Black holes

Abstract Black hole is an object so compact that it is inside its event horizon: a one-way surface through which particle and light can only traverse inward, and an exterior observer cannot receive any signal sent from inside. The Schwarzschild geometry is viewed in the Eddington-Finkelstein coordinates as well as in the Kruskal coordinates. Besides a black hole, the GR field equation also allows the solution of a white hole and a wormhole. The gravitational energy released when a particle falls into a tightly bound orbit around a black hole can be enormous. The physical reality of, and observational evidence for, black holes are briefly discussed. Quantum fluctuation around the event horizon brings about the Hawking radiation. This and the Penrose process in a rotating (Kerr) black hole comes about because of the possibility of negative energy particles falling into a black hole.