Speedy neutrinos? Not so fast. The shocking result that neutrinos apparently travelled from Switzerland to Italy faster than the speed of light may have been due to a malfunctioning fibre-optic cable, says OPERA, the Italian collaboration of physicists that made the first, surprising claim.
"If the mistake is confirmed, it was clearly due to a human error," says Luca Stanco, one of 15 members of the 160-strong OPERA collaboration who did not sign their names to the initial report of the results because they considered it too preliminary. "I tried to say, please be more careful," he adds.
The faulty cable suggestion is one of two possible problems with the initial result that OPERA (Oscillation Project with Emulsion-tRacking Apparatus) flagged today. The other would mean the neutrinos travelled even faster than initially thought.
The physicists of the OPERA collaboration shocked the world last September when it reported that neutrinos from a particle accelerator at CERN near Geneva, Switzerland, arrived 60 nanoseconds earlier at their detector in the Gran Sasso cavern in Italy than they would have if they had been travelling at the speed of light.
The result flew in the face of Einstein's theory of special relativity, which holds that nothing can accelerate beyond the speed of light. A particle violating that limit would be difficult to square with a century of experiments that have upheld the rest of the theory's predictions.
Neutrino timing
In September, the team said it had spent three years chasing every possible source of error they could think of. It even reran the experiment in November with an improved beam line. The result stuck ? but now it looks like it may actually be too good to be true, as many physicists expected.
"The OPERA collaboration has informed its funding agencies and host laboratories that it has identified two possible effects that could have an influence on its neutrino timing measurement," says a statement on CERN's website. "These both require further tests with a short pulsed beam. If confirmed, one would increase the size of the measured effect, the other would diminish it."
The first effect concerns "an oscillator used to provide the time stamps for GPS synchronizations". It could have led to an overestimate of the neutrino's time of flight, according to the OPERA statement. In other words, in this scenario, the neutrinos would have actually travelled even faster than OPERA initially reported. "We are not fully sure of the stability of the timing," says Stanco.
The second possibility suggests that neutrinos did not travel faster than light, as many physicists already suspected. It involves a cable connecting a GPS receiver to a computer above the Gran Sasso lab. The GPS signal was meant to ensure the clocks in Geneva agreed with the clocks in Gran Sasso, but its time stamp had to travel 8 kilometres from the surface to the underground lab. The team at OPERA had to subtract that signal's travel time from their final result to get the true neutrino flight time.
State unknown
The cable connecting the GPS receiver to the computer that sent the timestamp to Gran Sasso may have been malfunctioning when the measurements were performed, introducing 60 nanoseconds of delay. If it was, it would have made the neutrinos appear to arrive 60 nanoseconds too early.
"After the release of the result, OPERA rechecked the parts of the system. It turns out we didn't have the right answer from this connector," says Stanco. "It is not clear what the state of the connector was when the data was collected," he adds.
The next step is to rerun the experiment with the connector working properly ? something that will be done in May when CERN can provide a beam of neutrinos, says Stanco.
Which result is more likely? Stanco's personal belief is the second, cable explanation ? because it fits with scientific expectation. But he admits that "the reality may be different". "I think we should distinguish between our beliefs and the results of our experiments," he says.
In the light of a possible mundane explanation for the startling result, was OPERA too quick to announce its result in September? "I was against the publicity from the beginning," says Stanco. "In physics, mistakes happen. What matters is to be able to recognise them."
CERN defends its decision to host a symposium in which OPERA announced the result. "It is the normal order for any scientific process," says Arnaud Marsollier of the CERN press office. "Even in September, we were very cautious in saying this is not a discovery, this is something that is not understood and that we want to check again."
Einstein safe
CERN spokesperson James Gillies cautions that the real answer still isn't known. "There's a possible explanation, but we will not know for sure until further tests have been made with the beam," he says.
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