mxdplay4: "Stuff like sunlight exerting pressure because it has mass is actually pretty standard easy physics."
Even massless particles like photons can carry momentum.
Photons have relativistic mass due to their velocity, but no inherent mass like we are used to in everyday situations (resting mass). I'm not disagreeing with Loomis since his statement is correct in the major sense. But it just shows that this is a very difficult area to get your head around.
Photons do not have relativistic mass. They have zero mass period. They exert momentum effects because of their velocity. They ACT like they have mass, but do not have mass. "inherent mass" is not a term used by any scientist.
The key term here is relativistic mass. Photons have no rest mass (invariant mass, intrinsic mass, and proper mass are all terms for this), but they do have mass because they carry energy. By mass-energy equivalence they also have mass.
Relativistic mass (or apparent mass) is the sum of a particle's rest mass and the mass from kinetic energy. An observer who sees an object moving near the speed of light would measure the mass of that object to be larger than its rest mass (which is the mass measured when the object is stationary w.r.t. the measuring reference frame). This "extra" mass is actually the kinetic energy that the object possesses due to its motion.
Don't dig any deeper. You won't get out. The "extra" mass for real particles only, comes from the relavistic 4-vector expression for the total eneergy of a particle.
"inherent mass" is not a term used by any scientist.
You can use the term 'invariant mass' then. To be correct for the people who already understand it.
Neither term is relevant to light.