The branch of physics, closely related to classical thermodynamics, that studies the statistical and thermal properties of physical systems (e.g., gas) in terms of how statistical laws governing the component particles at the microlevel can govern the behaviour of the system at the macro level. James Clerk Maxwell (1831-1879 with his kinetic theory of gases was the starting point for statistical mechanics. One of its earliest applications was the attempt by Ludwig Boltzmann (1844-1906), its ultimate founder, to explain the thermodynamical properties of gases on the basis of the statistical properties of large assemblies of molecules. Joshiah W. Gibbs (1839-1903) followed this up by establishing the equivalence of statistical mechanics and thermodynamics through drawing an analogy with newtonian mechanics. With the advent of quantum mechanics, the preciseness of the premises of statistical mechanics (e.g., each particle has an exact position and momentum in a state space at a particular moment) was undermined. Thus, quantum mechanics came to absorb classical statistical mechanics. However, statistical mechanics played an important role in understanding the processes involved in transitions between two stable states.
See Bridge law (or principle), Classical thermodynamics, Newtonian mechanics, Quantum mechanics, Transition