[akomtu]

That's the model of photon, not the photon itself. A mathematical photon cannot decay. What prevents a photon from hitting a quantum bump on its way between galaxies and become an electron?

[cozzyd]

Indeed ultra high energy photons "decay" by interacting with the CMB.

[ben_w]

Using the word “decay” to include that feels like it can’t be right.

But I’m not a proper physicist, just an amateur.

[cozzyd]

Yeah, you're right, it's not the right word at all!

[ben_w]

Good to know — I was worried I might have stuck my foot in my mouth when I looked at your profile and saw your profession.