Question

What is an event horizon?​

Answer 1

Answer:

In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer.

In 1784, John Michell proposed that near compact massive objects, gravity can be strong enough that even light cannot escape. At that time, the Newtonian theory of gravitation and the so-called corpuscular theory of light were dominant. In these theories, if the escape speed of an object exceeds the speed of light, then light originating inside or from it can escape temporarily but will return. In 1958, David Finkelstein used General Relativity to introduce a stricter definition of a local black hole event horizon as a boundary beyond which events of any kind cannot affect an outside observer. This led to information and firewall paradoxes, which encouraged the re-examination of the concept of local event horizons and the notion of black holes. Several theories were subsequently developed, some with and some without event horizons. Stephen Hawking, who was one of the leading developers of theories to describe black holes, suggested that an apparent horizon should be used instead of an event horizon, saying "gravitational collapse produces apparent horizons but no event horizons". He eventually concluded that "the absence of event horizons mean that there are no black holes — in the sense of regimes from which light can't escape to infinity."

Explanation:

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Answer 2

In astrophysics, an event horizon is a boundary beyond which events cannot affect an observer. In 1784, John Michell proposed that near compact massive objects, gravity can be strong enough that even light cannot escape.