The explicit solution for the time evolution of the Nth rank spatial moment(The equation is abbreviated)(rj is the jth cartesian component of the position vector →r) of some field (The symbol is abbreviated) (→r; t) has been obtained. (The symbol is abbreviated) can be any spatially bounded massive field satisfying a relativistic first order linear (”Dirac-like”) differential equation, such as the Dirac bi-spinor. The solution for the Nth moment is a ”polynomial” in time of degree N, whose coefficients are oscillatory functions with only the frequency ω= mc^2/ħ being present. The spin of the field (The symbol is abbreviated) does not affect the form of the solution.