Kerr metric

The Kerr metric, also called the Kerr geometry, describes the geometry of empty spacetime around a rotating, uncharged, axially symmetric black hole with a quasispherical event horizon. It is an exact solution of the Einstein field equations of general relativity, which are highly non-linear and make exact solutions very hard to find. It generalizes the Schwarzschild metric of 1915 for a non-rotating body to a rotating one, and the exact solution for an uncharged rotating black hole remained unsolved until 1963, when Roy Kerr discovered it. It predicts frame-dragging, a distinctive prediction of general relativity that the Gravity Probe B experiment first measured in 2011, and it features an ergosphere in which all objects must rotate with the black hole.