What powers light?

Originally posted by humy
even if the Earth was transparent so that could happen, no. Remember, photons of light have no mass therefore it isn't like the potential energy that is lost when you drop an object with mass onto the surface of the Earth!

I see what you're getting at, but you're sort of half right and half wrong. The problem lies with mixing paradigms. Talking about potential energy is fine in Newtonian mechanics but no longer relevant in General Relativity. It's not actually the photon's masslessness that means that it has no potential energy, it's that the concept isn't used in General Relativity. In fact this is true of gauge theories in general, potential energy is a redundant concept, it's been replaced by connections in our paradigm theories.

In a semi-Newtonian model, so using special relativity in the asymptotic observers rest frame along with Newton's law of Gravity and Planck's formula to map the photons energy to it's frequency, one can assign a mass to the photon using Einstein's formula and proceed from there, in that approximation talking about potential energy makes sense. It's a kludge but it'll give good answers in the weak field limit.

In General Relativity the frequency drops because time runs faster as one goes further from the central mass. So there's no reason to start taking results from quantum theory, one can treat the light purely classically and the geodesic equations take care of everything. In that view there is no need to resort to potential energy even if one is looking at a massive out-moving particle.