Einsteins postulate is that the speed of light is constant but Einsteins postulate does not explain the quantum mechanics of how the speed is achieved . A photon does not have a propulsion mechanism therefore I propose the photons momentum is a consequence of gravitational transition . Gravitational transition being higher energy states progressing to lower energy state spatial points . Explaining the process Momentum hf = F<E Suggesting that photons are attracted by force to lesser energy state points rather than having an unique speed of its own. A comparison to spectral transitions .