A Lower Bound on Neutrino Mass
  The exchange of massless neutrinos between heavy fermions (e.g. $e,p,n$) gives rise to a long-range 2-body force. It is shown that the analogous many-body force can lead to an unphysically large energy density in white dwarfs and neutron stars. To reduce the energy density to a physically acceptable value, the neutrino must have a {\it minimum mass}, which is approximately $0.4\;eV/c^2$. Some recent questions relating to the derivation of this bound are also discussed. 
Ephraim Fischbach