fm, e2 / 4pi approx 1.44 MeV fm,
alpha = e2 / 4 pihbar c approx (1/137), c approx 3 x 1023 fm s-1.

Energies, masses, and momenta are usually quoted in MeV or GeV, and we shall follow this convention.