[nomad]

If the particle is at rest it is not a photon. In Special Relativity a photon is always a 4-D spacetime thread that is oriented so as to bisect the angle between the 4th dimension and an ordinary spatial direction for all observers. That's another way of saying that it is always moving at the speed c (~186,000 mi/sec). A photon has has energy (therefore mass) by virtue of E = hf and E = mc^2. E = energy, h is Planck's constant, f is frequency (color of light, for example), m is mass. Photons bend in a gravitational field, and have a gravitational field that is so small as to be inconsequential in practical matters (notice how small the mass is when you solve for m in the E = mc^2 equation, and noting that when you compute energy with the Planck constant times frequency, h ~ 6 x 10^-27 erg-sec).