Matt Lewandowski - Northwestern Univ. - Positivity of Amplitudes, Weak Gravity Conjecture, and Modified Gravity
E188 (Bldg 362)
We derive new positivity bounds for scattering amplitudes in theories with a massless graviton in the spectrum in four spacetime dimensions, of relevance for the weak gravity conjecture and modified gravity theories. The bounds imply that extremal black holes are self-repulsive, M/|Q| < 1 in suitable units, and that they are unstable to decay to smaller extremal black holes, providing an S-matrix proof of the weak gravity conjecture. We also present other applications of our bounds to the effective field theory of weakly broken Galileons, axions, and P(X) theories. Based on Phys. Rev. Lett. 123, 251103, arXiv:1902.03250.