In the first three parts of the book, the themes have been mechanics, thermodynamics, and electromagnetism, which can be grouped under the broader heading of “macrophysics”—that is, the physics of objects of ordinary size and larger. This fourth part of the book addresses for the first time the vastly different realm of “microphysics.” As used here, the term means the physics of molecules, atoms, and subatomic particles. Microphysics will be a major theme in the book from now on, particularly here in part 4, and then in parts 6 (quantum mechanics), 7 (nuclear physics), and 8 (particle physics).
Molecules (and the atoms they contain) are very small, incredibly large in number, chaotic in their motion, and difficult to isolate and study as individuals. But populations of molecules, like human populations, can be described by statistical methods. The strategy is to focus on average, rather than individual, behavior. Insurance companies do their business this way, and so do molecular physicists. The insurance company statistician might calculate the average life span for an urban population of males in a certain income bracket. The physicist might seek an average energy for a population of gas molecules occupying a certain volume at a certain temperature. The method works well enough for the insurance company to make a profit, and even better for the physicist because molecules are far more numerous and predictable than human beings. By determining energy, or an average value for some other mechanical property of molecules, the physicist practices what Gibbs called “statistical mechanics.”
The single chapter in this part of the book introduces the man who did the most to define, develop, and defend statistical mechanics. He was Ludwig Boltzmann, who wrote his most important papers on statistical mechanics in the 1870s. For Boltzmann, statistical mechanics was most profitable in discussions of the entropy concept. He found a molecular basis for the second