WHY THERE SHOULD BE 
NO INDEPENDENT PARTICLE MOTION IN NUCLEI
 
 There had been speculations concerning the possibility of shell 
structure in nuclei already in the 1930's, which constitute the early days
of modern nuclear physics. This period dates from the discovery of the 
neutron in 1932. The general belief was that nucleons, that is protons 
and neutrons, are the elementary building blocks of nuclei. In the 
1960's we learned, however, that each nucleon is itself composed 
of three quarks.  But that is another story. Now by about 1936 it had 
become accepted by the majority of nuclear physicists that shell 
structure could not be valid in nuclei. In fact, from the vantage point of 
that time, that was the correct conclusion. Let's briefly review just why
shell structure makes sense in atoms.  Here we have electrons moving 
in orbits. When an orbit is filled, we get a closed shell. But what does 
it take for the concept of orbits to make sense? It is that there is a
central field, due to the charge of the nucleus. We have, of course,
to also take into account the repulsion each electron feels due to the
other electrons, but, since the interactions between electrons are
weak and long range, the other electrons just modify the mean field, 
at least to a good approximation. We do not really have collisions 
between the electrons. So independent particle motion, i.e. shell
structure, makes sense in atoms.   
 
  For nuclei, the situation is quite different. There is no fixed center 
of force, and further the interactions between nucleons are strong and 
short ranged. That much was already known in the early 1930's. So at 
least at first sight, it does not make sense to talk about orbits, like in 
an atom, or like the planets around the sun. A better analogy is with
a liquid drop. The molecules are bound in the liquid, but they do not 
move in orbits. Rather they collide, with a mean free path small 
compared to the size of the system.  
So the nucleus should be better desribed by a liquid drop model than 
an independent particle model.  As long as I am talking about the
theory, it is fairly clear that, from what was known about the nuclear 
forces in the 1930's, we should, in fact, get something like a liquid
drop model, and no shell model.  
This is not the time to discuss this in more detail, but I want to
mention that in the 1950's we learned much more about the nuclear 
forces, including the fact that there is a repulsion at short distances. 
This is essential for getting a shell model, but that is another story, 
which certainly was not known at the time. The story is not finished 
to this day. But telling it would deflect me from the main task, which 
is to talk about Maria Mayer. 
 
  
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