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

by Steven Weinberg

Then in the second half of the nineteenth century the work of a new generation of theoretical physicists (including Maxwell in Scotland, Ludwig Boltzmann in Germany, and Josiah Willard Gibbs in America) showed that the principles of thermodynamics could in fact be deduced mathematically, by an analysis of the probabilities of different configurations of certain kinds of system, those systems whose energy is shared among a very large number of subsystems, as for instance a gas whose energy is shared among the molecules of which it is composed. (Ernest Nagel gave this as a paradigmatic example of the reduction of one theory to another.) In this statistical mechanics, the heat energy of a gas is just the kinetic energy of its particles; the entropy is a measure of the disorder of the system; and the second law of thermodynamics expresses the tendency of isolated systems to become more disorderly.