Starting with this equation, [itex]E^2 = p^2c^2 + m^2c^4[/itex] With a photon, m = 0... what happens when E starts to get large? Very large. As E approaches infinity, do strange things happen? Will a singularity form? If we let our photon have a mass, something very small, will the picture change at all? A very open question, since I'm having trouble here... What's the difference between mass and energy? Thanks for your time!