Dreams of a Final Theory: The Scientist's Search for the Ultimate Laws of Nature

by Steven Weinberg

The example of emergence that has been historically most important in physics is thermodynamics, the science of heat. As originally formulated in the nineteenth century by Carnot, Clausius, and others, thermodynamics was an autonomous science, not deduced from the mechanics of particles and forces but built on concepts like entropy and temperature that have no counterparts in mechanics. Only the first law of thermodynamics, the conservation of energy, provided a bridge between mechanics and thermodynamics. The central principle of thermodynamics was the second law, according to which (in one formulation) physical systems possess not only an energy and a temperature but also a certain quantity called entropy, which always increases with time in any closed system and reaches a maximum when the system is in equilibrium. This is the principle that forbids the Pacific Ocean from spontaneously transferring so much heat energy to the Atlantic that the Pacific freezes and the Atlantic boils; such a cataclysm need not violate the conservation of energy, but it is forbidden because it would decrease the entropy.