Hidden mechanical momentum and the field momentum in stationary electromagnetic and gravitational systems

The existence of hidden momentum, i.e., the mechanical momentum of a body the constituents of which move in a stationary manner but the center of mass of which is at rest in an external static field of force, is shown to follow from the general requirements of relativistic mechanics of continuous media, independently of whether the external forces are electromagnetic or gravitational. The hidden mechanical momentum is compensated by the momentum of the static fields in the system, necessitating, in the gravitational case, the existence of a gravinetic quasistatic gravitational field, which, in analogy to the magnetostatic field, is generated by a quasistationary current of mass and acts on a moving mass.