Well, ICARUS flies even higher, and so far shows no sign of losing its wings.
Remember OPERA, the experiment that claimed neutrinos sent from the CERN lab in Switzerland to the Gran Sasso lab in Italy arrive earlier than they were expected to? And that a couple of weeks ago had to admit they’d found a couple of problems that were large enough to scrap their result for the moment, and that require additional investigation?
And remember ICARUS, OPERA’s neighbor in the same Gran Sasso lab in Italy, which measured the energies of neutrinos from the CERN neutrino beam, and showed they were not altered in flight? And thus proved that if the neutrinos really were traveling faster than light, they did not exhibit anything like the variant of Cerenkov radiation that was suggested by and calculated by Cohen and Glashow?
Now, ICARUS’s result from the fall didn’t directly refute the OPERA experiment (despite some claims, even by them) but it certainly added to the aura of extreme implausibility that surrounded the whole story.
Well, this time ICARUS refutes OPERA. Essentially, they did the same measurement as OPERA-2, as I called the short-pulse variant of OPERA’s original experiment. They took data at the same time as OPERA-2, in the same neutrino beam, in the same laboratory. It took them a while to do all the distance and timing calibrations that OPERA had done many months ago, but they’re finished now. And whereas OPERA-2 gets the same result as OPERA-1— an early arrival of 60 nanoseconds (billionths of a second) — ICARUS finds a result consistent with an on-time arrival. Same measurement, different answer. At least one experiment made a mistake; and one result is vastly more plausible than the other, so I think the consensus is pretty clear in the matter.
In particular, ICARUS measured the arrival of 7 neutrinos while OPERA-2 was busy measuring 20 (as described in section 9 of their preprint). They find these neutrinos arrived on average on time, with an uncertainty of about 10 nanoseconds. This puts their result at least 4 standard deviations (or “sigmas”) away from OPERA’s result, even if you treat the errors very conservatively. It’s a clear, statistically significant disagreement between the two experiments.
This is the way it works in science all the time. A first experiment makes a claim that they see a striking and surprising effect. A second experiment tries to verify the effect and instead shows no sign of it. It’s commonplace. Research at the forefront of knowledge is much more difficult than people often realize, and mistakes and flukes happen on a regular basis. When something like this happens, physicists shrug and move on, unruffled and unsurprised.
The only thing that makes this story unusual is that this particular (non-)effect was at the heart of one of the most famous statements of twentieth century physics — the universal speed limit suggested in Einstein’s historic 1905 paper — and so it hit the headlines in a big way. Otherwise, it was just like many other examples I’ve seen in my career.
By the way, if you insist on calling OPERA a “CERN experiment”, which is at best misleading and at worst completely wrong, you’d better call ICARUS a “CERN experiment” too. Same beam, same neutrinos, same accelerator team. And in fact there are a couple of people from CERN who signed the ICARUS paper. CERN’s reputation should not suffer for OPERA’s errors.
So ICARUS’s result likely brings us to the final aria of this OPERA, and I think it is finally fair to alert the stage manager to prepare to lower the curtain. Oh, there’s a tad more minor drama yet; OPERA needs to clarify whether the suspected causes of their 60 nanosecond time shift are the real causes, or if there are yet others; a rerun of the experiment by ICARUS and OPERA and maybe others is perhaps still justified, to get a clean result that will allow the community to put this whole story into the filing cabinet and lock it shut. But basically, barring a big mistake by ICARUS, it’s over.
The silver lining? The experimental particle physics community has learned how to make long-range distance and timing measurements that are more precise and more accurate than were ever possible before. Don’t be surprised if this knowledge turns out to be useful, in some unexpected way, in future experiments.