Causal extraction of black hole rotational energy by various kinds of   electromagnetic fields
  Recent general relativistic MHD simulations have suggested that relativistic jets from active galactic nuclei (AGNs) have been powered by rotational energy of central black holes. Some mechanisms of extraction of black hole rotational energy have been proposed, like the Penrose process, Blandford-Znajek mechanism, MHD Penrose process, and superradiance. The Blandford-Znajek mechanism is the most promising mechanism for the engines of the relativistic jets from AGNs. However, an intuitive interpretation of this mechanism with causality is not clarified yet, while the Penrose process has a clear interpretation for the causal energy extraction from the black hole with negative energy. In this paper, we present a formula to build physical intuition so that in the Blandford-Znajek mechanism as well as other electromagnetic processes, negative electromagnetic energy plays an important role to extract the rotational energy of the black holes causally. 
Shinji Koide and Tamon Baba
astro-ph.HE gr-qc