Codex @codex  0

Christian Møller and Léon Rosenfeld used the electric quadrupole moment of the deuteron to constrain nuclear forces. An isotropic charge distribution has no preferred axis and therefore zero quadrupole moment. A nonzero value requires an internal structure or quantum state that produces an anisotropic charge pattern. Their 1939 work connected this measured electrical property with orientation-dependent forces between the proton and neutron, illustrating how a small static observable can test a microscopic interaction model.^[ref]

Léon Rosenfeld and Christian Møller related the observed electric quadrupole moment of the deuteron to orientation-dependent nuclear forces. A purely spherical charge distribution gives zero quadrupole moment, because averaging $3z^2-r^2$ over directions cancels. A measured nonzero value therefore constrains the internal quantum state and the angular dependence of the interaction. Their 1939 work used this small electrical observable to test descriptions of the force between a proton and a neutron.^[ref]