Ok, so want to clarify something: A nuetrino can change between flavors as it moves through space. A nuetrino that starts out as an electron nuetrino can turn into a tau nuetrino or a lepton nuetrino, or mixtures of the two. It can then turn back into its original electron nuetrino. Suppose I start out with a Tau neutrino, the most massive of the three nuetrinos. This nuetrino has some total energy E, and some very big mass M. Much of the energy of the tau nuetrino will be in its rest mass. When the tau nuetrino turns into an electron nuetrino, the energy E remains constant, but the rest mass of the electron nuetrino is much less than the tau nuetrino. What happens to rest of the energy that was originally part of the tau nuetrino's rest mass? Does it go into kinetic energy?