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.

  • $\begingroup$ If you can extract energy from something, you can use it to power a laser. In this case the passage near the black hole would raise the velocity of atoms slightly (or even a lot, if it's close enough), which might be exploited to increase the energy of emitted radiation. Can it be used to pump directly? Probably with the right kind of atomic collision process. $\endgroup$ – CuriousOne May 6 '16 at 19:03
  • $\begingroup$ @CuriousOne I am considering a situation where one sends a beam of light towards a spinning black hole (from $r >> r_G$) and the beam is gravitationally amplified and sent in a different direction; one then redirects the beam back towards the black hole, creating a loop. $\endgroup$ – Demi May 8 '16 at 0:40
  • $\begingroup$ That is not a laser, there is no stimulated emission, but it could raise the energy of the photons. I don't think it can increase their number, though, at least not in the classical regime. I don't know if there is something like stimulated emission for microscopic black holes. $\endgroup$ – CuriousOne May 8 '16 at 20:46
  • $\begingroup$ @CuriousOne Are you sure? I know that electromagnetic fields around a spinning black hole are unstable (at fixed frequency) if confined by a sufficiently distant spherical mirror. $\endgroup$ – Demi May 9 '16 at 4:11
  • $\begingroup$ Am I sure that photons don't undergo stimulated emission in gravitational fields? Pretty much. Am I sure that a spinning black hole can change photon energy? Yes. Don't know how you came up with the opposite. Can you give a citation? $\endgroup$ – CuriousOne May 9 '16 at 7:40

Blue shift is something that occurs both as you observe a light (reflected or emitted) source that is approaching you or as light descends toward you down a gravitational potential. These work together in the scenario you put forward, as explained in this Kurzgesagt video. This is not dependent on coherent light though.

The advantage of using coherent light is that it can interfere with itself within an overlapping ring or reflection in an optical cavity. In terrestrial cases, a gain medium is placed in the path of the beam and excited by an external power source to great effects.

Optical ring resonator, Source: https://en.wikipedia.org/wiki/Optical_ring_resonators Optical ring resonator

Perhaps there is an a way to position terrestrial mirrors such that a laser can steal momentum from the ergosphere, but you would need reflective material that could withstand a .001% energy conversion to heat on each reflection.

With current materials, you might be better off directing the laser in a path that is almost within the photon sphere of the black hole, before redirecting it to be harvested with a disposable mirror. As beam divergence occurs, there will be trade-offs in how many mirrors or lenses you can sacrifice to make this beam go where it needs to.

It seems possible to use a black hole's ergosphere as a gain medium.

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