The general theory of relativity predicts the existence of gravitational radiation or gravitational waves. Just as an accelerated charge produces electromagnetic radiation in the form of electric and magnetic fields, an accelerated mass produces gravitational radiation in the form of vibrations in the curvature of space-time analogous to the ripples in a pond.

The classical problem, which Albert Einstein solved in 1916, was to consider the radiation pattern from a thin rod rotating about an axis perpendicular to its center. The corresponding situation in electromagnetism produces a radiation pattern whose leading term is given by the electric-dipole moment. The mass-dipole moment is the time rate of change of the momentum of the spinning rod. Because this quantity vanishes by momentum conservation, the power of the radiation scales as the quadrupole moment, and its details are considerably more subtle.

The power of the radiation...