If a photon has no rest mass how could it be affected by Newtonian gravity? Isn't a zero mass going to kinda blow up the equations?
If you think of mass as a substance, as was the case within Newtonian mechanics, then that is an inportent distinction.
However, if you think about mass as it is described today, a small part emerges from fundamental particle interactions with the Higgs mechanism and the rest may emerge from the motion of complex charged particles and atoms through the ZPF. In both of these cases mass is not a defined substance. Instead it is an emergent phenomena... And photons while they have no rest mass contribute to and are affected by, the overall gravitational mass of a system.
Part of the problem that comes up in this type of discussion is that the definitions of mass and inertial are intertwined and too often, though we generally make the transition to a post Einstein relativistic view, we very often hold on to the everyday and old Newtonian concepts and definitions of mass and inertia.
The truth is that we really have no clear and definite definition of what mass is. Is it a fundamental component of objects, or perhaps a phenomena, which emerges from the interaction between objects? Or perhaps as some would suggest, even the interaction between objects and the ZPF, which itself might be thought of as, the "substance" of space/space-time itself?
If the Higgs boson and the the Higgs mechanism are in fact confirmed, it may lend more credibility to the second explanation and a definition of mass as an emergent phenomena, rather than fundamental "substance".
The case for a quantum source of gravity seems a harder nut to crack. Still there are those who continue work in that direction.
