Ablack holeis a geometrically defined region ofspacetimeexhibiting such stronggravitationaleffects that nothing—includingparticlesandelectromagnetic radiationsuch as light—can escape from inside it. The thepry ofgeneral relativitypredicts that a sufficiently compactmasscan deformspacetimeto form a black hole. The boundary of the region from which no escape is possible is called theevent horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. In many ways a black hole acts like an idealblack body, as it reflects no light. Moreover,quantum field theory in curved spacetimepredicts that event horizons emitHawking radiation, withthe same spectrumas a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin forblack holes of stellar mass, making it essentially impossible to observe.
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