Weak cosmic censorship, superradiance, and quantum particle creation

Abstract

Since 1970's, gedanken experiments have been devised to challenge the weak cosmic censorship conjecture (WCCC), which is the expectation that spacetime singularities will be hidden from faraway observers by event horizons so that classical predictability in a spacetime is preserved. These experiments involve the interaction of an extremal or a slightly sub-extremal black hole with a test particle or field, attempting to destroy the horizon, i.e. to create a so-called naked singularity. They usually conclude that WCCC cannot be violated starting from an extremal black hole, but may be violated starting from a slightly sub-extremal one, if backreaction and self-force effects are neglected. Some other works also analyze these effects. Starting 2007, a string of papers argue if WCCC can be violated by classically forbidden interactions occuring via the quantum nature of the particles associated with the fields; and where backrection and/or superradiance are pointed out as effects working in the direction of preserving the WCCC. We correct/modify a backrection argument, and furthermore point out that superradiance does not prevent {\em single particles} from being captured by the black hole; even if this capture would lead to WCCC violation. Then we consider the spontaneous emission (which we call the Zel'dovich-Unruh effect) of particles by the black hole, and find that at least for scalars, it can be understood without second quantization. It also completely invalidates the mentioned single- or few-particle thought experiments. However, the conclusions of our previous work on (at least) scalar fields interacting with black holes, i.e. that WCCC may be violated starting from slightly subextremal black holes, remains valid in this (semi)classical framework.