Black Holes

Black holes are general-relativistic solutions with an event horizon, a causal boundary from which no future-directed null geodesic reaches future null infinity. The canonical families are Schwarzschild (mass only), Reissner-Nordström (mass and charge), Kerr (mass and angular momentum), and Kerr-Newman (all three), with the no-hair theorem restricting stationary vacuum solutions to these parameters alone. Key structural results include the area theorem, Hawking radiation as a quantum consequence of the event horizon, the laws of black-hole thermodynamics, and the Penrose process for extracting rotational energy from the ergosphere of a Kerr black hole. The information paradox, concerning whether quantum information is preserved through gravitational collapse and evaporation, remains one of the deepest open questions in theoretical physics.