  We examine the recent statement by A. Dolgov and I. Novikov that superluminal
propagation of light, which is induced by the quantum corrections to the photon
propagation in gravitational field, permits non-causal signal propagation. For
this purpose we examine the possibility of the existence of non-causal signals
in the model case, when characteristic cone of the signal is outside the usual
light cone in Minkowski space-time, as it take place in the case of the photon
propagation in gravitation field with quantum corrections taking into account.
In such model the signal propagates along invariant intervals. It is shown that
there are no non-causal signals in the considered model. The recent statement
of A. Dolgov and I. Novikov was obtained by using additional supposition that
velocity of the signal with respect to the emitter is independent from its
motion. Such supposition does not valid in the case of the photon propagation
in gravitational field.
