No observational proof of the black-hole event-horizon

TL;DR

This paper argues that no astronomical observation can provide a definitive proof of a black-hole event horizon; one can only gather strong circumstantial evidence. It reviews ADAF-based arguments using quiescent X-ray transients, along with critiques involving winds, CDAFs, and alternative emission mechanisms, concluding that dimness is not a proof. It also examines the absence of X-ray bursts and the gravastar scenario as potential horizonless alternatives, emphasizing that observational indistinguishability from black holes complicates any positive proof. The authors advocate leveraging future probes of spacetime geometry, such as gravitational-wave measurements by LISA, and other high-precision tests, to constrain black-hole solutions, while acknowledging that true horizon proof may remain unattainable.

Abstract

Recently, several ways of obtaining observational proof of the existence of black-hole horizons have been proposed. We argue here that such proof is fundamentally impossible: observations can provide arguments, sometimes very strong ones, in favour of the existence of the event horizon, but they cannot prove it. This applies also to future observations, which will trace very accurately the details of the spacetime metric of a body suspected of being a black hole.