Basic Terminologies Related To Black Holes [ Black Hole - 01 ]

This artist’s impression depicts a rapidly spinning supermassive black hole surrounded by an accretion disc. This thin disc of rotating material consists of the leftovers of a Sun-like star which was ripped apart by the tidal forces of the black hole. The black hole is labeled, showing the anatomy of this fascinating object. (Credits: ESO)
Event Horizon: The event horizon is the critical boundary beyond which nothing, even light cannot escape. In the reference frame of the in-falling matter, everything is fine. But, to an outside observer, things appear differently because of gravitational time dilation. As the gravitational pull increases, light from the in-falling material starts becoming redshifted and as the material reaches the event horizon, due to tremendous redshift, it fades away. So, an outside observer can never witness the formation of the event horizon of the black hole. We discussed the concept of the gravitational redshift in the 6th article of the series.

Singularity: According to General Relativity, a gravitational singularity exists at the center of a black hole. Singularity is the region where the spacetime curvature becomes infinite. The singular region has zero volume and is thought to have infinite density. The appearance of singularities in general relativity is commonly perceived as signaling the breakdown of the theory.

Photon sphere: A photon sphere or photon circle is an area or region of space where gravity is so strong that photons are forced to travel in orbits. It is a spherical boundary of zero thickness in which photons that move on tangents to that sphere would be trapped in a circular orbit about the black hole. For the non-rotating black holes, the photon sphere has a radius 1.5 times the Schwarzschild radius. The rotating black holes, on the other hand, possess 2 photon spheres: one rotating in the same direction as the black hole, and the other one rotating in the opposite direction.

Ergosphere : Rotating black holes are surrounded by a region of spacetime in which it is impossible to stand still. This region of spacetime is called the ergosphere. Objects and radiation can escape normally from the ergosphere. Through the Penrose process, objects can emerge from the ergosphere with more energy than they entered. This energy is taken from the rotational energy of the black hole causing the latter to slow.

Innermost Stable Circular Orbit: According to Newtonian gravity, the test particles can orbit stably at some arbitrary distances from a central object. However, in general relativity, there exists an innermost stable circular orbit (often called the ISCO), inside of which, any infinitesimal perturbations to a circular orbit will lead to inspiral into the black hole.