New constraints on the physics of OPERA FTL neutrinos

What Cohen and Glashow did last week was to generalize this idea to point out a new physical phenomenon (new at least to me) and use it to argue that OPERA’s result is self-inconsistent. They argue that the very effect of faster-than-light travel that OPERA claims to observe would have caused distortions in its neutrino beam that clearly were not observed. Moreover, Cohen and Glashow also pointed out that at least two other experiments studying higher energy neutrinos put even stronger constraints on the possibility of anything similar to what OPERA observed.

The article is fascinating, so read the whole thing. Dr. Strassler describes an elegant approach to constraining the types of modifications to Relativity that are consistent with the OPERA data. This approach was developed by Andrew Cohen and Sheldon Glashow.

Cerenkov radiation is emitted by electrically charged particles moving faster than light in a medium. Relativity says we shouldn’t see Cerenkov radiation in a vacuum, but it is an important effect in materials where light slows down, such as water, and particles can exceed the local speed of light. You may have seen that blue glow around a submerged reactor; that’s Cerenkov radiation, and the effect takes energy from the emitting particle.


Neutrinos have no charge, so they would not emit Cerenkov radiation (well, they have a very, very tiny type of charge so they can emit a very, very dim form of Cerenkov radiation). But neutrinos interact via the weak force, and what Cohen and Glashow did was show that such particles can emit an analogous type of radiation if they exceed the speed at which electrons can travel in a medium. This radiation would remove energy from the neutrino beam in a way that would be very easy for the OPERA experiment to see. But OPERA’s results do not show the energy removal signature of Cohen-Glashow radiation.

Observations of neutrinos from a distant supernova have put strong constraints on neutrino speed for lower energies than OPERA. Two other experiments have observed neutrinos 100 to 1000 times more energetic than OPERA’s neutrinos, and they do not see the Cohen-Glashow radiation energy loss.

So, we must choose between OPERA’s FTL neutrinos or Cohen and Glashow’s weak force radiation effect. It is not impossible that both could be true, but if so, it will place strong constraints on the kind of modifications that can be made to Special Relativity.

In short, OPERA’s FTL results became more unlikely, but have not yet been ruled out. I was struck by the elegance of the Cerenkov radiation analogy involving the weak force to put tighter constraints on the physics of FTL neutrinos, if they exist.

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