First of all, sorry for the conceptual simplifications, but they seem
necessary...:
Well, in matter-energy transformations, the photons' total energy is
equivalent to the original matter:
mc^2 = sum(hv)
So m=sum(hv)/c^2 is the mass wich was transformed in c-velocity (always)
photons... (or vice-versa)
Ex: In simpler e+ / e- anihilation, we have after 1 photon with >= 1024
keV, wich equals to two 512 keV rest mass- equivalent eletrons ( + and - )
plus kinetic energies....
In the contrary effect, 1 photon of energy necessarily >= 1024 keV is
transformed in a pair positron-negatron, wich variant kinetic energies but
with "normal" velocities, very far from c (generally, slow eletrons).
Suggestion: Study these transformations.... who knows the answer you want is
there .....???
Does anybody know how much time lasts such transformations ??? And what
happens during them ???
Gomes.