Black Holes
A black hole (the term was coined by John Archibald Wheeler in 1967) is a closed surface through which gravity prevents light from propagating. Insofar as relativity prohibits anything from traveling faster than light, it follows that nothing can escape through the surface of a relativistic black hole. That said, in general relativity the notion of energy is problematic, and energy and hence mass can be extracted by classical and quantum processes. Classically the interior of a black hole contains a singularity: Along certain paths physical quantities become ill-behaved (e.g., the gravitational field may become infinite). While nothing can pass back through the surface of a black hole, it is possible in certain models to travel into other universes. All of these properties have philosophically disturbing implications that have strongly influenced the development of physics, especially since there are solid theoretical and experimental reasons to believe that black holes are not merely hypothetical, but actually exist.
History
Black holes (henceforth BHs) arise in the general theory of relativity (GTR). However something similar is possible in Newtonian physics. John Michell (1784) and Pierre Simon Laplace (1796) pointed out that, as a ball thrown upwards with insufficient speed will eventually fall back to Earth, if a star of a given mass were smaller than a certain size (in modern parlance, its critical radius) then even light corpuscles, emitted from the surface at the speed of light, would eventually be pulled back to its surface.
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