Species and Strings

TL;DR

The paper argues that weakly-coupled string theory can be viewed as a theory of N = 1/g_s^2 particle species, with the species scale L_N = N^{1/(D-2)} l_D setting the fundamental UV cutoff for gravity with many species. By combining black hole thermodynamics, holography, and string/M-theory relations, the authors derive a universal bound S > N and show how the Bekenstein-Hawking entropy can be interpreted as entanglement entropy when the UV cutoff is the species scale. They analyze the roles of Chan-Paton factors, Regge excitations, and M-theory duality to demonstrate that in string theory the effective number of species is finite and controlled by the string coupling, and in the strong coupling limit the theory reduces to pure gravity in higher dimensions with the eleven-dimensional Planck length setting the resolution limit. The results unify a holographic perspective on information and entropy with the string/M-theory spectrum, suggesting a consistent mapping between gravity with many species and string/M-theory.

Abstract

Based on well-known properties of semi-classical black holes, we show that weakly-coupled string theory can be viewed as a theory of N = 1/g_s^2 particle species. This statement is a string theoretic realization of the fact that the fundamental scale in any consistent D-dimensional theory of gravity is not the Planck length l_D, but rather the species scale L_N = N^1/(D-2) l_D. Using this fact, we derive the bound on semi-classical black hole entropy in any consistent theory of gravity as S > N, which when applied to string theory provides additional evidence for the former relation. This counting also shows that the Bekenstein-Hawking entropy can be viewed as the entanglement entropy, without encountering any puzzle of species. We demonstrate that the counting of species extends to the M-theory limit. The role of the species scale is now played by the eleven-dimensional Planck length, beyond which resolution of distances is gravitationally-impossible. The conclusion is, that string theory is a theory of species and gets replaced by a pure gravitational theory in the limit when species become strongly coupled and decouple.