I was wondering if light can travel faster when it is within a region of moving spacetime. I know in general, that when you move a medium such as water, like in a river, water waves travel slower when against the current and faster when along with the current.
Is this true with light? It seems to me that when light is travelling in the ergosphere of a black hole (a region where spacetime moves at the speed of light), it would travel 2 times faster. Fact check me about the ergosphere.
The speed of light relative to a local observer is always c, but relative to a far away observer it can indeed receed faster than c; for example, a photon receding at the hubble radius has a speed of 2c relative to the observer in the center of the frame, and a photon travelling in your direction at that distance has a speed of 0c relative to you (the proper distance relative to you stays fixed), but still 1c relative to a local observer who is also located at the hubble radius, see Susskind, Cosmology 2 at timestamp 1:30:30. The same goes for the ergosphere or the horizon of a black hole, relative to a local observer it still has exactly 1c.
