The discussion clarifies the distinction between apparent horizons and event horizons in the context of spacetime. The apparent horizon is a local concept where light rays do not move radially outward, and it is coordinate dependent, meaning it can change based on the slicing of spacetime. In contrast, the event horizon is a fundamental property that defines the boundary beyond which light cannot escape to null infinity, relying on the entire history of spacetime. Apparent horizons are always located inside event horizons, and while they can evolve discontinuously, event horizons evolve smoothly.
PREREQUISITESPhysicists, cosmologists, and students of general relativity seeking to deepen their understanding of horizon concepts and their implications in spacetime analysis.
Nabeshin said:The apparent horizon is, roughly speaking, the surface at which light rays shined radially outward do not move radially outward (inside the apparent horizon, they move inwards.) This implies that the apparent horizon is a local concept, in the sense that it relies only on an experiment in some small region of spacetime. Furthermore, apparent horizons are coordinate dependent horizons which can change or even vanish depending on your slicing of spacetime.
Event horizons, by contrast, are the surfaces beyond which light can never propagate to null infinity. This definition requires you to know the entire history of the spacetime to determine whether or not the light can escape, and in this sense it is a fundamental causal property of the spacetime which does not depend on coordinates.
Other differences between the two are that apparent horizons, if they exist, are always inside event horizons. The two horizons also coincide for a static spacetime. The apparent horizon can evolve discontinuously, while the EH evolves smoothly.