The branch of physics that applies statistical principles to the mechanical behavior of large numbers of small particles (such as molecules, atoms, or subatomic particles) in order to explain the overall properties of the matter composed of such particles. The kinetic theory of heat is an example of statistical mechanics; the laws of thermodynamics can all be explained using statistical mechanics. Both classical physics and quantum mechanics have been used in the development of statistical mechanical theories. ◇ Bose-Einstein statistics explains the behavior of large numbers of bosons, which are particles that can simultaneously occupy the same quantum state (such as photons in a laser beam). ◇ Fermi-Dirac statistics explains the behavior of large numbers of particles that obey the Pauli exclusion principle (such as electrons) and cannot simultaneously occupy the same quantum state.