““If they were as we described them, the existence of black holes would contradict the principles of thermodynamics. One of these principles says that heat always flows from hotter to cooler bodies. The classical non-quantum mechanical black hole will eat up energy in any shape, including thermal energy. Therefore, it is colder than-or at least as cold as-any other matter existing. Thermodynamics will admit temperatures that are barely above absolute zero, but not this lower limiting temperature itself. The problem that a classical black hole poses is therefore this: It cannot be warmer than absolute zero; otherwise there might be colder objects to which it would pass thermal energy. But like all other objects, it cannot reach absolute zero itself. Thus it is impossible for us to assign any temperature at all to a black hole, and that clashes with the laws of thermodynamics. It takes quantum mechanics to resolve this contradiction. Stephen Hawking devised an answer in 1974: While the black hole itself does not radiate, its immediate surroundings can give off thermal energy by radiation, like any other mass. To an observer at some distance, the radiation appears to issue from the black hole itself.””
