I know that as things "fall" towards each other, they lose potential energy, that's fine. What I want to know is when a photon falls towards a mass does the photon also lose potential energy? I would assume so because potons have an equivalent mass. From the equation E = hf, can I therefore assume that the frequency of a photon actually lowers when it approaches the Earth? If so then it seems to also follow that it's momentum also lowers from the equation E = pc, this then seems to suggest that photons lose mass when approaching other mass and that an impulse is applied to the photon to cause this loss of momentum. But this would mean that the impulse was acting away from the mass, ie: the photon was experiencing a repulsive force upon approaching some matter. Is this a positive gravitational force?