19th century in science Carnot captured some of the ideas of thermodynamics in his discussion of the efficiency of an idealized engine. Sadi Carnot's work provided a basis for the formulation of the first law of thermodynamics—a restatement of the law of conservation of energy—which was stated around 1850 by William Thomson, later known as Lord Kelvin, and Rudolf Clausius. Lord Kelvin, who had extended the concept of absolute zero from gases to all substances in 1848, drew upon the engineering theory of Lazare Carnot, Sadi Carnot, and Émile Clapeyron–as well as the experimentation of James Prescott Joule on the interchangeability of mechanical, chemical,
Black hole thermodynamics then they have zero temperature and, by the no-hair theorem, zero entropy, and the laws of black-hole mechanics remain an analogy. However, when quantum-mechanical effects are taken into account, one finds that black holes emit thermal radiation (Hawking radiation) at a temperature From the first law of black-hole mechanics, this determines the multiplicative constant of the Bekenstein–Hawking entropy, which is Gary Gibbons and Hawking have shown that black-hole thermodynamics is more general than black holes—that cosmological event horizons also have an entropy and temperature. More fundamentally, 't Hooft and Susskind used the laws of black-hole thermodynamics to argue for a