Gravity, Supergravity and Superstrings (GRASS) seminar: Signatures of Black Hole Thomson Dipoles

The most general black holes (BHs) of the Einstein—Maxwell theory carry electric and magnetic charge. Extremal magnetic BHs, in particular, are stable against pair creation, neutralisation by accretion and Hawking radiation, and could have survived until today even if they were formed in the early universe. In this talk I will show that, when placed on an ionised homogeneous medium, rotating magnetic BHs tend to accrete matter in such a way that the medium becomes polarised and the hole reduces its angular momentum in a mechanism that resembles the collapse of a Thomson dipole. Energy and angular momentum can be extracted from the hole via an enhanced Penrose process, and I will discuss the relation of the latter to high-energy cosmic rays. I will also show that (weak) cosmic censorship cannot be violated in the process of matter accretion by an extremal magnetic BH, as long as the infalling matter satisfies the null energy condition. Finally, I will conclude by discussing briefly recent progress in deriving master wave equations governing gravitational wave generation on magnetic BH spacetimes.