I’m not a physicist but have been reading it with interest. I must admit I don’t quite understand the detail of your question but I think the main question is “how is it known that the absorption frequencies are the same as the emissive frequencies?”
As far as I understand, the absorption...
It is not that a photon has no mass. A photon has no “rest-mass” but it does have a unified entity called “mass-energy”, which has the same properties as rest mass does, as far as gravitation is concerned.
Consider a pair of electron and positron. They have rest mass, which other masses...