Other meanings of Event horizon
Astrophysics
In general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. It is most commonly associated with black holes, where it marks the point of no return: anything crossing it, including light, cannot escape. The concept also applies to cosmological horizons, such as the observable universe's particle horizon.
An event horizon is a null surface that separates spacetime into two regions: one from which causal signals can reach a distant observer, and one from which they cannot. For a black hole, it is a sphere around the singularity, with radius given by the Schwarzschild radius for a non-rotating, uncharged black hole: rs = 2GM/c2.1 The horizon is not a physical surface but a mathematical boundary; an observer crossing it would not feel anything special locally, but they would be unable to communicate with the outside universe.
Besides black hole horizons, there are cosmological event horizons, such as the one around an observer in an accelerating universe, beyond which light emitted now will never reach us. There is also the particle horizon, which marks the limit of what we can have seen so far, and the future horizon, which bounds what we will ever see. In addition, in the context of the holographic principle, the horizon of a black hole encodes the information of its interior, a key idea in quantum gravity.2
The existence of event horizons is supported by observations of black holes, such as the shadow of M87* imaged by the Event Horizon Telescope in 2019, which shows a dark region consistent with a photon ring around a horizon.3 Additionally, gravitational wave detections by LIGO and Virgo are consistent with mergers of black holes with horizons. However, some alternative theories propose horizonless objects, but they are less favored.
One lesser-known aspect is that the event horizon of a rotating (Kerr) black hole is not spherical but oblate, and it is surrounded by an ergosphere where spacetime is dragged. Another is the concept of a 'naked singularity'—a singularity without an event horizon—which is forbidden by the cosmic censorship hypothesis. Also, the entropy of a black hole, proportional to the area of its horizon, leads to the holographic principle. Furthermore, the 'firewall' paradox questions whether the horizon is a smooth region or a place of high-energy particles, a topic of ongoing debate.4
The event horizon is a global concept: it is defined with respect to the entire future of the spacetime, not locally. This means that in a dynamical spacetime, the horizon may not be known until the end of time. In contrast, an apparent horizon is a local concept, defined by the behavior of outgoing light rays. The two coincide in stationary spacetimes but differ in general. This distinction is important in numerical relativity and in understanding gravitational collapse.
The term 'event horizon' also appears in other contexts, such as in the film 'Event Horizon' (1997), but this article focuses on the astrophysical concept.
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