"SR says that nothing with mass can travel at "c".
The only thing that can travel at "c" is a photon.
Because a photon has no rest mass, it is not possible to define a F of R for a photon.
In view of these considerations, the infinite time factor that would be relevant to the F of R of a photon has no meaning?"

There are some who say that; at rest the photon has a duration that is infinitely small (1 Planck time limit?). At c its duration is infinitely long (1 universe time limit?)

I don't know. I haven't spent much time on EMW. I sure hope that I can get around to it (1 life time limit?)