SOME OPEN QUESTIONS
 

 
 NUCLEAR FORCES
 A lot of progress has been made in understanding the workings of the
nuclear shell model in terms of more fundamental principles. In principle,
we should start out using a realistic nucleon-nucleon interaction and deal 
with the many body problem. That is what is done in atomic physics, where 
we are dealing with simple Coulomb interactions. However, in nuclei, there
are problems: 

 
 WHAT SCATTERING ALONE DOES NOT TELL US

 
ROLE OF ENERGY NON-CONSERVATION
Nucleon-nucleon scattering only tells us about the so-called 
"On energy-shell" properties of the interaction. On energy-shell means 
energy-conserving. It is possible for the interactions to violate
energy conservation for a short time, consistent with the Heisenberg 
uncertainty principle. Such energy non-conserving scatterings has an effect
 in systems involving more than two nucleons. Thus two interactions which
lead to the same scattering properties can lead to different properties of
for systems of three and more nucleons.  

 
ROLE OF THREE BODY FORCES
Another problem is the possible existence of  non-additive 
interactions, i.e. three body forces. Such interactions are known to 
occur in atoms.. 

 
MESON EXCHANGE AND QUARK MODEL AS GUIDE
For these reasons, it is necessary to have some theoretical guidance 
in formulating the basis of  nuclear structure. Meson exchange theory
does provide some such guidance. Also, low energy QCD is helpful 
in a different connection, that of establishing priorities.

 
 SUPERMULTIPLET SYMMETRY
 In the limit of  large number of quark colors, the most important component 
of the nuclear interaction preserves supermultiplet (SU4) symmetry.
 That's another way of saying that the spin-independent part of the 
interaction dominates over any spin-dependence. The next most important
part of the interaction is the spin-orbit coupling. And finally, least important
is the spin-dependent part of the interaction, including the tensor force.
This argument does not hold at large interparticle distances. Here the tensor 
interaction, which is due to one pion exchange,  is the largest component of
the nucleon-nucleon interaction large distances. However, at shorter 
distances, exchange of rho and heavier mesons reduces the strength of the 
tensor interaction. 
 
 
ROLE OF SPIN-DEPENDENT INTERACTIONS
 
All this implies that it is quite necessary to treat off-shell and many 
body effects, especially for the spin-independent part of the interaction. 
On the other hand, while the spin-orbit and tensor interactions can 
absolutely not be ignored, it may not be necessary to treat these with
the same degree of precision. 
      

 
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 S.A.M. Updated 9-2-96