[UPDATE: Some extensive comments added below.]
[UPDATE: Journalists and Bloggers: PLEASE NOTE: OPERA is not a CERN experiment. The CERN laboratory does not deserve the bad press it is getting (though they certainly unwisely put themselves in a position to receive it.)]
Many of you are probably already aware of various rumors running around By now you are all aware of yesterday’s initial report that the OPERA experiment — famous for announcing that neutrinos traveling from CERN to the Gran Sasso Laboratory in Italy arrived 60 nanoseconds (billionths of a second) earlier than expected, thus suggesting that neutrinos can travel slightly faster than the speed of light — had found a problem with a cable connector that exactly explains the 60 nanosecond timing shift.
But the immediate source of this rumor was a science journalist, and the article was based on an anonymous source who is not described as being in the OPERA experiment. And the details quoted in the article didn’t add up, in my view. Given the number of wrong reports and rumors that I have read over the past months about this experiment, my reaction was to wait.
I didn’t have to wait long.
Presumably Perhaps to avoid misinformation from hitting the headlines, it appears that OPERA has released a statement that indicates that the article from earlier today yesterday is not true. [Update: To be clearer, I probably should have written, ``true in its details.''] But this statement itself contains big news. Unfortunately I myself have still not obtained an original version of their statement, so I can’t independently verify what I’ve read. If the quotations I have read of their statement are correct, The statement clearly calls the experiment’s result into question. However, in contrast to the original news article, it does not state that an issue causing a 60 nanosecond shift has been unambiguously identified. On the contrary, what it states is that two issues, not one, affecting the result in opposite directions (i.e., one making the neutrinos seem to have arrived earlier, and one making them seem to have arrived later), have cropped up. And it would appear that at least one, and perhaps both, aren’t fully understood yet. This suggests that, for the moment, the experimental situation is less clear than before, not more so. Rather than the cause of the effect having been identified, the resulting murky waters would instead mean that the systematic uncertainties on the measurement would then be larger than were stated in the OPERA paper, reducing the stated statistical significance of the result (which was about 6 standard deviations.)
Unfortunately the relevant details have not yet been provided. We haven’t been told whether these new uncertainties are on the scale of ten nanoseconds, sixty nanoseconds, or a thousand nanoseconds. And that means we can only guess, at this point, as to what the implications are — whether the significance of the result is reduced from six standard deviations to four, two, or essentially zero. On top of that, further investigation by OPERA may reduce these uncertainties, while along the way the main result may change from what it was originally. So we can have our suspicions, but we can’t draw clear conclusions without more information from OPERA.
[UPDATE: For clarity: Of course most physicists suspect the experiment is wrong, but from the very beginning most of us strongly suspected it was wrong — as are (historically) most experiments with a radical result, even by very good scientists. Even the OPERA people suspected it, but couldn’t find the key mistake, nor (until now) could anyone find any mistakes.]
[SECOND UPDATE: For further clarification as to what I’m getting at, see this new post.]
The best hint we have that this is not minor news is the use of the words “could significantly affect” in the OPERA statement. Here is the statement, as quoted from the journal Nature, with comments, boldface and color highlights added by me:
The OPERA Collaboration, by continuing its campaign of verifications on the neutrino velocity measurement, has identified two issues that could significantly affect the reported result. The first one is linked to the oscillator used to produce the events time-stamps in between the GPS synchronizations. [UPDATE: This is the one which would have made the neutrinos appear to arrive later, and thus apparently slower.] The second point is related to the connection of the optical fiber bringing the external GPS signal to the OPERA master clock. [UPDATE: This is the one which would have made the neutrinos appear to arrive earlier, and is therefore the prime suspect for having caused the effect; in this regard the original news article was correct. Notice this is an optical fiber, not a standard wire.]
These two issues can modify the neutrino time of flight in opposite directions. While continuing our investigations, in order to unambiguously quantify the effect on the observed result, the Collaboration is looking forward to performing a new measurement of the neutrino velocity as soon as a new bunched beam will be available in 2012. An extensive report on the above mentioned verifications and results will be shortly made available to the scientific committees and agencies.
Nature also quotes Caren Hagner, a member of OPERA at the University of Hamburg, as saying: “For the moment the collaboration decided not to make a quantitative statement, because we have to recheck and discuss the findings more thoroughly.”
In short —
assuming — even the OPERA experimenters apparently do not yet understand the situation to their satisfaction. Obviously we have to wait until they know what’s going on before we, outside the experiment, can draw correct conclusions.
[UPDATE: I think many of you must be wondering why I am so cautious here. It’s this: notice that OPERA say something odd: that they’ve found problems and they look forward to repeating the measurement. If OPERA had found the problem and were confident that they had done so, they would say that, and there would be little need to repeat the measurement (though it might be nice to do so.) It would seem to me to be far more upsetting, frustrating and embarrassing to them, not less, to say (as they basically do) that they can’t yet figure out how big these newly identified effects are, and imply (as they basically do) that they are not sure that they can, and instead must repeat the measurement — a do-over. I don’t know if that implication is really there or not, but it is odd that they seem to focus on the repeat of the measurement. So there is a bit more here than meets the eye, and I’m waiting to find out what it is before making what would otherwise be irresponsible statements.
Note also that just because they know there is an important problem with an optical wire doesn’t mean they know yet that it is the problem that caused the effect that they measured. (You personally may suspect that it is, but there’s no way for you to know it if they don’t.) For all they say, and for all we know, there might be yet another issue, one that they haven’t yet identified, that’s the main cause of the 60 nanosecond shift.
So don’t get me wrong; I’ve thought this experiment was almost certainly wrong from the beginning, and so did the vast majority of my colleagues. As I’ve emphasized many times on this site, most experiments with a radical result turn out to be wrong, and this one was particularly implausible. From my point of view, the whole story since September has mainly been a question of finding the cause. But the cause has not yet been confirmed; it may have been found, or it may not yet have been found.
That said, almost anyone would agree the experiment result is now on life-support, because any meaningful reduction in the original statistical significance (from an optimistic 6 standard deviations) basically takes most remaining credibility out of what was a highly implausible claim to start with.]
As I wrote at the beginning of this year, the OPERA, ICARUS and BOREXINO experiments (which are all quite close to each other in Italy’s Gran Sasso Laboratory) will soon conduct a long re-run of what I called OPERA-2, the second version of the OPERA experiment that was run using short pulses of neutrinos. This will give OPERA a chance to redo their experiment with these newly identified problems fixed, while ICARUS and BOREXINO will get a chance to try to avoid any mistakes, or at least make their own independent and unrelated set of mistakes. But obviously if the original OPERA measurement becomes less convincing, or even completely unconvincing, the odds for a violation of Einstein’s relativity will look even worse than they did before.
So I am afraid this does not appear to be the dramatic end of the OPERA that some people have claimed today. However, with this turn of events, few will be surprised if it proves to be the beginning of the last act.