[ben_w]

I understand the wariness.

However, in relativity a photon cannot decay: because it travels at the speed of light and has infinite time dilation, it does not subjectively experience the passage of time in which the possibly of decay could exist.

[hilbert42]

OK, fair enough. But what happens in a homogeneous lossless dielectric with say a velocity factor of about say 0.6?

And what would happen in some theoretical meta material where say, values for say µ and ε were lower than their vacuum counterparts?

OK, it's a red herring, but interesting to contemplate.

[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.