  The weak gravity conjecture states that quantum gravity theories have to
contain a charged state with a charge-to-mass ratio bigger than unity. By
studying unitarity and causality constraints on higher derivative corrections
to the charge-to-mass ratio of extremal back holes, we demonstrate that heavy
extremal black holes can play the role of the required charged state under
several assumptions. In particular, our argument is applicable when the higher
spin states Reggeizing graviton exchange are subdominant in the photon
scattering. It covers (1) theories with light neutral bosons such as dilaton
and moduli, and (2) UV completion where the photon and the graviton are
accompanied by different sets of Regge states just like open string theory. Our
result provides an existence proof of the weak gravity conjecture in a wide
class of theories, including generic string theory setups with the dilaton or
other moduli stabilized below the string scale.
