Alan Chodos, in his commentary in the December 2011 issue of PHYSICS TODAY (page 8), summarized the OPERA experiment that supposedly found neutrinos traveling at a speed of c + δc, where c is the speed of light and δc ≈ 7 × 105 cm/s. He also discussed some theoretical speculations and objections, but he ended his commentary with the odd comment that “if the OPERA result fails to survive, that will not prove that neutrinos don’t travel faster than light.” Then he presented his own ideas of tachyonic (faster-than-light) neutrinos that would support the “apparent lack of Lorentz invariance in the neutrinos’ superluminal propagation” (see the article by Olexa-Myron Bilaniuk and E. C. George Sudarshan, PHYSICS TODAY, May 1969, page 43).
Chodos didn’t mention that regardless of neutrino properties, the most serious problem with the OPERA result is that it entails a failure of causality. Since the clocks in the rest frame of the experiment are synchronized by GPS in accordance with special relativity, which is accepted as valid, consider the corresponding observations with clocks synchronized in a frame of reference moving with velocity c – δc′ along the CERN beam direction, where 0 < δc′ < δc. In that frame, one would find that each signal at the CERN graphite target detector occurs after a corresponding signal at the Gran Sasso neutrino detector, which would be manifestly absurd.1,2 Recently it has been found that OPERA’s faster-than-light result was an error due to “a faulty cable connection.”3 Moreover, an independent research group, ICARUS, has announced that neutrinos obey nature’s speed limit.4
