What interaction causes the photon to slow down?

[mikehelland]

"So you think photons slow down, which implies they change invariant mass. Basically they obtain restmass, they become a different kind excitation in terms of QFT. And we know these have to interact with ordinary matter, we can see them in telescopes. But if you know anything about QFT, you know if you rotate a feynman diagramm you also get a valid feynman diagramm, and ammong other things due to a 90° rotation absorbtions can become emissions. So the question is, if there is interaction with these slow photons, why didn't we see these slow photons created in particle physics experiments?"

[mikehelland]

nothing causes the redshifts, they are not an effect of something; they are fundamental to nature, their own cause.