Neutrinos Travel Faster Than Light, According to One Experiment - Science AAAS

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Fat lady singing? The OPERA particle detector may have spotted neutrinos traveling faster than light, which would bring down the curtain on special relativity as an exact theory.

Credit: OPERA collaboration

If it's true, it will mark the biggest discovery in physics in the past half-century: Elusive, nearly massive subatomic particles called neutrinos appear to travel just faster than light, a team of physicists in Europe reports. If so, the observation would wreck Einstein's theory of special relativity, which demands that nothing can travel faster than light.

In fact, the result would be so revolutionary that it's sure to be met with skepticism all over the world. "I suspect that the bulk of the scientific community will not take this as a definitive result unless it can be reproduced by at least one and preferably several experiments," says V. Alan Kostelecky, a theorist at Indiana University, Bloomington. He adds, however, "I'd be delighted if it were true."

The data come from a 1300-metric-ton particle detector named Oscillation Project with Emulsion-tRacking Apparatus (OPERA). Lurking in Italy's subterranean Gran Sasso National Laboratory, OPERA detects neutrinos that are fired through the earth from the European particle physics laboratory, CERN, near Geneva, Switzerland. As the particles hardly interact at all with other matter, they stream right through the ground, with only a very few striking the material in the detector and making a noticeable shower of particles.

Over 3 years, OPERA researchers timed the roughly 16,000 neutrinos that started at CERN and registered a hit in the detector. They found that, on average, the neutrinos made the 730-kilometer, 2.43-millisecond trip roughly 60 nanoseconds faster than expected if they were traveling at light speed. "It's a straightforward time-of-flight measurement," says Antonio Ereditato, a physicist at the University of Bern and spokesperson for the 160-member OPERA collaboration. "We measure the distance and we measure the time, and we take the ratio to get the velocity, just as you learned to do in high school." Ereditato says the uncertainty in the measurement is 10 nanoseconds.

However, even Ereditato says it's way too early to declare relativity wrong. "I would never say that," he says. Rather, OPERA researchers are simply presenting a curious result that they cannot explain and asking the community to scrutinize it. "We are forced to say something," he says. "We could not sweep it under the carpet because that would be dishonest." The results will be presented at a seminar tomorrow at CERN.

The big question is whether OPERA researchers have discovered particles going faster than light, or whether they have been misled by an unidentified "systematic error" in their experiment that's making the time look artificially short. Chang Kee Jung, a neutrino physicist at Stony Brook University in New York, says he'd wager that the result is the product of a systematic error. "I wouldn't bet my wife and kids because they'd get mad," he says. "But I'd bet my house."

Jung, who is spokesperson for a similar experiment in Japan called T2K, says the tricky part is accurately measuring the time between when the neutrinos are born by slamming a burst of protons into a solid target and when they actually reach the detector. That timing relies on the global positioning system, and the GPS measurements can have uncertainties of tens of nanoseconds. "I would be very interested in how they got a 10-nanosecond uncertainty, because from the systematics of GPS and the electronics, I think that's a very hard number to get."

No previous measurements obviously rule out the result, says Kostelecky, who has spent 25 years developing a theory, called the standard model extension, that accounts for all possible types of violations of special relativity in the context of particle physics. "If you had told me that there was a claim of faster-than-light electrons, I would be a lot more skeptical," he says. The possibilities for neutrinos are less constrained by previous measurements, he says.

Still, Kostelecky repeats the old adage: Extraordinary claims require extraordinary evidence. Even Ereditato says that one measurement does not extraordinary evidence make.