Kerr Metric

The Kerr metric (or Kerr vacuum) describes the geometry of empty spacetime around an uncharged axially-symmetric black-hole with an event horizon which is topologically a sphere. The Kerr metric is an exact solution of the Einstein field equations of general relativity; these equations are highly non-linear, which makes exact solutions very difficult to find. The Kerr metric is a generalization of the Schwarzschild metric, which was discovered by Karl Schwarzschild in 1916 and which describes the geometry of spacetime around an uncharged, spherically-symmetric, and non-rotating body. The corresponding solution for a charged, spherical, non-rotating body, the Reissner–Nordström metric, was discovered soon afterwards (1916–1918). However, the exact solution for an uncharged, rotating black-hole, the Kerr metric, remained unsolved until 1963, when it was discovered by Roy Kerr. The natural extension to a charged, rotating black-hole, the Kerr–Newman metric, was discovered shortly thereafter in 1965. These four related solutions may be summarized by the following table:

where Q represents the body's electric charge and J represents its spin angular momentum.

According to the Kerr metric, such rotating black-holes should exhibit frame dragging, an unusual prediction of general relativity. Measurement of this frame dragging effect was a major goal of the Gravity Probe B experiment. Roughly speaking, this effect predicts that objects coming close to a rotating mass will be entrained to participate in its rotation, not because of any applied force or torque that can be felt, but rather because of the curvature of spacetime associated with rotating bodies. At close enough distances, all objects — even light itself — must rotate with the black-hole; the region where this holds is called the ergosphere.

The Kerr metric is often used to describe rotating black holes, which exhibit even more exotic phenomena. Such black holes have different surfaces where the metric appears to have a singularity; the size and shape of these surfaces depends on the black hole's mass and angular momentum. The outer surface encloses the ergosphere and has a shape similar to a flattened sphere. The inner surface marks the "radius of no return" also called the "event horizon"; objects passing through this radius can never again communicate with the world outside that radius. However, neither surface is a true singularity, since their apparent singularity can be eliminated in a different coordinate system. Objects between these two horizons must co-rotate with the rotating body, as noted above; this feature can be used to extract energy from a rotating black hole, up to its invariant mass energy, Mc2. Even stranger phenomena can be observed within the innermost region of this spacetime, such as some forms of time travel. For example, the Kerr metric permits closed, time-like loops in which a band of travelers returns to the same place after moving for a finite time by their own clock; however, they return to the same place and time, as seen by an outside observer.