Neutrino transport: no asymmetry in equilibrium
  A small asymmetry in the flux of neutrinos emitted by a hot newly-born neutron star could explain the observed motions of pulsars. However, even in the presence of parity-violating processes with anisotropic scattering amplitudes, no asymmetry is generated in thermal equilibrium. We explain why this no-go theorem stymies some of the proposed explanations for the pulsar ``kick'' velocities. 
Alexander Kusenko, Gino Segre, and Alexander Vilenkin