First we had the mystery of the "missing" solar neutrinos, except it turns out they weren't missing, just in disguise -- the three types of neutrinos can change flavors, or "oscillate," into other flavors. They can do this because -- gasp! -- they have a tiny bit of mass, despite the fact that physicists had assumed for decades that neutrinos were massless, like photons.
And now, it seems, neutrinos might be able to violate Albert Einstein's cosmic speed limit by traveling just a wee bit faster than the speed of light, based on a startling new result from the OPERA (Oscillation Project with Emulsion tRacking Apparatus) collaboration. Yeah, you heard me. All the other particles fall in line, heck, even photons obey the speed of light limit, but neutrinos just have to be special. It's like they think the rules don't apply to them.
Honestly, so much has been written about these results in the last few days, it's hard to know where to begin, but here's the gist: In experiments conducted between the European Centre for Nuclear Research (CERN) in Switzerland and a laboratory in Italy, neutrinos were clocked zipping along at 300,006 kilometers per second -- i.e., slightly faster that the speed of light.
Scientists blasted a laser-like beam producing billions upon billions of neutrinos from CERN to the Gran Sasso Laboratory 730 kilometres (453 miles) away. It takes fractions of a second for neutrinos to travel that distance, and when they get to OPERA, they strike the detector, which is comprised of 150,000 bricks of alternating lead plates and photographic emulsion films. To figure out just how fast they're going, you divide the distance between the two points by the measured time it took for the neutrinos to travel between them.
The result: the neutrinos arrived 60 nanoseconds earlier than the 2.3 milliseconds taken by light. "This result comes as a complete surprise," said physicist Antonio Ereditato, spokesman for the OPERA experiment. "We wanted to measure the speed of neutrinos, but we didn't expect to find anything special." Hah! You're studying neutrinos, dude. Expect the unexpected.
As Sean Carroll points out over at Cosmic Variance, this isn't one of those pesky three-sigma results that pop up all the time in particle physics -- most recently in the ongoing search for the Higgs boson -- and then just as quickly disappear as more data is added to the mix, because it turned out to be a statistical fluctuation.
A five-sigma result (five standard deviations) is usually sufficient to claim a discovery. The OPERA collaborators are reporting an impressive six-sigma result -- in other words, it's probably not due to a random statistical fluctuation.
So why aren't they enthusiastically claiming discovery from the highest mountaintop? They recognize that, as the saying goes, extraordinary results demand extraordinary evidence. "Whenever you touch something so fundamental, you have to be much more prudent," Ereditato told The Guardian. "A result is never a discovery until other people confirm it." That's why the team spent six months double, triple, and quadruple checking their analysis. "If there is a problem, it must be a tough, nasty effect, because trivial things we are clever enough to rule out."
Don't Believe the Hype (Yet)
I'm sorry to report that, for all the hoopla, the general consensus that has emerged over the last couple of days is that (a) it's a really interesting, potentially exciting result, but (b) it probably won't hold up over time. Even the OPERA team isn't entirely convinced they're right; they're putting their work out there and basically asking their colleagues to poke holes in it and find anything they've missed. These are world-class physicists, mind you, but nobody is perfect, particularly when it comes to such tricky measurements.
So, what could be the problem? Per Carroll:
There is another looming source of possible error: a “systematic effect,” i.e. some unknown miscalibration somewhere in the experiment or analysis pipeline. (If you are measuring something incorrectly, it doesn’t matter that you measure it very carefully.) In particular, the mismatch between the expected and observed timing amounts to tens of nanoseconds; but any individual “event” takes the form of a pulse that is spread out over thousands of nanoseconds. Extracting the signal is a matter of using statistics over many such events — a tricky business.
A similar method was used by scientists with the MINOS collaboration, which also saw hints of neutrinos traveling slightly faster than light in 2007, although with much smaller statistical significance -- so much so, that Fermilab physicist Joseph Lykken described the result as "inconclusive."
"It's a pretty messy way to try to test a fundamental property," Lykken told Discovery News. "You have a proton beam at CERN that makes the neutrinos, but you don't know which proton made which neutrino. This makes it tough to claim nanosecond timing of the neutrinos. OPERA says they can do this on a statistical basis. Maybe so, but normally in experiments you use something well understood to measure something messy, not the other way around."
Another objection: "In a way, this experiment has been done," according to Marc Sher, a particle physicist with William & Mary College. We can look to the neutrinos detected from Supernova 1987A, which arrived roughly three hours before the light from the exploding star reached the detectors. But that's not because neutrinos traveled faster than light. Rather, they were able to pass right through all the material forming an envelope around the dying star, whereas photons would have to work their way through.
Physicists did the calculations and expected a three-hour delay, and that's exactly what they observed with the neutrinos from SN1987A. However, as Sher (and many others) have pointed out, if the OPERA result is real, those neutrinos should have traveled much faster, so much so that they would have arrived even sooner -- say, in 1984. I think physicists probably would have noticed.
"Supernova neutrinos are known, experimentally, to travel at the same speed as light to better than a part in a trillion," Sher emphasized. "The OPERA claim is that they are traveling faster than light by a part in 30,000." And, well, that's problematic.
John Beacom of Ohio State University told Discovery News that the comparison to SN1987A neutrinos might not be the best one to make: "It's meaningless without knowing how the speed might vary with neutrino energy, distance, etc." If you want a possible good cross-check of the OPERA results, he suggests a closer analog would be to look to experiments like IceCube, which searches for high-energy neutrinos associated with gamma ray bursts.
Read more at Discovery News
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