Ergosphere

Ergosfera surrounding the horizon of events of a black hole in rotation.

The ergosphere (in English, ergosphere), also known as the ergosphere, is the outer region near the event horizon of a black hole. in rotation. In it, the black hole's gravitational field rotates along with it, dragging space-time along. It is a phenomenon theorized by the New Zealand physicist Roy Kerr and emanates directly from Einstein's theories of general relativity. The Kerr model of a black hole is based on the first and simplest model of a black hole, the Schwarzschild model.

Its name was proposed in 1971 by Remo Ruffini and John Archibald Wheeler during the Les Houches conferences, and derives from the Greek word ergon, meaning "work". It received this name because it is theoretically possible to extract energy and mass from this region. The ergosphere has an oblate spheroidal shape that touches the event horizon at the poles of a rotating black hole and extends to a larger radius at the equator. The equatorial (maximum) radius of an ergosphere corresponds to the Schwarzschild radius of a non-rotating black hole; the (minimum) polar radius can be as small as half the Schwarzschild radius in the case that the black hole is rotating at its maximum (at higher rates of rotation the black hole could not have formed).

Possible models of ergospheres

The Schwarzschild model

The first fundamental model of a black hole was that of the German Karl Schwarzschild. The Schwarzschild black hole is basically a temporary singularity in space-time with zero angular momentum and constitutes a simpler solution and the first of physical interest to be found to the equations of general relativity.

Kerr's Model

This model is a solution to the equations of general relativity for a rotating black hole. Such a singularity, unlike the Schwarzschild singularity, would have an annular shape. Real black holes found in nature must be rotating since, by conservation of angular momentum, they will rotate just as the parent star or object did. It is known that when dying stars lose a large part of the angular momentum, being expelled together with the matter ejected by the supernova explosion in which the black hole is formed. But despite this loss of momentum, a part of it remains. Such a hole would produce, in a certain region called the ergosphere, a zone of "drag" of spacetime. The ergosphere is an ellipsoidal structure, its semi-minor axis coinciding with its axis of rotation. The ergosphere is flattened, therefore, in the direction of the axis of rotation in a similar way as the Earth does due to its rotation.

The ergosphere and time travel

Within the ergosphere there is no rest. It is impossible for a body not to move, since space itself revolves around the singularity, so the matter that is in that region will rotate along with it. This fact according to the theory of relativity entails curious consequences. The observation of a body traveling fast enough on the ergosphere could give a relative velocity with respect to us even higher than the speed of light c. In that case, such an object would simply disappear from our sight.

Penrose process

Because the ergosphere lies outside the event horizon, objects in this region can escape the gravitational pull of a black hole. An object can gain kinetic energy by entering the gravitational field of a rotating black hole and then escaping from it, taking some of the black hole's energy with it. This process of absorbing energy from a rotating black hole is called the Penrose process and was developed in 1969 by mathematician Roger Penrose. The theoretical maximum energy extraction that can be extracted is 29% of the total energy. When the energy is absorbed, the black hole loses its spin and the ergosphere ceases to exist. This process is what could explain why black holes give off gamma-ray flares. Computer models have shown that the Penrose process is responsible for the high-energy particle emissions being observed from quasars and other active galactic nuclei.