With the death of Aage Niels Bohr on 8 September 2009, the world lost one of its finest theoretical physicists. Aage was a flame that was constantly burning, a penetrating intellect in a tireless struggle with the secrets of Nature. A brief chronology in the next two paragraphs hints at the extraordinary background for the unfolding of Aage’s unusual gifts.
Born on 19 June 1922, the fourth son of Margrethe and Niels Bohr, Aage grew up amid the remarkable circle of scientists — “Uncle Kramers,” “Uncle Klein,” Uncle Nishina,” “Uncle Heisenberg,” “Uncle Pauli,” and so forth—who came to work with his father. He spent his adolescence at the Carlsberg Mansion of Honor where from 1932 on his parents were hosts for scholars, artists, and persons in public life. With Nazi arrest imminent, the Bohr family fled to Sweden in 1943; Aage and Niels Bohr were flown to England, where both were brought into the nuclear weapons program “Tube Alloys.” Thence from 1943 to 1945, Aage was traveling with his father between London, Washington, DC, and Los Alamos, serving as his assistant and secretary and sharing day-to-day Niels Bohr’s thoughts and work related to the Manhattan Project.
Aage returned to Denmark in August 1945 and acquired the master’s degree from Copenhagen University (1946) with a thesis on aspects of atomic stopping. In 1949-50 he held a research position at Columbia University, where he shared office with James Rainwater and published on magnetic moments of nuclei with nonspherical shape. He began collaborating with Ben Mottelson in 1951. After completing the doctor’s thesis “Rotational States in Atomic Nuclei” (1954), Aage was appointed professor of physics at Copenhagen University (1956). After Niels Bohr’s death (1962), Aage became director of the Niels Bohr Institute, a post he held until 1970. Bohr and Mottelson provided a systematic exposition of their views on atomic nuclei in the monograph Nuclear Structure volume I, Single-Particle Motion (1969), and volume II, Nuclear Deformations (1975). The Nobel Prize for Physics was awarded in 1975 “to Aage Niels Bohr, Ben Roy Mottelson, and Leo James Rainwater for the 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.” Aage was director from 1975 to 1981 of the Nordic Institute for Theoretical Atomic Physics (Nordita). He began collaborating with Ole Ulfbeck in 1982, developing a graduate course, Quanta and the Constitution of Matter; and in five publications between 1986 and 2009, the origin and content of quantal indeterminacy was reassessed.
To those who did not know Aage, his trajectory may seem to present a paradox. For the first half of his scientific life he was a central figure and greatly admired leader of a research center to which nuclear physicists from the whole world must go at one time or another. He was also the reclusive thinker who devoted the last 25 years of his life to penetrating analysis of quantal concepts but was reluctant to put pen to paper on the subject. In the following we focus on aspects of Aage’s dichotomous scientific life.
Aage became a card-carrying physicist shortly after it had been convincingly demonstrated in the late 1940s that a range of atomic nuclei has properties such as binding energies and electric and magnetic moments that reflect independent-particle motion with a long mean free path of the nucleons in their mean field. This discovery came as a shocking surprise and a dilemma since it implied a shell structure for the nucleus with an analogy to the electronic shell structure for the atom. In contrast, the basis for interpreting the growing body of experimental evidence had been the liquid-drop and compound nucleus models, in which the forces between the nucleons lead to a strong coupling of their motion.
In 1950 a need to reconcile the two contrasting pictures was thus imminent. An important clue was provided by the nuclear electric quadrupole moments, which are sometimes more than an order of magnitude larger than can be attributed to a single proton and directly point to a deformation of the nucleus as a whole. The crucial recognition, also realized by Rainwater, was that the degeneracies of the spherical shell structure may in fact lead to an equilibrium with an anisotropic intrinsic single-particle density and mean field, for nuclei with particles in partially filled shells. A deformation in space suggests the possibility of collective rotation. The striking discovery in the Coulomb excitation process of such rotational band structure in the excitation spectrum provided an early foothold for the collective elements of the picture (1953).
The moments of inertia of the nuclear rotational states were found to be markedly smaller than the moments for rigid rotation that we expected for uncorrelated single-particle motion, thereby exhibiting correlations (1955). In work with David Pines (1958), it was suggested that the necessary correlations could be related to an energy gap in the quasiparticle excitation spectrum created by a pair binding, as in the Bardeen-Cooper-Schrieffer theory of electronic superconductivity.
It gradually emerged that we were, in fact, exploring a quite novel type of many-body quantal system, distinguished at the time by the unique possibility of detailed observations of individual quantal states and their transitions. That exploration became part of a broad development of quantal many-body concepts appropriate to the description of symmetry in a multitude of dimensions (spin-, isospin-, gauge-, orbital-space). The development ultimately revealed the ubiquity of collective features of the nuclear stuff, ranging from oscillation quanta of the fields in the new dimensions to the static deformations and profoundly significant zero-frequency modes of the fields (Goldstone boson). The dilemma of the contrasting pictures that set the development in motion had provided the possibility of a more comprehensive vision—very much in the spirit of Aage’s attitude to conflicts of any kind.
In 1978 Aage’s wife of 28 years, Marietta Bohr, died, a loss that affected Aage immensely. At this crossroad, Aage deliberately reoriented his life and scientific work and relinquished direct responsibility in the leadership at the Niels Bohr Institute and Nordita. He felt it a challenge to develop a comprehensive graduate course on quantal phenomena that came to bring the extended role of spacetime symmetry into the foreground of quantum mechanics itself. The inevitable reassessment of old issues in a new light, in particular the origin and content of quantal indeterminacy, led us over the next 20 years, first Aage and Ulfbeck and subsequently Aage with both of us, to seek a basic principle from which quantum mechanics develops in a compelling manner.
Quantum mechanics contends that events occur that are beyond causal analysis and yet are caused by a particle. The use of such contradictory concepts is avoided by accepting a complete break with causality in the interpretation of experimental evidence. By this break, clicks in counters come without a cause (“out of the blue’) as a basic axiom, referred to as fortuity, providing the hitherto missing principle underlying quantum mechanics. The profound implications of the principle, so much against ingrained notions and deeply held beliefs concerning atoms as things that cause the click, completely occupied Aage’s mind and strength for the last decade of his life. Still, in his last months he was struggling to find the proper formulation that would link rates of uncaused clicks, through the intermediary of complex-valued symmetry waves, with the probability structure inherent in spacetime symmetry.
Anyone who came in close contact with Aage was strongly influenced by his inclusive personality. Strange but true, never again shall we ask as we jokingly did at the end of the day, “When shall we three meet again?” The flame is defunct.
