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Measurements of neutrino speed have been conducted as tests of special relativity and for the determination of the mass of neutrinos. Astronomical searches investigate whether light and neutrinos emitted simultaneously from a distant source are arriving simultaneously on Earth. Terrestrial searches include time of flight measurements using synchronized clocks, and direct comparison of neutrino speed with the speed of other particles. Since it is established that neutrinos possess mass, the speed of neutrinos should be slightly smaller than the speed of light in accordance with special relativity. Existing measurements provided upper limits for deviations of approximately 10−9, or a few parts per billion. Within the margin of error this is consistent with no deviation at all. == Overview == It was assumed for a long time in the framework of the standard model of particle physics, that neutrinos are massless. Thus they should travel at exactly the speed of light according to special relativity. However, since the discovery of neutrino oscillations it is assumed that they are massive. Thus they should travel slightly slower than the speed of light, otherwise their relativistic energy would become infinitely large. This energy is given by :, ''v'' being the neutrino speed and ''c'' the speed of light. The neutrino mass ''m'' is currently estimated as being 2 eV/c², and is possibly even lower than 0.2 eV/c². According to the latter mass value and the formula for relativistic energy, relative speed differences between light and neutrinos are smaller at high energies, and should arise as indicated in the figure on the right. Time-of-flight measurements conducted so far investigated neutrinos of energy above 10 MeV. However, velocity differences predicted by relativity at such high energies cannot be determined with the current precision of time measurement. The reason why such measurements are still conducted, is connected with the theoretical possibility that significantly larger deviations from light speed might arise under certain circumstances. For instance, it was postulated that neutrinos might be some sort of superluminal particles called tachyons, even though others criticized this proposal. While hypothetical tachyons are thought to be compatible with Lorentz invariance, superluminal neutrinos have also been studied in Lorentz invariance violating frameworks as motivated by speculative variants of quantum gravity, such as the Standard-Model Extension according to which Lorentz-violating neutrino oscillations can arise. Besides time-of-flight measurements, those models also allow for indirect determinations of neutrino speed and other modern searches for Lorentz violation. All of those experiments confirmed Lorentz invariance and special relativity. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「measurements of neutrino speed」の詳細全文を読む スポンサード リンク
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