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Obituary: Ben Roy Mottelson (1927-2022)

»We are very sad to learn that Ben Mottelson passed away on 13 May 2022. He was a remarkable human being and physicist, and his deep thinking, warm personality, contagious enthusiasm, playfulness and laughter will be sorely missed,« write several of Ben Mottelson's former colleagues in their obituary of the Nobel Prize winner from Copenhagen‘s Niels Bohr Institute.

Ben was born in La Grange, in the western suburbs of Chicago. In World War II he was sent to Purdue University for training as a naval officer and he remained there after the war, receiving a BS degree in 1947. After completing his PhD at Harvard in 1950 under the supervision of Julian Schwinger, he received a Sheldon Traveling Fellowship, and chose to spend the year 1950-51 in Copenhagen at the Niels Bohr Institute (NBI), then UITF (Universitetets Institut for Teoretisk Fysik).

Fortunately for the development of physics, Copenhagen became his base for the rest of his life, and he became a Danish citizen in 1971. After short-term appointments at NBI, he became a staff member of the CERN Theoretical Study Group that operated in Copenhagen during the years 1953-1957 before the facilities at CERN’s long-term home in Geneva were inaugurated. In 1957 he became professor at the then newly established Nordita (then the Nordic Institute for Theoretical Atomic Physics, now the Nordic Institute for Theoretical Physics). He remained there for the rest of his career, apart from shorter periods at other institutions, and was Nordita’s director from 1981-1983.

Symbiosis between Ben Mottelson and Aage Bohr

In Copenhagen a close scientific symbiosis arose between Ben and Aage Bohr who, after a period in the USA, was taken up with the possibility of nuclei being aspherical. This notion, which was at odds with Niels Bohr’s picture of the nucleus as a liquid drop, was initially put forward by James Rainwater at Columbia. The challenge that Ben and Aage attacked was to understand the interplay between collective behavior of the nucleus as a whole and the motion of individual nucleons, for which there was recent experimental evidence. If nuclei were aspherical, one could expect there to be rotational bands, as in molecules, and these were investigated theoretically and found experimentally in the course of the 1950s. This led to a fruitful interplay between theory and experiment and, due to the presence of Ben and Aage, Copenhagen became a leading center internationally for nuclear research. The weekly “Experimental Group Meeting” at NBI was an important forum for discussions of nuclear physics, both theoretical and experimental.

The Nobel Prize

One puzzle was why the effective moment of inertia of deformed nuclei was less than the value for a rigid body. This was solved in 1958, after David Pines described in Copenhagen the then recent BCS (Bardeen Cooper, and Schrieffer) explanation of superconductivity in metals as being due to pairing of electrons. Ben, Aage and David realized that an analogous pairing of nucleons in nuclei could explain the reduced moments of inertia. Other modes of excitation of nuclei were also considered, and Ben and Aage’s unified model of the nucleus accounted excellently for the experimental properties of nuclei at low energies. For their work on disentangling the many facets of the behaviour of atomic nuclei, Ben and Aage, together with James Rainwater, received the Nobel Prize in Physics in 1975. In the latter part of this early period and beyond, Ben had a close collaboration with Ikuko Hamamoto, who played a major role in the creation of Ben and Aage’s two-volume classic, “Nuclear Structure”, which is still widely used today.

Following the Nobel Prize, Ben continued to work on problems in nuclear physics, especially on deformed nuclei, but his focus moved towards other small quantum systems. Small metal clusters turned out to have interesting properties, and in Copenhagen Ben had a fruitful theoretical collaboration with colleagues there. In 1971 Balian and Bloch had predicted that the density of levels in finite systems should exhibit a “supershell” structure superimposed on the shell structure familiar from atoms and atomic nuclei. To observe such effects demanded systems with one thousand or more particles, and was therefore impossible with nuclei. However, with metal clusters this could be done. It was a great triumph that experiments at NBI to measure the distribution of cluster sizes formed from sodium vapour demonstrated the existence of supershells for the first time.

Shortly afterwards, Ben realized that shell structure would also occur in semiconductor nanostructures. Back in the 1990s this was an entirely new field after these so-called “artificial atoms” had been created in the laboratory. With his curiosity and outstanding ability to see the big picture, Ben contributed to the field of nanoscience.

Ignored traffic lights

His enthusiasm and deep knowledge of many-body and few-body physics were inspiring to people working with him and a strong driving force in shaping and guiding nuclear physics and related areas. Another example is the physics of ultra-cold atomic Bose gases, where he worked intensely on the nature of vortices in rotating condensates and their connection to superfluidity. Likewise, his studies of pairing in ultra-cold Fermi gases set the stage for the later experimental discovery of pairing fluctuations in these systems.

Around the turn of the century, Ben worked with Aage Bohr again, this time together with Ole Ulfbeck, on the foundations of quantum mechanics, where they postulated fortuitousness as the guiding principle.

For his contributions, Ben received widespread recognition both in Denmark and abroad. Among honours apart from the Nobel Prize, he was an international member of the US National Academy of Sciences (1973), and a member of the Royal Danish Academy of Sciences and Letters (1974), and he received the Atoms for Peace Award (1969).

Throughout his career, Ben sought to avoid public attention, and it was his good fortune to be in China when the award of the Nobel Prize was announced. He was unpretentious and very approachable on a personal level. Ben read widely and one of his favorite places was the NBI library, very close to his office.

He devoted himself to science and, with a few exceptions avoided administrative positions. Nevertheless, he had great impact on international science, especially in the leading role he played in the establishment in Trento, Italy, of ECT, the European Centre for Theoretical Studies in Nuclear Physics and Related Areas, of which he became founding director, from 1993-1997.

Ben was a family person. He was married to Nancy née Reno until her early death in 1975, and they had three children. In 1983 he married Britta Siegumfeldt, who died in 2014. His warm sense of family also extended to the broader community of physicists, in Copenhagen, at ECT and beyond.

Ben had a playful nature: making kites and hot-air balloons were among his delights, as well as climbing trees. Ben was fearless when expressing his views on science or politics, and when on his bike, shunning bike helmets and ignoring traffic lights. Despite personal tragedies and his own health issues in the last years, he retained his characteristic good humour.

Although Ben is no longer with us, his legacy will continue to inspire us and coming generations.