The Kerr metric is a solution to Einstein’s field equations in general relativity that describes the spacetime geometry around a rotating (uncharged) black hole. Discovered by Roy Kerr in 1963, it generalizes the simpler Schwarzschild metric, which only applies to non-rotating black holes.
Unlike a static black hole, a Kerr black hole has angular momentum—it spins. This rotation causes spacetime to be dragged around the black hole, a phenomenon known as frame-dragging. The Kerr metric accounts for this twisting of spacetime and predicts the formation of the ergosphere, a region outside the event horizon where nothing can remain at rest.
The Kerr solution has deep implications in astrophysics, helping explain phenomena such as:
- The shape and behavior of accretion disks
- The launching of relativistic jets
- The possible extraction of energy via the Penrose process
The Kerr metric is essential for understanding the nature of most black holes in the universe, since real black holes are believed to rotate due to the conservation of angular momentum.