Neutrinos have a quantum mechanical property called "flavor." This flavor can transform as neutrinos move through space. A major challenge is to keep track of both the physical movement of the neutrinos and their change of flavor in astrophysical systems such as core-collapse supernovae and neutron star mergers. The complicated arrangement and large number of neutrinos in these systems make it nearly impossible to follow all or even a subset of the neutrinos.
