Saturday, April 2, 2011

The essence of the nuclear many-body problem

I quite like reading obituaries of scientists because one can actually learn some interesting science and sometimes learn something about how great scientists work.
The November 2010 issue of Physics Today has an obituary for Aage Bohr. [My issue just arrived in the snail mail.] He was the fourth son of Niels Bohr and received the Nobel Prize in 1975 for work in nuclear theory. The obituary gives a beautifully succinct summary of the nuclear many-body problem:
 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.
It is also interesting that the last 30 years of his life, Aage Bohr, shifted his focus to questions concerning the foundations of quantum theory; questions his father had focused on.
With regard to eventually following ones father, I see some parallels in myself. Lately I have become increasing interested in questions in molecular biophysics, including the role of water. My late father, a physical biochemist, spent much of his career working on water and proteins.

[Details of the figure are discussed here].

1 comment:

  1. Just wanted to say - I really like reading your blog. It is a very nice mix of scientific insights, practical advice, and sometimes, like here, also personal stories.