k$_{GLYPH<UpTee>}$ m GLYPH<LParen1> k$_{0}$$^{2}$$_{GLYPH<Minus>}$k$_{3}$ $^{2}$GLYPH<RParen1> 1 GLYPH<Slash1> 2 sessing different m . The quasi-interceptions, – i.e. the would-be interception of dispersion curves of two states taken as independent within a certain approximation,– would result in the mutual repulsion of the dispersion curves leading to formation of mixed states, polari- tons, an example of which is given by a photo-positronium - a mixed state between a photon and the electron-positron bound state created by it in a strong magnetic field [5, 17–19]. This situation is illustrated by Fig.1. The refraction index squared n 2 a is defined for photons of mode a on the mass shell (23) as It follows from (26) with m = f 1 / $^{2}$(0) = 0 that the refraction index is greater than unity - the statement common in standard optics of media (this is not, certainly, true for (massive) plasmon branches). Consequently, the modulus of the phase velocity in each mode v ph a = ( k$_{0}$/ | k | $^{2}$) k equal to 1 /n$_{a}$ is, for the photon proper , also smaller than the velocity of light in n 2 a ≡ | k | 2 k 2 0 = 1 + k 2 ⊥ − f$_{a}$ ( k 2 $_{⊥}$) k 2 0 . (30) 17 FIG. 1: Disposition of dispersion curves. The lower bold solid line is the massless ( photon ) dis- persion curve restricted from above by the light cone k 2 = 0, presented by the thin solid line. The upper solid bold line, representing a massive branch, cannot pass higher than the straight dotted line, originating from its crossing with the vertical axis according eq. (27). The dashed lines show a quasi-interception.