## Quick Reference

A rotating black hole, as distinct from a non-rotating Schwarzschild black hole, named after the New Zealand mathematician Roy Patrick Kerr (1934– ), who first described their properties in 1963. Black holes are expected to rotate rapidly, since the stars that formed them would have been rotating; hence they will be Kerr black holes. Several consequences arise from the addition of rotation to a black hole. First, the event horizon becomes elliptical, and its surface area becomes less than that for a static black hole of the same mass. If the black hole were rotating sufficiently quickly, the area of the event horizon would reduce to zero, leaving the central singularity visible from outside (a naked singularity). Second, there is a region around a rotating black hole, the ergosphere, in which objects are forced to spin around the black hole. The outer edge of the ergosphere is the static limit. Third, a new, inner event horizon forms, and it becomes possible to travel through the black hole, and emerge into a new universe or perhaps another part of our own Universe, through this second event horizon. Rotating black holes with electric charge are called Kerr–Newman black holes, named after Kerr and the American mathematical physicist Ezra Ted Newman (1929– ), but in practice black holes are unlikely to have any significant electric charge.

*Subjects:*
Astronomy and Astrophysics.

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