Is a laser that uses a rotating black hole's ergosphere as the gain medium possible? The idea is that the coherent light feeds off of the black hole's rotation. Assume that the black hole is rotating at a substantial fraction of $c$, as is realistic.
Edit: The original context is that a really advanced civilization that orbits a black hole wants to extract energy from the BH without having to send something valuable (like a spaceship) near the BH, or wants to create an extremely powerful directed energy weapon to defend its system against hostile attackers. You can make really big mirrors in space, so this seemed like a useful concept.