[jdbburg]

The structure of the photon (here meaning the fact that it can fluctuate into an electron positron pair) is what leads to the speed of light being c and not something even larger. So your intuition is somewhat correct, except that the effect is "already taken into account" when we first learned what the speed of light was. Also, a photon can do this fluctuating even if it doesnt have a lot of energy, since the electron positron pair can be virtual - i.e. they have almost no mass and live only for an instant before annihilating back into a photon

[vlovich123]

Hmm... is the right mental model then that the photon has a speed higher than C in its photon state but on average it's C due to the fluctuation into an electron/positron pair? I would imagine that's the wrong model because there's contradictions that arise (e.g. the speed of light would vary with the energy of the photons so that those not fluctuating would have a higher speed but that's not the case AFAIK). Is this just "quantum mechanical weirdness" or do we have an "intuitive" explanation for how the speed of light and this fluctuation interplay?

[tsimionescu]

I don't think this is right at all. C is a universal constant, and it is not in fact known for sure whether light travels at c or just below (whether photons have a mass). However, individual photons in certain conditions can absolutely move at speeds lower than c. If photons of different energy move at different speeds, this would be detectable by firing the photons at the same far-away mirror, and measuring how much time it takes for each photon to return (on average).

[shoemakersteve]

That's really interesting. What's this effect called? I'd love to learn more about it. This topic fascinates me to no end!