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 axisymmetric solution to the vacuum field equations. Recent and anticipated advances in observations of black holes throughout the electromagnetic spectrum have both secured our understanding of their basic properties and opened new opportunities for devising tests of gravity theories. In this talk, I will show that observations of continuum and line spectra from accreting black-hole candidates with current and future instruments can provide the first direct test of the no-hair theorem. I will also discuss how the imaging observations of the inner accretion flow around the black hole in the center of the Milky Way can help us map the spacetime in the vicinity of its horizon and lead to a detailed test of the Kerr solution.