Bounding Violations of the Weak Gravity Conjecture

Abstract

The black hole weak gravity conjecture (WGC) is a set of linear inequalities on the four-derivative corrections to Einstein--Maxwell theory. Remarkably, in four dimensions, these combinations appear in the $2 \to 2$ photon amplitudes, leading to the hope that the conjecture might be supported using dispersion relations. However, the presence of a pole arising in the forward limit due to graviton exchange greatly complicates the use of such arguments. In this paper, we apply recently developed numerical techniques to handle the graviton pole, and we find that standard dispersive arguments are not strong enough to imply the black hole WGC. Specifically, under a fairly typical set of assumptions, including weak coupling of the EFT and Regge boundedness, a small violation of the black hole WGC is consistent with unitarity and causality. We quantify the size of this violation, which vanishes in the limit where gravity decouples and also depends logarithmically on an infrared cutoff. We discuss the meaning of these bounds in various scenarios. We also implement a method for bounding amplitudes without manifestly positive spectral densities, which could be applied to any system of non-identical states, and we use it to improve bounds on the EFT of pure photons in absence of gravity.