Relativity-9: The importance of corroborating evidence in science

(For previous posts in this series, see here.)

In my series on the logic of science, I recounted how philosopher of science Pierre Duhem had pointed out as far back as 1906 that the theories of science are all connected to each other and changes in one area will have unavoidable effects on others that should be discernible. In this case, if neutrinos in the OPERA experiment did in fact travel faster than the speed of light, then we should be able to look at some other effects that should occur and see if they are observed.

Cherenkov glow.jpegOne of them is the ‘Cherenkov effect’. This effect says that when something travels faster than the speed of light, it should emit a certain kind of radiation that is analogous to the shock waves that are produced when something travels faster than the speed of sound. This is known as the ‘sonic boom’ that we can hear when jet planes break the speed of sound. It also occurs when bullets are fired at speeds greater than the speed of sound but because bullets are so small the sonic boom is too weak for us to hear it.

The Cherenkov effect is well known and has been studied and confirmed. How can this be if it requires something to travel faster than the speed of light? Recall that the speed of light barrier in Einstein’s theory is that in a vacuum. When light travels through any medium (light, water, atmosphere), it is slowed down by the interactions of the medium with the light particles. Other particles such as electrons are also slowed down by the medium but they may not be to the same extent, in which case it can be possible for some particles in a medium to travel faster than the speed of light in that same medium. If they do so, they should emit the light equivalent of the sonic boom and this is called Cherenkov radiation. The spectrum of light emitted lies mainly in the ultraviolet region and its overlap with the visible spectrum produces a characteristic blue glow. One can see this in the cooling water that surrounds nuclear reactors, as in the image on the right, and in this video of a pulse of radiation being sent into the cooling liquid.

In a paper, Andrew Cohen and Sheldon Glashow calculate that high energy, faster-than-light neutrinos as produced in the OPERA experiment would lose much of their energy due to Cherenkov radiation, mainly by the production of electron-positron pairs, on their way from CERN to Gran Sasso. But that does not seem to have happened, according to a different experiment at Gran Sasso (known as ICARUS) that works with the same neutrino source as the OPERA experiment.

Another concern involving consistency is with the supernova SN1987A that was observed in 1987. It turned out that a cluster of 24 neutrinos were detected in three different detectors on the Earth about three hours before the supernova was observed, i.e. before the light signals reached Earth. That difference was not put down to the neutrinos traveling faster than the speed of light but to the fact that the neutrinos, while created at the same time as the light, escaped from the exploding star three hours before the light did due to their low interactivity with matter, and so had a head start on the journey to Earth, even though they traveled in free space at the same speed as light. The measured time difference was consistent with our understanding of the processes involved in a supernova.

If the neutrinos had speeds greater than that of light by even the small amount given by the OPERA experiment, then because of the huge distance of the supernova from Earth (about 168,000 light years), the supernova neutrinos should have reached Earth about 4.7 years before we saw the supernova. If neutrinos in the OPERA experiment had in fact, been traveling faster than the speed of light, why had they not done so in other situations, such as the 1987 supernova?

The working model of science is that things behave in a law-like, repeatable manner and not idiosyncratically. If we observe something in one situation, we expect to see it happening again in similar situations. If a deviation from law-like behavior is observed, we assume that this is due to the existence of another, deeper, hitherto unknown law whose effect only became apparent because of some conditions that had been incorrectly assumed to be unimportant.

In this case, one could postulate that since the OPERA neutrinos have a thousand times as much energy as the supernova neutrinos, faster-than-light speeds only arise for such high-energy neutrinos. Of course, such a new explanation requires new corroborative evidence and so the discussion will go on as explanations and evidence play out their dialectical relationship until a consensus emerges. That is how science works.

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  1. Jared A says


    Is there a more general version of Cherenkov radiation than is typically taught to a physics student? I thought that it was only caused by charged particles and that neutrinos are electrically neutral. Is this a different type of Cherenkov radiation than is normally seen in reactors?

  2. Jared A says

    Nevermind, spending less than 5 minutes on wikipedia is enough to instruct me that uncharged particles give rise to the Askaryan effect.

  3. says

    Yes, Cherenkov radiation is caused by charged particles. What the papers linked to seem to suggest is that the high-energy neutrinos should produce electron-positron pairs and it is these particles that should produce the radiation.

  4. says

    On the neutrinos track, there might be some particles (or coherent collections of these?) which might absorb a neutrino at one side, while on the other side, in the same time emitting a similar neutrino. Thus we get an illusion of a faster-than-light movement of a neutrino which seemingly “jumped over” another particle with no time…
    If so, could the neutrinos run even faster in heavier materials (?like Earth’s core)?
    I wrote about this in Physics Forum, too, trying to make it a little bit funny (link added).

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