Black Hole Quantum Mechanics in the Presence of Species

TL;DR

This paper revises the bound on the number of particle species in quantum black holes, introducing a new fundamental length scale that resolves the species problem and highlights differences between gravitational and non-gravitational species.

Contribution

It demonstrates that the species bound persists in quantum theory, establishing a new length scale and clarifying the self-UV-completion of gravity versus non-gravitational species.

Findings

The species bound leads to a new length scale, L_{species} = sqrt{N_{species}} L_P.

Gravitational species' lightest black holes are at the unitarity violation scale.

Non-gravitational species require additional physics to restore unitarity.

Abstract

Recently within the context of a microscopic quantum theory, the Black Hole's Quantum N-Portrait, it was shown that continuous global symmetries are compatible with quantum black hole physics. In the present paper we revise within the same framework the semi-classical black hole bound on the number of particle species N_{species}. We show that unlike the bound on global charge, the bound on species survives in the quantum picture and gives rise to a new fundamental length-scale, L_{species} = sqrt{N_{species}} L_P$, beyond which the resolution of species identities is impossible. This finding nullifies the so-called species problem. This scale sets the size of the lightest quantum black hole in the theory, Planckion. A crucial difference between the gravitational and non-gravitational species emerges. For gravitational species, the lightest black holes are exactly at the scale of perturbative unitarity violation, which is a strong indication for self-UV-completion of gravity. However, non-gravitational species create a gap between the perturbative unitarity scale and the lightest black holes, which must be filled by some unitarity-restoring physics. Thus, self-UV-completion of gravity implies that the number of non-gravitational species must not exceed the gravitational ones.