My question here involves Delbruck Scattering specifically but my curiosity is more general. Delbruck Scattering is the scattering of a photon off of the Coulomb field of a nucleus via the creation and annihilation of real and virtual electron-positron pairs. The process can occur at energies below 1.022 MeV, the pair production threshold. I know that Delbruck scattering has real and imaginary scattering amplitudes (Kasten, 1986). My question then is two-fold, 1) What does it mean that a scattering amplitude has a real part and an imaginary part? 2) Does one or the other part (Re or Im) have any bearing on the fact that D-scattering can occur below 1.022 MeV, i.e. at these lower energies I would assume the particles are virtual, does that mean that the real part of the scattering amplitude was smaller than the Imaginary part? Is there any relationship between the realness or virtualness of particles and the realness or imaginariness of the cross section for their production?