I'm still struggling with the concept of a mass of a photon. I understand that photons have momentum because of their inertial mass...m = p/v. This makes sense to me but what does it mean to say they have zero 'rest mass'. If they have momentum at some time t, and thus inertial mass at that time, how can they have zero 'rest mass' at some later time? Has mass not been destroyed? Where did this inertial mass go? Thanks.