Testing the No-Hair Theorem with Observations of Astrophysical Black Holes in the Electromagnetic Spectrum

TL;DR

This paper discusses how current and future electromagnetic observations of black holes can be used to directly test the no-hair theorem, which states that black holes are described solely by their mass and spin.

Contribution

It proposes a method to test the Kerr metric of black holes using electromagnetic observations, advancing the empirical verification of general relativity.

Findings

Imaging and spectroscopic data can test the Kerr metric.

Observations can potentially confirm or challenge the no-hair theorem.

Abstract

The Kerr spacetime of spinning black holes is one of the most intriguing predictions of Einstein's theory of general relativity. The special role this spacetime plays in the theory of gravity is encapsulated in the no-hair theorem, which states that the Kerr metric is the only realistic black-hole solution of the vacuum field equations. Recent and anticipated advances in the observations of black holes throughout the electromagnetic spectrum have secured our understanding of their basic properties while opening up new opportunities for devising tests of the Kerr metric. In this paper, we argue that imaging and spectroscopic observations of accreting black-holes with current and future instruments can lead to the first direct test of the no-hair theorem.