Ben Roy Mottelson was a distinguished American-Danish theoretical physicist whose pioneering work revolutionized our understanding of atomic nuclei. Born in Chicago on July 9, 1926, he earned his bachelor's degree from Purdue University in 1947 and completed his PhD in nuclear physics at Harvard University in 1950 under Julian Schwinger. Following his doctoral studies, Mottelson accepted a fellowship at the Niels Bohr Institute of Theoretical Physics in Copenhagen where he began his groundbreaking research on nuclear structure. He subsequently became a permanent member of the theoretical study group at CERN in Copenhagen and was appointed professor at the Nordic Institute for Theoretical Atomic Physics in 1957, a position he held until 1994, while also serving as visiting professor at the University of California, Berkeley in 1959. Mottelson became a naturalized Danish citizen in 1971, cementing his lifelong commitment to scientific research in Denmark.
Mottelson's most significant contribution was his collaborative work with Aage Bohr and James Rainwater that revealed the non-spherical geometry of atomic nuclei, fundamentally challenging the previously accepted theory that all nuclei were perfectly spherical. Through detailed theoretical analysis and experimental verification, they demonstrated that the motion of subatomic particles could distort the shape of the nucleus, with profound implications for understanding nuclear structure across all elements. Their development of the unified model of the nucleus established the critical connection between collective motion and particle motion within atomic nuclei, providing a comprehensive theoretical framework that accounted for the variety of nuclear excitations. This revolutionary work, which was published in a series of papers between 1952 and 1953, earned Mottelson, Bohr, and Rainwater the Nobel Prize in Physics in 1975, with the Nobel Committee specifically recognizing their "discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection".
Following his Nobel Prize, Mottelson continued to make substantial contributions to nuclear physics, particularly in the study of deformed nuclei and the application of pairing effects inspired by superconductivity theory. He played a pivotal role in introducing the pairing effect to explain differences in energy levels between even and odd atomic nuclei, drawing analogies from the Bardeen-Cooper-Schrieffer theory of superconductivity. Mottelson's scientific leadership extended beyond his own research as he devoted significant effort to fostering international scientific collaboration, most notably through his instrumental role in establishing the European Centre for Theoretical Studies in Nuclear Physics and Related Areas in Trento, Italy, where he served as founding director from 1993 to 1997. The profound impact of his work continues to shape nuclear physics research worldwide, and his legacy as a theoretical physicist of extraordinary depth and insight endures through the foundational principles he established regarding the structure and behavior of atomic nuclei. Ben Roy Mottelson passed away on May 13, 2022, at the age of 95, leaving behind a scientific legacy that transformed our understanding of the fundamental building blocks of matter.
