Giant dipole resonance and evolution of concepts of nuclear dynamics

Fifty years ago, in his paper on "Quadrupole and dipole γ emission from nuclei", A.B. Migdal introduced implicitly the concept of a dynamic collective model in nuclear physics and used this concept to predict a giant dipole resonance. Evolution of the theory of this resonance has had an enormous influence on the formation of modern concepts relating to the dynamics of nuclei. A brief historical introduction is followed in this paper by an account of the conceptual aspects of the subsequent evolution of the ideas on the nature of the giant dipole resonance. This evolution has followed a complex path from the initial identification of a nucleus with a liquid drop to its representation as a system of independent nucleons. Recent investigations have made it possible to understand the interrelationships between these apparently diametrically opposed concepts, to bring them closer together, and to demonstrate the equivalence of the description of the giant dipole resonance with either concept.