I'm not sure this is resolved. see: https://en.wikipedia.org/wiki/Quantum_electrodynamics#Conclu...
> Or is it that energy can have mass prior to being converted under some conditions?
Using https://en.wikipedia.org/wiki/Mass%E2%80%93energy_equivalenc... you can (theoretically) convert between energy and mass, and then measure everything as https://en.wikipedia.org/wiki/Electronvolt. I have yet to see anything create protons from energy, so I am hesitant to do this.
Won't a proton/antiproton annihilation also create two photons? If so, wouldn't that also work in reverse?
I expect it would make a lot more than just two photons. See: https://en.wikipedia.org/wiki/Annihilation#Proton%E2%80%93an...
> If so, wouldn't that also work in reverse?
It would have to be demonstrated.
Won't a proton/antiproton annihilation also create two photons? If so, wouldn't that also work in reverse?