Tuesday, November 19, 2013

ergosphere


The ergosphere is a region located outside a rotating black hole. Its name is derived from the Greek word ergon, which means “work”. It received this name because it is theoretically possible to extract energy and mass from the black hole in this region. The radius of ergosphere at equator corresponds to Schwarzschild radius of non-rotating black hole and with minimum radius at poles (down to a half of Schwarzschild radius for maximally rotating black hole)

The ergosphere is ellipsoidal in shape and is situated so that at the poles of a rotating black hole it touches the event horizon and stretches out to a distance that is equal to the radius of the event horizon. Within the ergosphere, spacetime is dragged along in the direction of the rotation of the black hole at a speed greater than the local speed of light in relation to the rest of the universe. This process is known as the Lense-Thiring effect or frame-dragging. Because of this dragging effect, objects within the ergosphere are not stationary with respect to the rest of the universe unless they travel faster than the speed of light, which is impossible based on the laws of physics. But in truth, particles are not moving with that speed, it is the spacetime of the ergosphere that moves with a speed higher than the speed of light. A suspended plumb, held stationary outside the ergosphere, will experience infinite/diverging radial pull as it approaches the static limit. At some point it will start to fall, resulting in gravitomagnetically induced spinward motion. Another result of this dragging of space is the existence of negative energies within the ergosphere.

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