Fundamental strings as black bodies

We show that the decay spectrum of massive excitations in perturbative stri ng theories is thermal when averaged over the (many) initial degenerate sta tes. We first compute the inclusive photon spectrum for open strings at the tree level showing that a black body spectrum with the Hagedorn temperatur e emerges in the averaging. A similar calculation for a massive closed stri ng state with winding and Kaluza-Klein charges shows that the emitted gravi ton spectrum is thermal with a "grey-body" factor, which approaches one nea r extremality. These results uncover a simple physical meaning of the Haged orn temperature and provide an explicit microscopic derivation of the black body spectrum from a unitary S matrix. (C) 1999 Elsevier Science B.V. All rights reserved.