Mechanical properties of the light wave with phase singularity.

General analysis of energy transport and momentum density of a paraxial light beam is presented. Usually there exist transverse components of which the tangential one corresponds to energy circulation around the beam axis. This circulation produces the mechanical angular momentum of the beam that exists independently on polarization state of radiation. In particular, non-zero angular momentum is typical for the beam with wave front dislocation; e.g., angular momentum of a Laguerre-Gaussian beam with axial symmetry and zero magnitude at the axis has the same value that a plane circularly polarized wave of equal energy holds. This case is shown to be only a special representative of the wide class of paraxial beams holding a sort of vortical motion. The general analytical hallmark of this class is formulated in terms of the Wigner function moments and is found to consist in asymmetry of the matrix of mixed space-angle intensity moments. On this base some phenomena connected to generation, propagation and interaction of wave front dislocations manifest a demonstrative mechanical analogy and allow apparent interpretation Principal possibility to observe the beam vorticity in experiments where the light phase is modulated by mechanical torques of reaction applied to optical elements is discussed.