Non-perturbative consistency of theories without superextremal particles

Determine whether four-dimensional Einstein–Maxwell theories coupled to matter that contain no superextremal charged particles (i.e., no species with mass m less than charge q in Planck units) remain consistent once non-perturbative quantum-gravity effects in 1/G_N are included, despite the perturbative indications that near-extremal black holes avoid superextremality, obey the second law, and lack large-entropy extremal remnants.

Background

The weak gravity conjecture argues that quantum gravity should contain at least one particle with charge-to-mass ratio satisfying m < q (in Planck units) to prevent stable extremal remnants and ensure discharge of charged black holes. In this paper, the authors analyze the evaporation of near-extremal charged black holes and show that perturbative quantum-gravity effects (Schwarzian corrections) modify the density of states so that it vanishes at extremality and alter Hawking radiation rates, removing apparent semiclassical pathologies.

They therefore suggest that, in the absence of superextremal particles, near-extremal black holes may still be perturbatively consistent: black holes avoid becoming superextremal, the second law of thermodynamics is respected, and large-entropy extremal remnants are not present. However, they note that a full assessment requires understanding non-perturbative effects in 1/G_N, leaving open whether such theories are consistent at the non-perturbative level.