Absorption of photons and fermions by black holes in four dimensions

The absorption of photons and fermions into four-dimensional black holes is described by equations which in certain cases can be analyzed using dyadic index techniques. The resulting absorption cross sections for near-external black holes have a form at low energies suggestive of the effective string model. A coupling to the effective string is proposed for spin-0 and spin-$1/2 fields of pure $N=4$ supergravity which respects the unbroken supersymmetry of extreme black holes and correctly predicts dilaton, axion, and fermion cross sections up to an overall normalization.