timmdeeg said:
However did they take into consideration that neutrinos have a mass long time ago?
Because, as
@Ibix said, the solar neutrino problem is evidence that neutrinos are not massless. Whereas we have no evidence at all that photons are not massless.
Briefly, the solar neutrino evidence is this: we can calculate the rate of nuclear reactions in the Sun's core from its energy output, which in turn gives us the rate at which neutrinos are produced in the Sun's core. All of those neutrinos are electron neutrinos. We have set up detectors on Earth that detect electron neutrinos, but those detectors only see about 1/3 as many electron neutrinos as expected.
The only way we know of to account for this is that the electron neutrinos from the sun have a nonzero amplitude to turn into muon or tau neutrinos as they travel--or, more generally, that neutrinos with a definite energy (such as neutrinos coming from particular nuclear reactions in the Sun) do not have a definite flavor ("electron", "muon" , or "tau"), but have amplitudes for being each of the three different species, and those amplitudes vary with time as the neutrinos travel. So calling the neutrinos produced by nuclear reactions in the Sun "electron neutrinos" is not quite correct: they are actually not in the "electron neutrino" flavor eigenstate. They are mixtures of all three flavors that just happen to be produced at the point in the time variation of the flavor amplitudes at which all of the amplitude is in the "electron" flavor. By the time they have traveled from the Sun to the Earth, the variation of the flavor amplitudes with time has turned them into a mixture of all three flavors, in roughly equal proportions, and our detectors only detect the "electron" flavor portion, so we only detect about 1/3 of them.
The point is that this variation of flavor amplitude with time can only happen if neutrinos are not massless. This is because if neutrinos are massless, the flavor eigenstates and the energy eigenstates have to be the same. (The reasons for this are fairly detailed technically.) So evidence that the flavor eigenstates and the energy eigenstates are not the same is evidence for neutrinos not being massless.
