The Higgs is supposed to couple to fermions but not photons. Nevertheless, photons add mass to systems in SR. Also kinetic energy (KE) of fermion motion adds inertia and mass to systems in SR theory. So, how does the Higgs field "tell" that this type of mass from photons and KE IS mass, and give it inertia? This is a real problem. Two photons bouncing around in a container (in its COM frame) give it an extra invariant mass E/c^2 where E is the photon energy (even one photon, if trapped, will do this, but I want to make this simple--any pair of photons has invariant mass so long as they don't go in the same direction) The same is true of the kinetic energy of any particle bouncing around in such a container. It's kinetic energy E adds to the container mass as E/c^2, since it adds energy without adding any net momentum (invariant mass is total energy/c^2 where momentum is zero, as here in the COM frame). This is most easily seen in a container of gas, where the container gets not just the rest masses of particles of gas, but mass from their total kinetic energies also. Now, a container like this acts as a "particle" and has inertia given by the Higgs field. How does the Higgs field see the invariant mass, and particularly the parts of invariant mass that are "made" of kinetic energy and massless photons? Clearly the mechanism works, since Higgs sees the full mass of neutrons and protons, and not just the 1% of their mass that is rest mass of their quarks. The Higgs field sees quark kinetic energies also, so it's like my bottle problem. There's something odd here, as there must be coupling between any sort of invariant mass and Higgs, and yet the invariant mass in many systems, like the kinetic energy of such a system in its COM frame, can't be "located" in space. The mass is in the system, but where is it? How does the Higgs field know?