How many black holes fit on the head of a pin?

TL;DR

This paper challenges the traditional view by showing that multi-black-hole configurations can have greater entropy than single black holes, implying that microscopic D-brane systems may not accurately count the microstates of individual black holes.

Contribution

It demonstrates that the entropy of multi-black-hole systems can surpass that of single black holes, questioning the microstate counting assumptions in string theory.

Findings

Multi-black-hole configurations can have higher entropy than single black holes.

D-brane microstates may correspond to multi-centered black hole arrangements.

Single black hole microstate counting may be incomplete or misleading.

Abstract

The Bekenstein-Hawking entropy of certain black holes can be computed microscopically in string theory by mapping the elusive problem of counting microstates of a strongly gravitating black hole to the tractable problem of counting microstates of a weakly coupled D-brane system, which has no event horizon, and indeed comfortably fits on the head of a pin. We show here that, contrary to widely held beliefs, the entropy of spherically symmetric black holes can easily be dwarfed by that of stationary multi-black-hole ``molecules'' of the same total charge and energy. Thus, the corresponding pin-sized D-brane systems do not even approximately count the microstates of a single black hole, but rather those of a zoo of entropically dominant multicentered configurations.