In general relativity, an **event horizon** is a boundary in spacetime beyond which events cannot affect an outside observer. It is the shell of the "points of no return", the points at which the gravitational pull becomes so great as to make escape impossible, even for light.

## Black holesEdit

An event horizon is most commonly associated with black holes. Light emitted from inside the event horizon can never reach the outside observer. Likewise, any object approaching the horizon from the observer's side appears to slow down and never quite pass through the horizon,^{[1]} with its image becoming more and more redshifted as time elapses. This means that the wavelength is getting longer as the object moves away from the observer.^{[2]} The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.

## Horizon typesEdit

More specific types of horizon include the related but distinct absolute and apparent horizons found around a black hole. Still other distinct notions include the Cauchy and Killing horizon; the photon spheres and ergospheres of the Kerr solution; particle and cosmological horizons relevant to cosmology; and isolated and dynamical horizons important in current black hole research.

## ReferencesEdit

- ↑ Chaisson, Eric (1990).
*Relatively Speaking: Relativity, Black Holes, and the Fate of the Universe*. W. W. Norton & Company. p. 213. ISBN 978-0393306750. - ↑ Bennett, Jeffrey; Donahue, Megan; Schneider, Nicholas; Voit, Mark (2014).
*The Cosmic Perspective*. Pearson Education. p. 156. ISBN 978-0-134-05906-8.