Liouville's Theorem

(1) In mechanics, a theorem asserting that the volume in phase space of a system obeying the equations of mechanics in Hamiltonian form remains constant as the system moves. Liouville’s theorem was established in 1838 by the French scientist J. Liouville.

The state of a mechanical system defined by the generalized coordinates q₁, q₂, . . ., q_N and the canonically conjugate generalized momenta p₁, p₂, . . ., p_N (where N is the number of degrees of freedom of the system) can be considered as a point with rectangular Cartesian coordinates q₁, q₂, ..., q_N; p₁, p₂, . . ., p_N is a 2N-dimensional space called the phase space. The evolution of the system in time is represented as the motion of this phase point in the 27V-dimensional space. If phase points entirely fill some region of the phase space at the initial moment of time and pass over into another region of the space in the course of time, then the corresponding phase volume, according to Liouville’s theorem, will be the same. Thus the motion of the points that represent the state of the system in the phase space resembles that of an incompressible fluid.

Liouville’s theorem permits introduction of a distribution function of the particles of the system in phase space and is the basis of statistical physics.