Powering Quasars
The Penrose process and related magnetohydrodynamic effects in the ergosphere are believed to power some of the most energetic phenomena in the universe, such as quasars and active galactic nuclei. The accretion disk interacts with the rotating spacetime, transferring angular momentum and releasing vast amounts of gravitational potential energy.
The Extremal Limit
There is a maximum spin rate for a black hole, known as the extremal limit, where \(a = GM/c\). At this limit, the two horizons merge. Nature likely prevents black holes from exceeding this limit, preserving the cosmic censorship conjecture. Observations of X-ray spectra from accretion disks allow astronomers to measure the spin of stellar-mass and supermassive black holes.