A fourfold system. An electric quadrupole, for example, would be two dipoles with one reversed with respect to the other. Quadrupole interactions between electrons in atoms lead to some of the forbidden lines in the spectra of nebulae. Mass distributions such as an oblate spheroid or a binary star system will also have a quadrupole moment. General relativity predicts that a rotating mass system having a quadrupole moment will radiate gravitational waves which are quadrupole in nature. Thus if a gravity wave were to pass by a system with four masses at the corners of a square, the two along one diagonal would move towards each other, while the other two would move apart. The cosmic microwave background, although very close to being isotropic, shows a very small quadrupole variation.
Subjects: Astronomy and Astrophysics.