Energy Extraction from a Kerr Black Hole - An Ultimate Power Source in   the Universe?
  It is widely believed that some classes of high-energy transients may be powered by the rotational energy of a rapidly spinning black hole. The energy extraction mechanism commonly discussed involves macroscopic magnetic fields that are produced by currents flowing in a disk or torus surrounding the black hole. In spite of large efforts that have led to impressive progress in our understanding of the physics of magnetized flows in Kerr geometry, several important issues remain unresolved: the nature of the load in the Blandford-Znajeck model, the causality and stability of a force-free magnetosphere, global current closure, and the role of boundary conditions on the channels through which the extracted energy is released. This chapter provides an account of recent work that addresses some of those open questions. 
Amir Levinson