Time delay in electromagnetic scattering

The average time delay in electromagnetic scattering by a spherically symmetric, reflection-invariant, nonabsorbing scatterer is evaluated, extending results previously derived for scalar nonrelativistic scattering. For the lth partial wave in each polarization, the time delay is the spectral average of dη_l/dω over the incident wave packet (where η_l is the corresponding phase shift and ω the angular frequency). This is one-half of the Eisenbud-Wigner nonrelativistic expression. The relationship with angular time delay in a given direction is also discussed.