[Toffo (OP)]

Let's say I have a fast spinning black hole, and I'm extracting energy from it using a method by which energy can be extracted from a spinning black hole, Penrose Process or something like that.

I'm trying to keep the black hole's mass and angular momentum constant by feedind the black hole with photons whose angular momentum points to same direction as the black hole's angular momenum. Is this possible?

[jeffreyH]

Let's say I have a fast spinning black hole, and I'm extracting energy from it using a method by which energy can be extracted from a spinning black hole, Penrose Process or something like that.

I'm trying to keep the black hole's mass and angular momentum constant by feedind the black hole with photons whose angular momentum points to same direction as the black hole's angular momenum. Is this possible?

If you want to be forever trapped in the ergosphere, yes. In which case you will not be able to transmit the energy you gain to a region outside the ergosphere.

[Toffo (OP)]

If you want to be forever trapped in the ergosphere, yes. In which case you will not be able to transmit the energy you gain to a region outside the ergosphere.

What? Why? I think I'll use a Hawking radiation radiating black hole, the rotation of the black hole gives the radiated photon gas some angular momentum, I install light mills that extract rotation energy out of the photon gas, afterwards I dump the photon gas back to the black hole.

[Toffo (OP)]

If I have a beam of photons with parallel spins, by using that beam I can apply a large torque to a fast spinning black hole, if the number of photons is large. That would cause a large increase of spinning energy of the black hole. Energy of the beam does not have to be large, photons may be arbitrarily low energy photons.

[evan_au]

...cause a large increase of spinning energy of the black hole. Energy of the beam does not have to be large...

The kinetic energy of a spinning black hole is immense. A black hole can have the mass of a star, up to 1% the mass of a galaxy. And they can spin incredibly fast without flying apart, as the escape velocity exceeds the speed of light.

Conservation of Energy suggests that you cannot add more energy to a black hole than the energy of the beam.
To add any detectable amount energy to a black hole would require a beam of truly astronomical energy (or incredibly high power, for an extremely long time).

You also need to consider what fraction of the momentum of the photon will impart linear momentum or angular momentum of the black hole. I imagine that absorbing the photon beam would impart some linear momentum to the black hole, so it would not all appear as angular momentum of the black hole? And generating the beam would impart a considerable momentum on the beam source, so it would be inclined to go flying off into space.

I suspect that the energy required to power the beam would be better spent providing useful energy for your interstellar civilization.

[yor_on]

Depends on definitions. "Inside this ergosphere, the time and one of the angular coordinates swap meaning (time becomes angle and angle becomes time) because timelike coordinates have only a single direction (and remember the particle is necessarily rotating with the black hole in a single direction only). Because of this weird and unusual coordinate swap, the energy of the particle can assume both positive and negative values as measured by an observer at infinity." Now, if that is correct, which I personally hold to be incorrect, then maybe?

http://www.gu.se/forskning/publikation?publicationId=195725

[jeffreyH]

Depends on definitions. "Inside this ergosphere, the time and one of the angular coordinates swap meaning (time becomes angle and angle becomes time) because timelike coordinates have only a single direction (and remember the particle is necessarily rotating with the black hole in a single direction only). Because of this weird and unusual coordinate swap, the energy of the particle can assume both positive and negative values as measured by an observer at infinity." Now, if that is correct, which I personally hold to be incorrect, then maybe?

http://www.gu.se/forskning/publikation?publicationId=195725

yor_on this has to be one of the most interesting posts I have read. I have just determined these exact results for myself.