Horizon (general relativity)
Encyclopedia
There are several types of spacetime
horizons that play a role in Einstein's theory of general relativity
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Spacetime
In physics, spacetime is any mathematical model that combines space and time into a single continuum. Spacetime is usually interpreted with space as being three-dimensional and time playing the role of a fourth dimension that is of a different sort from the spatial dimensions...
horizons that play a role in Einstein's theory of general relativity
General relativity
General relativity or the general theory of relativity is the geometric theory of gravitation published by Albert Einstein in 1916. It is the current description of gravitation in modern physics...
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- Absolute horizonAbsolute horizonIn general relativity, an absolute horizon is a boundary in spacetime, defined with respect to the external universe, inside of which events cannot affect an external observer. Light emitted inside the horizon can never reach the observer, and anything that passes through the horizon from the...
, a boundary in spacetime in general relativity inside of which events cannot affect an external observer - Apparent horizonApparent horizonIn general relativity, an apparent horizon is a surface that is the boundary between light rays that are directed outwards and moving outwards, and those directed outwards but moving inwards.Apparent horizons are not invariant properties of a spacetime...
, a surface defined in general relativity - Cauchy horizonCauchy horizonIn physics, a Cauchy horizon is a light-like boundary of the domain of validity of a Cauchy problem...
, a surface found in the study of Cauchy problems - Celestial horizonCelestial horizonThe celestial horizon, also called the rational horizon, is a great circle parallel to the horizon, in which a plane at right angles to Zenith and Nadir lines are passing through the centre of the earth and intersects the celestial sphere, the center of which is the center of the Earth....
, a great circle parallel to the horizon - Cosmological horizon, a limit of observability
- Event horizonEvent horizonIn general relativity, an event horizon is a boundary in spacetime beyond which events cannot affect an outside observer. In layman's terms it is defined as "the point of no return" i.e. the point at which the gravitational pull becomes so great as to make escape impossible. The most common case...
, a boundary in spacetime beyond which events cannot affect the observer - Killing horizonKilling horizonA Killing horizon is a null hypersurface on which there is a null Killing vector field .Associated to a Killing horizon is a geometrical quantity known as surface gravity, \kappa...
, a null surface on which there is a killing vector field - Particle horizon, the maximum distance from which particles can have travelled to an observer in the age of the universe