Gluons, too, but there are reasons why free gluons never propagate significant distances.
Also possibly (as in, we haven’t ruled it out) the lightest of the neutrinos. It’d be kind of weird if one neutrino is massless while the others have mass, but then again, it’s kind of weird that photons are massless while the Z has mass.
Yes. This was the point I was trying to make.
I read somewhere that quantum mechanics does not explain the speed of light but instead assumes it.
I can’t find the link to what I read though so grain of salt and all that.
Sure, and as above, this notion of “observable” is not really the same as in the QM sense of observing some phenomena at subatomic scales.