Hi. Since photons lose energy when moving against gravity (because of gravitational redshift), there must be some other energy that is increased, i.e. the energy converts to another form. That other energy must be potential energy, right? But for a photon to obtain potential energy, it must perform a work. And for the photon to perform a work, there must be some force acting on it while it is moving. The only force I can come to think of is the force of gravity. But for the gravity to act upon it, doesn't it need to have a mass?