If photons don't have mass, why do their paths "bend" in a gravitational field? This is question #8 in the FAQ and the answer provided is this: I'm trying to understand the last sentence. I must be having a mental block: why would thinking of "relativistic mass" cause identical geodesics? Is this statement saying light of all frequencies follows the same geodesics? (edit: that part seems ok) If a given region of spacetime has some initial curvature, then as photons of different energy (or relativistic mass) pass that region should they not cause a slight change in the curvature .....and I therefore the geodesics for photons of different energy would be slightly different...?? (edit: No they are not..had to think about this; so it's just the "relativistic mass" that's now confusing me.