Abstract:
It is proved that, in nonlinear electrodynamics, the velocity of propagation of a small timedependent disturbance (which can be assumed to be a signal) in an external field depends on the strength of the field and can exceed the velocity of light in a vacuum. This implies that,
in an external field, the order (in time) of the emission and reception of a signal can depend
on the reference system if the usual Lorentz-transformation formula holds. The possibility
is considered of avoiding this contradiction by the introduction of a space-time metric dependent
on the field or by the introduction of a new selection principle permitting the exclusion
of nonlinear Lagrangians which allow signal velocities greater than the velocity of light.