[Neri]

As I understand it, Special Relativity has light propagated rectilinearly and at constant speed as judged from inertial frames of reference. General Relativity has light (like everything else) following a "world line" dependent upon spatio-temporal location in gravitational fields generated by massive objects. The straight line of Euclidean space is like the world line of the non-Euclidean space-time of the gravitational field only in the sense that both, in their respective realms, are the shortest distance between two points. Whether a thing is moving in a straight line in Euclidean space or in a world line in a gravitational field, it is still moving. The real question is how is motion possible in the first place?