Of Particular Significance

OPERA’s Presentation: My Initial Comments

Matt Strassler 9/23/11

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; however, see his comment below.] 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.

Now, some brief comments on the theoretical implications, in addition to what I said in Tuesday’s and Thursday’s posts.

You may have heard some people say that neutrinos traveling faster than light would mean that Einstein’s theory is completely wrong and implies that instantaneous communication and even time travel would be possible. Balderdash! this is loose and illogical thinking. If Einstein’s theory were exactly correct AND neutrinos could travel faster than light, then this would follow. But if neutrinos travel faster than light, then Einstein’s theory is wrong at least in some part, and until you know exactly how it needs to be modified, you can draw no such conclusions.

As I emphasized in yesterday’s post, Einstein’s principles should be divided for current purposes into two parts:

  1. There is a universal speed limit.
  2. Light travels at this speed limit.

Observing that some neutrinos travel faster than light could mean there is no speed limit at all, or simply that light does not travel at the speed limit. And there are much more complex logical possibilities. Some of these would require only rather small (though still revolutionary) adjustments to current theoretical physics. Others would be more disruptive to current thinking. But it is certainly not true (as some physicists have said in public fora) that it requires going back to the drawing board as far as theoretical physics and Einsteinian relativity are concerned. It will depend on which of the various logical possibilities is actually operating. Hyperventilating about the impending collapse of existing theoretical physics is a tad inappropriate at this time.

In fact, over the years quite a few theorists (including very mainstream and well-respected scientists at major universities) have considered the possibility that Einstein’s principles might be slightly violated, and some of that work from the 1990s (and probably earlier [?]) suggested that studying neutrino properties would be a good way to look for signs of such violations. So it is not as though these issues have never come up before in theoretical physics… though I think it fair to say that everyone has viewed this exciting possibility as a long-shot.  (And most of us probably still do, until this experiment is confirmed.)

There is certainly more to say about the theoretical situation, but I am still learning about what the experts already know.

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A decay of a Higgs boson, as reconstructed by the CMS experiment at the LHC