I hedged my bets in my most recent post on the subject, but I think the answer is now clear: the faster-than-light neutrino results were indeed in error. In brief, there were two problems that combined to yield an error of about 60 nanoseconds, which pushed the apparent speed of the neutrinos above the speed of light, which caused the uproar last fall.
The longer version (and Matt Strassler has a lot more details) gets a little complicated, but I will summarize:
- The main culprit was a fiber optic cable that was slightly out of alignment. This is not quite a “loose wire”, as it sometimes has been described: it’s far more subtle and harder to check than that, but it’s still fundamentally a simple technical problem. (My prediction that the effect was due to something really subtle turns out not to be correct!) When the alignment of the fiber optic cable is correct, the timing shrinks by 73 nanoseconds, larger than the original 60 nanosecond effect.
- A second, smaller source of error is something known as the “clock problem”, relates to the relative timing of events between two detectors. However, since the cable produces the larger effect, I’m not particularly concerned with the clock problem here (since I don’t have a lot of time to write today!).
I admit, I’m more than a little disappointed in this outcome. While I didn’t expect the faster-than-light result to stand for a variety of reasons, I assumed the most obvious potential hardware problems would be the first thing to check before announcing possibly paradigm-breaking experimental results to the public. Yes, it’s not just a loose connection, but given that they were able to find the problem within a few months indicates that some people at OPERA were likely already thinking the fiber optic cable could be a problem. In other words (as Matt Strassler also says), why didn’t anyone check this sort of thing before telling the world their results?
It has become a scientific cliché that extraordinary claims require extraordinary evidence (Carl Sagan is probably the best-known propagator). While it was to their credit that the original OPERA press releases were cautious about jumping to conclusions based on their data, in my mind the whole situation still feels like the students who would come to me after lab with results showing blatant violations of physical laws, wanting to know what they did wrong (usually when it’s too late to fix), but unwilling to check everything over themselves.
In any case, the whole OPERA saga is over. No Götterdämerung here; it more resembles a mistaken-identity farce as written by Gilbert and Sullivan, performed by the Marx Brothers.