but only two, as it should be for a photon. The point is that the massless branch of the dispersion curve for mode 1 does not correspond to any real elementary excitation, except for two special cases. It follows from (9) and (17) that κ 1 | k$_{µ}$ =0 = k $_{2}$∂ L ( F , G ) ∂ F . (29) Hence the light cone k 2 = 0 is a guaranteed solution to the dispersion equation (14) for mode 1 in the vicinity of the origin k$_{µ}$ = 0. Can there exist massless excitations in mode 1 other than k 2 = 0? The answer is ”no”, because from (29) it follows that k 2 = 0 is the only possibility for a dispersion curve of mode 1, as it approaches the origin k$_{µ}$ = 0. Now, from eq. (8) it is seen, that both electric and magnetic fields in mode 1 disappear at k 2 = 0 , since on this mass shell the elementary excitation is pure gauge, unless either k $^{2}$E2 − kF $^{2}$k ∼ k 2 ⊥ = 0 or k $^{2}$B2 + kF $^{2}$k ∼ k 2 3 − k 2 0 = 0, in which cases the common factor k 2 can be normalized out from ♭ (1) µ (8). These exceptional cases propose kinematical conditions for degeneracy of the polarization tensor to be discussed in Section V. For the first of them, the one of parallel propagation, k 2 ⊥ = 0 , the mode-1 photon is actual, while, on the contrary, mode-2 photon no longer exists, the mode-2 excitation becoming massive, as it is argued below in Section V. The overall number of massless degrees of freedom, therefore, is again two. Note that although k$_{⊥}$ = 0 may seem to be an isolated point, as a matter of fact this is not the case: every nonparallel propagation k$_{⊥}$ = 0 reduces to perpendicular propagation k$_{3}$ = 0 by a Lorentz boost along axis 3 , which does not lead us out of the special frame. In the second exceptional case, k 2 3 − k 2 0 = 0, again the mode-1 photon is actual, but the mode-3 photon does not exist according to Section V. So the number of massless degrees of freedom is two in this case, too. We concluded above in this Section that the causality requires that in the plane ( √ k 2 0 − k 2 $_{3}$, k$_{⊥}$ ) the photon dispersion curves ( m = 0) are located outside or coincide with the light cone: k 2 ≥ 0. (Remind that the light cone k 2 = k 2 − k 2 0 = 0 is the mass shell of a photon in the vacuum without an external field.) However, unlike the case, indicated below eq. (25), a violation of this ban would not lead to a complex-energy ghost or directly signalize the vacuum instability, but would mean a presence of superluminal wave, known as tachyon. On the other hand, massive branches of the dispersion curves as restricted by the condition (27) with m = f 1 / $^{2}$(0) = 0 may well cross the light cone and pass to its exterior. They may even quasi-intercept with the massless (photon) branches or with branches pos- 16