The static limit is the hypersurface around a rotating black hole , which is the boundary of the area within which any body (or photon) can no longer be at rest relative to a remote observer. In order to keep from falling onto the surface of the event horizon, a body under the surface of the static limit must rotate with a positive angular velocity around the black hole (in the direction of rotation of the black hole). For a non-rotating hole , the event horizon and the static limit coincide, while for an unperturbed rotating one, they only touch poles, representing, in Boyer-Lindquist coordinates, ellipsoids of rotation, the region between which is called the ergosphere . In the Kerr black hole, the static limit is determined by the equation . The appearance of the ergosphere is an example of the extreme manifestation of the effect of dragging inertial reference frames by rotating bodies.
Physical sense
Physically, the presence of a static state limit means that one can remain at rest, having reached the static state limit, only having the speed of light. The bodies that have lower speed are involved in spiral movement with a gradual approach to the event horizon, from which there is no principal return. In contrast to a body that is beyond the event horizon, it is still possible to break out of the surface of the static limit - at a certain angle and speed of entering the ergosphere, you can throw a part of the mass into a black hole, but the remaining mass, having received a powerful acceleration, will be thrown out of the static limit.
You can see the deep similarity of a rotating black hole and the known effect of the vortex - for example, a giant whirlpool . The gravitational field of the Kerr-Newman black hole resembles a cosmic whirlpool. A spaceship flying past is sucked into the center like an ordinary boat. As long as the ship is beyond the static limit, it can still move “wherever it wants.” In the region (shown in gray) between the static limit and the event horizon, he is already forced to rotate in the same direction as the black hole; his ability to move freely decreases with further suction, but he can still get out, moving along an unwinding spiral. The interior of the horizon is shown in red: it is no longer possible to escape from there, even moving at the speed of light. The situation is beautifully illustrated by the story of Edgar Poe “The Falling down to Malström ” (1840).
Interesting Facts
Theoretically, the “most ecological method of obtaining energy”, called the Penrose process, is possible - a garbage truck approaching a black hole dumps trash into it and is removed from the vicinity of a black hole, having more energy than before this maneuver. Thus, it is possible to convert into kinetic energy up to half the rest mass of the discharged garbage, which makes the process very tempting, but still difficult to implement.