The most distinguishing feature of the supermassive black hole: its event horizon. The point of no return for in-falling matter, it is about 15 million kilometres across, or one-tenth of the distance between Earth and the sun – minuscule in astronomical terms.
In general relativity, an event horizon is a boundary in spacetime, most often an area surrounding a black hole, beyond which events cannot affect an outside observer. Light emitted from beyond the horizon can never reach the observer, and any object that approaches the horizon from the observer’s side appears to slow down and never quite pass through the horizon, with its image becoming more and more redshifted as time elapses. The traveling object, however, experiences no strange effects and does, in fact, pass through the horizon in a finite amount of proper time.
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