Electromagnetic extraction of energy from Kerr black holes.

It is shown that if the magnetic field and angular momentum of a Kerr black hole are large enough, the vacuum surrounding the hole is unstable because any stray charged particles will be electrostatically accelerated and will radiate, with the radiation producing electron-positron pairs so freely that the electromagnetic field in the vicinity of the event horizon will become approximately force-free. Equations governing stationary force-free electromagnetic fields in Kerr spacetime are derived, and it is found that energy and angular momentum can be extracted from a rotating black hole by a purely electromagnetic mechanism. A perturbation technique is outlined for calculating approximate solutions under certain circumstances, and solutions are obtained for a split monopole magnetic field as well as for a paraboloidal magnetic field. The present concepts are applied to a model of an active galactic nucleus containing a massive black hole surrounded by an accretion disk.