Those wacky neutrinos are at it again, mystifying physicists by refusing to behave like they’re supposed to.
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 CERN (European Centre for Nuclear Research) in Switzerland and a laboratory in Italy, neutrinos were clocked zipping along at 300,006 kilometers (186,000 miles) per second — i.e., slightly faster than the speed of light.
Scientists blasted a laser-like beam producing billions upon billions of neutrinos from CERN to the Gran Sasso Laboratory 730 kilometers (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 composed 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.
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“These bursts are typically short, from fractions of a second to several seconds, and they often have sharp features on shorter timescales,” said Beacom.
IceCube uses complicated models to separate background signals from terrestrial atmospheric (lower energy) neutrinos coming from all directions at once. It looks for neutrinos with high energy arriving from a specific direction, within a specific time frame.
IceCube hasn’t yet detected any gamma ray burst neutrinos, but that might be because IceCube researchers have set the wrong limits on their models; they would be looking in the wrong time frames when sifting through their data. If IceCube continues to find nothing, OPERA might just be correct. But if IceCube finds even one gamma ray burst neutrino within its parameters — well, that wouldn’t be so good for OPERA.
But What If It’s Right?
So, OK, it’s probably going to go away with time. But just for giggles, let’s assume that OPERA’s result ends up being verified? What does this mean, really?
One thing that it wouldn’t mean is that relativity is completely wrong, and now time travel and instantaneous communication will be possible. As physicist Matt Strassler put it (rather Spock-like) in his own bloggy commentary: “Balderdash! This is loose and illogical thinking.” And OPERA is not claiming that their work has overturned Einstein. “I would never say that,” Ereditato told Science. “We are forced to say something. We could not sweep it under the carpet, because that would be dishonest.”
That said, several physicists over the last couple of decades have seriously looked at the possibility of slight violations of relativistic principles, i.e., Lorentz invariance, and even suggested that neutrinos might be a good place to look for it.
This principle states that everyone will measure the speed of light the same, regardless of their frame of reference. It’s the most central tenet of special relativity; it’s why length contracts and time dilates, so that the speed of light remains constant. Maybe this isn’t the case, however, and we need to tweak special relativity a bit.
While reluctant to speculate, for good reason, Lykken opined that if the result does hold up, “I would bet it is a sign of warped extra dimensions.” Those extra dimensions — a key element in theories of quantum gravity — would provide a shortcut of sorts for neutrinos, enabling them to travel just a wee bit faster than photons.
Or perhaps certain high-energy neutrinos are really tachyons, those hypothetical particles first proposed in the 1960s as traveling faster than the speed of light. That’s an idea that’s been kicking around since a 1985 paper by Alan Chodos, Ari Hauser and Alan Kostalecky first proposed it.
Specifically, they predicted that if neutrinos interacted with an unknown field lurking in the quantum vacuum, they would be able to travel faster than light. “With this kind of background, it is not necessarily the case that the limiting speed in nature is the speed of light,” Kostelecky told The Guardian. “It might actually be the speed of neutrinos and light goes more slowly.”
Yet even Kostelecky advocates caution: “It’s such a dramatic result, it would be difficult to accept without others replicating it.”
So that’s where we stand on faster-than-light neutrinos. Physicists independent of the OPERA collaboration will poke around looking for systemic errors in the analysis, while other neutrino experiments will scramble to see if their own results confirm or disprove OPERA’s exciting find. In the meantime, heed the words of Strassler: “Hyperventilating about the impending collapse of existing theoretical physics is a tad inappropriate at this time.”