The second law specifies that heat will not pass spontaneously from a colder to a hotter body without some change in the system. Or, as Planck himself generalized it in his Ph.D. dissertation at the University of Munich in 1879, that “the process of heat conduction cannot be completely reversed by any means.” Besides forbidding the construction of perpetual-motion machines, the second law defines what Planck’s predecessor Rudolf Clausius named entropy: because energy dissipates as heat whenever work is done—heat that cannot be collected back into useful, organized form—the universe must slowly
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