Kerr Black Holes, Spin & Frame Dragging: Structures & Vocabulary

Real astrophysical black holes are expected to rotate. The Kerr solution describes an ideal rotating uncharged black hole, whose spin changes horizon geometry, orbital structure, energy extraction possibilities, and the motion of nearby matter and light.

Rotating mass drags local inertial frames, an effect called frame dragging. Near a rapidly spinning black hole, this alters orbital frequencies and precession. Electromagnetic fields in magnetized plasma can couple to the rotating spacetime and help power jets by extracting rotational energy.

Relativistic X-ray reflection spectra, continuum fitting, quasi-periodic timing behavior, and horizon-scale observations constrain spin-dependent accretion geometry. Frame dragging has also been measured weakly around Earth, confirming the underlying relativistic effect outside the black-hole regime.

The spacetime contains an event horizon and an outer ergoregion where remaining stationary relative to distant space is impossible. Prograde and retrograde orbits behave differently because of frame dragging.