Fundamental Strings as Black Bodies

TL;DR

This paper demonstrates that the decay spectra of massive string excitations are thermal, revealing a microscopic origin of black body radiation at the Hagedorn temperature through explicit calculations in string theory.

Contribution

It provides the first explicit derivation of black body spectra from string decay processes, connecting thermal emission to fundamental string states and the Hagedorn temperature.

Findings

Photon spectrum for open strings is thermal at the Hagedorn temperature.

Graviton emission from closed strings shows a thermal spectrum with a grey-body factor.

Results offer a microscopic understanding of black body radiation in string theory.

Abstract

We show that the decay spectrum of massive excitations in perturbative string theories is thermal when averaged over the (many) initial degenerate states. We first compute the inclusive photon spectrum for open strings at the tree level showing that a black body spectrum with the Hagedorn temperature emerges in the averaging. A similar calculation for a massive closed string state with winding and Kaluza-Klein charges shows that the emitted graviton spectrum is thermal with a "grey-body" factor, which approaches one near extremality. These results uncover a simple physical meaning of the Hagedorn temperature and provide an explicit microscopic derivation of the black body spectrum from a unitary $S$ matrix.