Exchange currents

Recent developments in the theory of nuclear exchange current operators are reviewed. The relation between the nuclear interaction and the associated electromagnetic and weak exchange current operators is elaborated. The review is focused on those exchange current operators which appear as consequences of chiral symmetry and gauge invariance and for which there is clear experimental evidence. The exchange current operators are described as they appear both within the conventional meson exchange framework as well as within the compatible, but more fundamental, topological soliton model approach. The latter, which allows a coherent simultaneous treatment of both nucleons and nuclei, leads to many phenomenologically instructive connections between weak, electromagnetic and strong interaction operators. Finally the role of exchange current operators in relativistic treatments of the nuclear transition operators is reviewed. (orig.)