Comment on "Empty Black Holes, Firewalls, and the Origin of Bekenstein-Hawking Entropy"

TL;DR

This paper critiques a proposed model of black hole horizons, arguing that it fails to reproduce Bekenstein-Hawking entropy and lacks a proper thermodynamic interpretation due to the absence of a true horizon.

Contribution

It demonstrates that the surface fluid model with zero energy density does not match black hole entropy and questions the applicability of thermodynamics without a horizon.

Findings

Entropy per unit area does not match Bekenstein-Hawking entropy.

Lack of a horizon prevents the Unruh effect from being considered.

Temperature varies with angle, undermining local thermodynamic equilibrium.

Abstract

Saravani, Afshordi and Mann \cite{SAM} considered a surface fluid with vanishing energy density on the stretched horizon of a black hole, taken as the new boundary of spacetime. We show that their entropy per unit area of the fluid does not correspond to the Bekenstein-Hawking entropy, both for a Schwarzschild black hole and for a Kerr-Newman black hole. Due to the lack of a horizon, we argue that the Unruh effect could not be taken into account and the concept of local thermodynamic equilibrium is not sustained because of the $\theta$-dependence of the temperature.