I was reading about pair-instability supernovae, which is when gamma rays produced in the core of a large star become energetic enough to produce electron-positron pairs, which causes a loss of radiation pressure.
Am I correct in assuming that if a gamma ray of sufficient energy were traveling through free space it would oscillate between being a gamma ray and being a (perhaps virtual) electron-positron pair? Would the time it spends in the latter form cause it to take longer to get from point A to point B than expected from the speed of light?
According to this question Is pair production only with $\gamma$ photons? it requires another body nearby such as an atomic nucleus to exchange momentum with. OK - so suppose the gamma ray is traveling through space, but there are a few atoms per cubic meter of ionized gas. Would the effect occur then? Is the result different than the speed of light through the medium or the effect through scattering? (I.e. difference between actually bouncing off atoms vs. passing near atoms.)