Extracting information on black hole horizons

TL;DR

This paper explores how photon orbits outside Kerr black hole horizons encode information about the inner horizon, potentially allowing observers to detect and analyze black hole horizons through emitted spectra.

Contribution

It introduces the concepts of horizon confinement and replicas, demonstrating that photon orbits near the rotation axis carry information about the inner horizon, which is detectable externally.

Findings

Photon orbits near the rotation axis encode inner horizon information.

Photon frequencies can reveal details about the black hole's internal structure.

Photon orbits exist close to the Kerr geometry's rotation axis.

Abstract

We present some features of Kerr black hole horizons that are replicated on orbits accessible to outside observers. We use the concepts of horizon confinement and replicas to show that outside the outer horizon there exist photon orbits whose frequencies contain information about the inner horizon and that can, in principle, be detected through the emission spectra of black holes. It is shown that such photon orbits exist close to the rotation axis of the Kerr geometry. We argue that these results could be used to recognize and further investigate black holes and their horizons