Some Comments on the Faster Than Light Neutrinos

with 2 comments

Matt Strassler

The OPERA experiment has now presented its results, suggesting that a high-energy neutrino beam has traveled 730 kilometers at a speed just a bit faster than the speed of light.  It is clear the experiment was done very carefully.  Many cross-checks were performed.  No questions were asked for which the speaker did not have at least a reasonable answer.
Some preliminary comments on the experiment (none of which is entirely well-informed, so caution…)

  • They have to measure times and distances to an accuracy of 1 part in a few hundred thousand. This is hard, not impossible, and they have worked with metrology experts to carry these measurements out.
  • The timing measurement is not direct; it has to be made in a statistical fashion. The proton beam pulses that make the neutrino beam pulses [read more about making neutrino beams here] are not sharp spikes in time, but are distributed in time over ten thousand nanoseconds. (Recall the measured early arrival of the neutrinos is only 60 nanoseconds.) And so one cannot measure, for each arriving neutrino, how long it took to travel. Instead one has to measure the properties of the proton beam pulses carefully, infer the properties of the neutrino pulses, measure the timing of the many arriving neutrinos, and work backwards to figure out how much time on average it took for the neutrinos to arrive. This sounds tricky.[Thanks to Ryan Rohm for calling my attention to this a few days ago.]That said, the experimenters do show some evidence that their technique works.  But this could be a weak point.
  • I am a bit concerned about the way in which statistical and systematic errors are combined. The theory for statistical errors is well-defined; one assumes random fluctuations. In combining two statistical errors E1 and E2, one says that the overall error is the square root of E1-squared + E2-squared.  This is called “adding errors in quadrature.”  But systematic errors are much less well-defined, and it is not clear you should combine them in quadrature, or combine them with statistical errors in quadrature. The OPERA experiment combines all errors in quadrature, and says they have a measurement at 6 standard deviations away from the speed of light. If you instead combined systematic errors linearly with statistical errors (E1+E2 instead of as above) you would get 4 standard deviations. If you combined all the systematic errors with each other linearly, and then with the statistical error linearly, you would get 2 standard deviations (though that is surely too conservative). All this is to say that this result is not yet so significant that different and more conservative treatments of the uncertainties would all give a completely convincing result. This is just something to keep in mind when evaluating such an exceptional claim; we need exceptional confidence……

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Written by physicsgg

September 23, 2011 at 6:54 pm

2 Responses

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  1. The press coverage of this event has been absurd. Special Relativity does not forbid faster than light signals. Such signals are apparently forbidden by the principle of causality. Causality is a philosophical rather than a physical principle so it would not be surprising if it were found to be wanting.

    Wikibooks has an excellent article on Special Relativity and causality in its textbook on Special Relativity. See,_causality_and_Special_Relativity

    It is possible that the neutrino results are true or that they are false but either way Einstein did not “get it wrong”.


    September 24, 2011 at 6:50 pm

  2. If possible, make the neutrino beam in pulses. Even if cannot make ON/OFF pulses, make UPPER THRESHOLD / LOWER THRESHOLD, either at the beam generator and beam detector – the two sets of THRESHOLD can be different. The duration of pulses will be accurate at the destination detecting sensors. Hope this would get rid of many facing doubts.


    January 26, 2012 at 2:08 am

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