Event horizon

An event horizon is a boundary in spacetime beyond which events cannot send signals to distant observers. Its location depends on the entire spacetime, not merely on local conditions.

Schwarzschild radius: The radius of the event horizon around a nonrotating, uncharged mass in general relativity. For a Schwarzschild black hole, the horizon lies at this characteristic radius.
General relativity: Einstein's theory of gravity, in which matter and energy shape spacetime and free-falling bodies follow its geometry. Event horizons arise from the causal structure of relativistic spacetime.
Apparent horizon: A surface on which outgoing light rays have zero instantaneous expansion on a chosen spatial slice. Unlike an event horizon, it can be defined locally and can shift with the choice of time slicing.
Hawking radiation: Thermal radiation predicted to arise from quantum fields in the curved spacetime of a black hole. It gives black holes a temperature and links horizons to quantum effects.
Light cone: The set of possible paths that light signals can follow from an event through spacetime. At the horizon, the future-directed light paths no longer lead outward to distant observers.
Spacetime: The four-dimensional structure combining three dimensions of space with one dimension of time. A horizon is a boundary defined by which spacetime events can causally connect.
Rindler horizon: A causal boundary experienced by uniformly accelerated observers in flat spacetime. It limits an observer's access without requiring a black hole or curved spacetime.
Black hole information paradox: The conflict between quantum-mechanical information conservation and the apparent information loss in black hole evaporation. Whether information can escape or remain encoded challenges the classical picture of a one-way horizon.
Penrose diagram: A spacetime diagram that compresses infinite distances and times while preserving causal structure. It depicts the horizon as the boundary between events that can and cannot reach infinity.
Causality: The relation in which one event can influence another through physical interactions or signals. An event horizon separates events by whether their effects can reach distant observers.

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