He moved to the Institute for Theoretical Physics
in Copenhagen, Denmark on
the Sheldon Traveling Fellowship from Harvard, where he began work as
a nuclear physicist. The U.S. Atomic Energy Commission followed with
another fellowship which granted Mottleson more time to continue his
work in Copenhagen. In 1953, he accepted a research position at the
European Organization for Nuclear Research. In 1957, he became a professor
at the newly founded Nordic Institute for Theorectical Atomic Physics
in Copenhagen.
Mottelson and colleague Aage Bohr helped prove the
theories of James Rainwater regarding the structure of atomic nuclei.
The trio shared the 1975 Nobel
Prize in Physics for their work.
The following press release from the Royal Swedish
Academy of Sciences describes Mottleson’s work:
The Royal Swedish Academy of Sciences has decided
to award the Nobel Prize in Physics for 1975 in equal shares to Professor
Aage Bohr, Denmark, to Professor Ben Mottelson, Denmark and to Professor
James Rainwater, USA, 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.
At the end of the Forties nuclear physics had advanced
to a stage where a more detailed picture of the structure of the atom
nucleus was beginning to emerge and it was becoming possible to assess
its properties. The models scientists were working with then were,
however, fairly deficient and contradictory to a certain extent. The
oldest was the drop model, in which the nucleus was regarded as a
drop of liquid, where the nucleons correspond to the atoms in the
liquid. Some properties of the nucleus, particularly those associated
with the "magic numbers", show however that the individual
nucleons definitely affect the behaviour of the nucleus. This discovery,
which is demonstrated in the scale model, was awarded the 1963 Nobel
Prize in Physics.
As time passed it was found that the nucleus has
properties, which cannot be explained by these theories. Perhaps the
most striking were the very marked aberrations from spherical symmetry
in the distribution of charge observed in certain nuclei. It was also
pointed out by several research scientists that this might indicate
that certain nuclei are not spherical but are deformed as an ellipsoid,
but no one could give a reasonable explanation of this phenomenon
The solution of the problem was first presented
by James Rainwater of Columbia University, New York, in a short paper
sent for publication in April 1950. In this he observes the interplay
between the greater proportion of the nucleons, which form an inner
nucleus, and the outer, the valence nucleons, and he points out that
the valence nucleons can influence the shape of the inner nucleus.
Since the valence nucleons move in a field which is determined by
the distribution of the inner nucleons, this influence is mutual,
If several valence nucleons move in similar courses, this polarizing
effect on the rest of the nucleus can be so great that the nucleus
as a whole becomes permanently deformed Expressed very simply, it
can be said that as a result of their rotation certain nucleons expose
the "walls" of the nucleus to such high centrifugal pressure
that it becomes deformed, Rainwater also attempted to calculate this
effect theoretically and got results that corresponded with experimental
late on the distribution of the charge.
Aage Bohr, working in Copenhagen, but at this time
on a visit to Columbia University, had, quite independently of Rainwater,
been thinking along the same lines In a paper, sent for publication
about a month after Rainwater's, he formulates the problem in a more
general, but from the physical viewpoint loss lucid, way
These relatively vague ideas were further developed
by Bohr in a famous work from 1951, in which he gives a very comprehensive
study of the coupling of oscillations of the nuclear surface to the
movements of the individual nucleons. By means of an analysis of the
theoretical formula for the kinetic energy of the nucleus he could
predict the different types of collective excitations: vibrations
obtained by a periodic change of the shape of the nucleus around a
certain mean value and the rotation of the whole nucleus around an
axle at right angles to the symmetry axle.
Up to then advances made had been purely theoretical
and the new ideas largely lacked experimental foundation. The very
important comparison with experimental data was done in three works,
written jointly by Bohr and Mottelson, and published in the years
1952-1953. The most spectacular finding was the discovery that the
position of energy levels in certain nuclei could be explained by
the assumption that they form a rotation spectrum. The conformity
between theory and experiment was so complete that there could be
no doubt of the accuracy of the theory. This gave stimulus to new
theoretical studies, but above all to very many experiments to prove
the theoretical predictions. This dynamic development very soon led
to a deepened understanding of the structure of the atomic nucleus.
In the research done since then, Bohr and Mottelson
have been central figures, and have definitely inspired research in
this field, although they themselves have not published many works.
However, when it comes to principles, perhaps the most important discovery
during this period originates from them (in collaboration with Pines).
This concerns the fact that nuclear matter has properties reminiscent
of superconductors.