A recent suggestion has been made that neutrinos may not be massless, they may have mass. The waves may propagate at different speeds and thus will get out of phase. Need to show that neutrinos have mass. Direct experiments to detect the mass of neutrinos, inconclusive. Interesting points massless particles travel at the speed of light, particles with mass cannot.
Supernova explosions produce neutrinos and photons.
If neutrinos are massless, they should reach the Earth at the same time as the photons reach the Earth from Supernova explosions. This experiment was tried for SN1987A, but the theory of Supernova eplosions wasn't good enough and there weren't enough neutrinos to get a reliable answer. This situation will definitely improve however. Direct experiments to look for neutrino oscillations in terrestrial accelerators and using the products of cosmic rays interacting with the atmosphere of the Earth. Work needs to be followed up upon.