Pontecorvo's solution to the solar neutrino problem involved an idea that he had first considered in 1957, and developed over the following decade. This was the idea that neutrinos may convert into other types of neutrinos, a phenomenon known as neutrino oscillation. Somewhere between the Sun and the Earth, electron neutrinos might transform into muon neutrinos. An important point was that for this to happen, neutrinos could not have zero mass, and therefore could not travel at the speed of light. The existence of the oscillations was finally established by the Super-Kamiokande experiment in 1998 and later confirmed by other experiments.