Including the black hole entropy in the entropy ledger gives a more useful law, the generalized second law of thermodynamics (GSL) (Bekenstein 1972, 1973, 1974): the sum of ordinary entropy S_o outside black holes and the total black hole entropy never decreases and typically increases as a consequence of generic …

Since a “quantum” black hole emits heat and light, it therefore has a temperature. This means black holes are subject to the laws of thermodynamics. The entropy of a black hole is then related to the surface area of its event horizon. The second law again states that the entropy of a black hole system cannot decrease.

The second law of thermodynamics requires that black holes have entropy. If black holes carried no entropy, it would be possible to violate the second law by throwing mass into the black hole.

It can never leave that region. For all practical purposes the matter has disappeared from the universe. Once inside the black hole’s event horizon, matter will be torn apart into its smallest subatomic components and eventually be squeezed into the singularity.

Black holes are freezing cold on the inside, but incredibly hot just outside. The radiation from the material masks the tiny amount of radiation escaping from the hole itself, and so what the astronomers observe is the very hot outside environment, rather than the freezing cold environment inside.

Black holes are freezing cold on the inside, but incredibly hot just outside. The internal temperature of a black hole with the mass of our Sun is around one-millionth of a degree above absolute zero.

The most massive black holes in the Universe, the supermassive black holes with millions of times the mass of the Sun will have a temperature of 1.4 x 10-14 Kelvin. That’s low. Almost absolute zero, but not quite.