Ben Mottelson

A home full of argument

Ben Roy Mottelson was born in Chicago on July 9, 1926, the second of three children of Goodman Mottelson, an engineer with a university degree, and Georgia Mottelson, née Blum. His family was Jewish. He grew up in the village of La Grange, a western suburb of Chicago, in a home where, as he later wrote, scientific, political and moral issues were freely and vigorously discussed.

He went to Lyons Township High School in La Grange and graduated during the Second World War. The United States Navy sent him to Purdue University for officer training, and he stayed on to earn a bachelor’s degree in 1947. He then went to Harvard for graduate work in nuclear physics under Julian Schwinger, one of the great theorists of the century, and received his doctorate in 1950.

Copenhagen for a year, and for life

With a Sheldon Travelling Fellowship from Harvard, Mottelson chose to spend 1950 and 1951 at the Institute for Theoretical Physics in Copenhagen, later the Niels Bohr Institute, drawn by its place in the history of modern physics and its tradition of international cooperation. He never really left. A fellowship from the U.S. Atomic Energy Commission kept him there for two more years, and from 1953 to 1957 he worked in the theoretical study group of CERN, the new European laboratory for particle physics, which was based in Copenhagen while its home in Geneva was being built.

When the Nordic Institute for Theoretical Atomic Physics, Nordita, was founded in Copenhagen in 1957, Mottelson became a professor there, a post he held for most of his career; he was its director from 1981 to 1983. He became a Danish citizen in 1971 while keeping his American passport.

He spent the spring of 1959 as a visiting professor at the University of California, Berkeley, but Copenhagen remained his base. The Niels Bohr Institute and Nordita were magnets for physicists from all over the world, and over the decades Mottelson helped train and inspire a stream of young theorists from the Nordic countries and far beyond. He was elected an international member of the U.S. National Academy of Sciences in 1973 and a member of the Royal Danish Academy of Sciences and Letters in 1974, and he received the John Price Wetherill Medal in 1974 and the Marian Smoluchowski Medal in 1980.

A partnership with Aage Bohr

In Copenhagen Mottelson began a collaboration with Aage Bohr, the son of Niels Bohr, that lasted from 1950 until Aage Bohr’s death in 2009. Mottelson described it as a dialogue between kindred spirits attuned by a long period of shared experience. The two were a famous contrast: Bohr reserved, Mottelson outgoing, playful and full of enthusiasm.

The problem they took on was the nature of the atomic nucleus. By 1950 physicists had two very different pictures of it. In the shell model, protons and neutrons move independently in orbits, like electrons in an atom, which explained why certain numbers of particles make especially stable nuclei. In the liquid drop model, the nucleus behaves like a drop of fluid, which explained nuclear fission. Each worked for some things and failed for others. The American physicist James Rainwater had suggested that the outer particles might pull a nucleus out of its spherical shape.

Nuclei that spin and change shape

Bohr and Mottelson brought the two pictures together. They showed that the independent motion of individual protons and neutrons can deform the nucleus as a whole into a shape like a rugby ball, and that the deformed nucleus can then rotate and vibrate collectively, while the shape in turn shapes the orbits of the particles. In papers in 1952 and 1953, including their landmark article “Collective and individual-particle aspects of nuclear structure,” they showed that the energy levels of many nuclei follow the pattern expected of a spinning body, a rotational band like those seen in molecules.

Experiments at the Copenhagen cyclotron and elsewhere quickly found the predicted rotational bands. In 1958, after the BCS theory of superconductivity appeared, Bohr, Mottelson and David Pines showed that protons and neutrons in a nucleus pair up much as electrons do in a superconductor. That pairing explained why deformed nuclei rotate as if they were lighter than a rigid body and why nuclei with even numbers of particles differ from odd ones.

The result, known as the Bohr–Mottelson or unified model, became the standard framework for understanding the structure of atomic nuclei. Their two-volume book, Nuclear Structure, published in 1969 and 1975, became a bible of the field for generations of nuclear physicists.

Mottelson always insisted that theory must stay close to measurement. Colleagues recalled that in his lectures at the Niels Bohr Institute in the 1960s and 1970s, attended by students from many countries, he stressed learning from experimental data and keeping theorists and experimentalists talking to each other, and that he worked closely with experimental groups around the world. The predicted rotational bands were found in the early 1950s using a technique called Coulomb excitation at the Copenhagen cyclotron, a quick confirmation that helped the model win acceptance.

Why it matters

Atomic nuclei are the source of nuclear energy, of the radioactive isotopes used in medical diagnosis and cancer treatment, and of the elements forged in stars. Using any of them well depends on understanding how nuclei are built, how they hold together and how they respond when energy is added. The Bohr–Mottelson model gave physicists the language and the tools to describe the shapes and excitations of nuclei, and it remains part of the foundation on which nuclear science is built.

Their work also showed how ideas travel across physics. The pairing borrowed from superconductivity proved to be a general feature of systems of many interacting particles, from nuclei to neutron stars to clouds of ultracold atoms, and Mottelson spent much of his later career following those connections. In 1975 he shared the Nobel Prize in Physics with Aage Bohr and James Rainwater for the discovery of the connection between collective motion and particle motion in atomic nuclei. He had received the Atoms for Peace Award in 1969.

New quantum worlds

Mottelson liked to speak of “the wonderful world of finite quantal systems,” and in his later decades he applied lessons from the nucleus to other small quantum objects. He worked on the shell structure of electrons in tiny clusters of metal atoms, which experiments at the Niels Bohr Institute on sodium clusters showed to have a “supershell” pattern, and on semiconductor nanostructures known as quantum dots, or artificial atoms.

He was among the first to see that clouds of atoms cooled to near absolute zero could serve as clean laboratories for many-body quantum physics, a kind of “artificial nuclei.” His 1999 work on rotating Bose–Einstein condensates helped start a new research field, and he studied pairing in ultracold gases of fermions. With Aage Bohr and Ole Ulfbeck he also explored the foundations of quantum mechanics. From 1993 to 1997 he was the founding director of the European Centre for Theoretical Studies in Nuclear Physics and Related Areas, ECT*, in Trento, Italy.

Jewish roots and private life

Mottelson came from a Jewish family in the Chicago area, the child of a household that valued learning and argument. He did not write publicly about religious practice, and his life, spent mostly in Denmark from the age of 24, was centred on physics, family and the international community of the Niels Bohr Institute. He is listed among Jewish Nobel laureates in physics by the Jewish Virtual Library.

In 1948 he married Nancy Jane Reno; they had three children, Malcolm, Daniel and Martha. Nancy died in 1975, the year of his Nobel Prize, and in 1983 he married Britta Marger Siegumfeldt, who died in 2014. He was in China when the Nobel Prize was announced.

The man

Colleagues remembered Mottelson as warm, unpretentious and approachable, with a contagious enthusiasm and an extraordinary physical intuition. In discussions with him, they said, authority had no role to play and physics always came first. He regularly attended the weekly meetings of the experimental groups at the institute, questioning speakers and encouraging young and senior colleagues alike, and he stressed that theorists must learn from experimental data.

He made kites and hot-air balloons, climbed trees, and rode his bicycle to the institute, often ignoring traffic lights, almost to the end of his life. At 86, on a sailing trip across the Øresund, he objected when the others wanted to reef the sails in a strong wind, because he wanted to go fast and get wet.

Legacy

Ben Mottelson died in Copenhagen on May 13, 2022, at the age of 95. His legacy is the modern picture of the atomic nucleus as a small quantum object that can deform, spin and vibrate, a picture that every nuclear physicist learns and that underlies the study of nuclear energy, isotopes and the origin of the elements. He also left a model of how international science can work: an American who made his life in Denmark, worked for decades in a Nordic institute open to the world, and helped found a European research centre in Italy.

Seven decades after it appeared, the 1953 article he wrote with Aage Bohr is still cited by researchers, and their partnership, which lasted almost sixty years, remains one of the great collaborations in the history of physics. Colleagues summed him up as a man who played a decisive role in the understanding of atomic nuclei and who was, at the same time, an inspiring and warm human being.

Sources & further reading

Ben R. Mottelson, Biographical, NobelPrize.org · Ben Roy Mottelson, Physics Today obituary · Ben Roy Mottelson (1927–2022), Niels Bohr Institute · Ben Roy Mottelson 1926–2022, CERN Courier · Ben Roy Mottelson, Wikipedia · Researched from the sources above and checked against them. Spotted an error? Tell us.

A life of tikkun olam. This is part of tikkun olam, the Jewish idea of repairing the world, and every one of us can add to it.