  In nonlinear electrodynamics, by implementing the causality principle as the
requirement that the group velocity of elementary excitations over a background
field should not exceed unity, and the unitarity principle as the requirement
that the residue of the propagator should be nonnegative, we find restrictions
on the behavior of massive and massless dispersion curves and establish the
convexity of the effective Lagrangian on the class of constant fields, also the
positivity of all characteristic dielectric and magnetic permittivity
constants. Violation of the general principles by the one-loop approximation in
QED at exponentially large magnetic field is analyzed resulting in complex
energy tachyons and super-luminal ghosts that signal the instability of the
magnetized vacuum. General grounds for kinematical selection rules in the
process of photon splitting/merging are discussed.
