Do neutrinos change speed in neutrino oscillations? The process of neutrino oscillations is not very intuitive, hence a question. If an electron neutrino on its way from the sun turns into a tau neutrino, then for the energy to conserve, the speed would have to become slower. This however would change the momentum. For a single particle in an empty space, it seems impossible to change its rest mass without violating either energy or momentum conservation. Does this mean the rest mass of all neutrino flavors is the same or is there a better explanation from the standpoint of the physical sense?
 A: At the core of quantum mechanics is a most counter-intuitive truth: Although we might presume conservation is "disobeyed" over an unobserved part of an experiment, we find it really was obeyed when we observe it. In the case of neutrinos, no matter what flavor is detected we will see that both momentum and energy are conserved. If its a heavier flavor that's detected, then we'll also see that the time from emission to absorption is longer (the neutrino went more slowly). This QM principle is possible because observation of an experiment and alteration of an experiment are inseparable. Observation and alteration are synonymous. We are welcome to imagine any crazy behavior for something that is not observed as long it obeys consistent laws when it is observed. For example, I could say that the emission event 'knew' where the neutrino would be absorbed, and accordingly chose the neutrino flavor to emit at emission time. Notice that the recoil information from an emission event only tells you the momentum of the neutrino, not its velocity. So you can't tell the flavor at emission time.
