LAUSR

Ben Roy Mottelson passed away on May 13 of this year at the age of 95. Ben was a giant of theoretical physics, having, with Aage Bohr (son of Niels Bohr), reshaped our understanding of the atomic nucleus in all its aspects, both static and dynamic. At the same time he had a great impact on international theoretical physics and influenced generations of nuclear physicists. Ben was born in La Grange, Illinois, a western suburb of Chicago. Sent to Purdue University for naval officer training in World War II, he remained there after the war, receiving his Bachelor of Science degree in 1947. In 1950, after completing his doctoral thesis, “The ground states of lithium 6 and lithium 7,” at Harvard under the supervision of Julian Schwinger, he went in 1950 to 1951 to the University Institute for Theoretical Physics, now the Niels Bohr Institute (NBI), in Copenhagen, which became his base for the rest of his life. Ben became a Danish citizen in 1971, and was elected as a Foreign Member of the National Academy of Sciences in 1973. At the time of Ben’s arrival in Copenhagen, the common picture of the atomic nucleus was that it was a liquid drop composed of a fluid of neutrons and protons, an idea originating with Niels Bohr. This picture well explained the elementary energetics of nuclei as well as dynamical properties, such as nuclear fission. However, nuclear “shell effects,” analogous to atomic shell effects (e.g., tight binding of nuclei for the “magic numbers” of neutrons or protons 20, 28, 50, 82, and so forth, which had recently been put in evidence by Maria Goeppert Mayer and Hans Jensen) seemed to be in conflict with this picture. Furthermore, as questioned by James Rainwater at Columbia University and taken up by Aage Bohr, could some nuclei be aspherical? The challenge that Ben and Aage met was to put together a unified picture of the nucleus, to understand the interplay between collective behavior of the nucleus as a whole and the motion of individual nucleons, for which there was gathering experimental evidence. If nuclei were aspherical, one could expect them to behave as rotating tops, with bands of rotational excitations, as in molecules; such bands were investigated theoretically and found experimentally in the course of the 1950s. In addition, certain excited states of nuclei corresponded to quadrupole vibrations about a spherical equilibrium. Studies of these states opened up a particularly fruitful interplay between theory and experiment in nuclear physics. By the mid-1950s, however, two puzzles were apparent: the first was why the measured moments of inertia of deformed nuclei were less than those of a rigid body nucleus, and—seemingly unrelated—why certain states of deformed nuclei required particularly high energies to excite. Their common solution would unexpectedly tie together nuclear and condensed matter physics. In 1958, after David Pines brought to Copenhagen the then new Bardeen, Cooper, and Schrieffer explanation of superconductivity in metals as arising from pairing of electrons, Ben, Aage, and David quickly realized that an analogous pairing of neutrons or of protons in nuclei could explain the reduced moments of inertia, in the same way that the moment of inertia of a bucket of superfluid liquid helium is less than the classic rigid body value, and that the high excitation energies were the analog of the energy gap in a Ben Roy Mottelson. Photo courtesy of Ola J. Joensen, Niels Bohr Institute, Copenhagen, Denmark.