Oh I do so. You don't. Show me a photon at rest and I'll agree with your assertion that it has zero mass. My preferred view is that both mass and energy contribute to gravity, a photon has energy, so it contributes to gravity, so we can view that energy as mass. Now we can talk sensibly about the best definition of mass as regards rest mass v relativistic mass... http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html "It might be thought that it would be better to regard the relativistic mass as the actual mass of photons and light, instead of invariant mass. We could then consistently talk about the light having mass independently of whether or not it is contained. If relativistic mass is used for all objects then mass is conserved and the mass of an object is the sum of the masses of its part. However, modern usage defines mass as the invariant mass of an object mainly because the invariant mass is more useful when doing any kind of calculation. In this case mass is not conserved and the mass of an object is not the sum of the masses of its parts. For example the mass of a box of light is more than the mass of the box and the sum of the masses of the photons (the latter being zero). Relativistic mass is equivalent to energy so it is a redundant concept. In the modern view mass is not equivalent to energy. It is just that part of the energy of a body which is not kinetic energy. Mass is independent of velocity whereas energy is not... ...but please don't try to pretend that this is not a valid debate via facile attempts to discredit. Yeah, too right.