Can Neutrinos Travel Faster Than Light?

The news from researchers with the international OPERA experiment that neutrinos can travel faster than the speed of light was greeted with both admiration and skepticism at a press conference in Geneva last week.

The OPERA experiment observed a neutrino beam generated by particle accelerators underground at CERN in Geneva and sent about 450 miles to Italy's National Institute of Nuclear Physics Gran Sasso Laboratory. The OPERA result is based on the observation of more than 15,000 neutrino events measured at Gran Sasso and appears to indicate that the neutrinos travel at a velocity of 20 parts per million above the speed of light, which has been considered nature's cosmic speed limit.

Einstein's 1905 restricted theory of relativity states that nothing could exceed the speed of light in a vacuum, which is at odds with the OPERA results, which found that neutrinos travel slightly but significantly ahead of the time it would take light to cover the same distance in a vacuum.

"This result comes as a complete surprise," said OPERA spokesperson Antonio Ereditato of the University of Bern. "After many months of studies and cross checks, we have not found any instrumental effect that could explain the result of the measurement."

“If this measurement is confirmed, it might change our view of physics," said CERN Research Director Sergio Bertolucci, who added that the results need to be confirmed independently.

On Friday, CNRS (National Center for Scientific Research) researcher Dario Autiero presented the findings to an auditorium full of particle physicists at CERN, the European Organization for Nuclear Research.

"We have established synchronization between CERN and Gran Sasso that gives us nanosecond accuracy, and we’ve measured the distance between the two sites to 20 centimeters,” Autiero said. “Although our measurements have low systematic uncertainty and high statistical accuracy, and we place great confidence in our results, we’re looking forward to comparing them with those from other experiments."

The OPERA collaboration teamed with experts in metrology from CERN and other institutions to perform a series of high-precision measurements of the distance between the source and the detector, and of the neutrinos' time of flight. The distance between the origin of the neutrino beam and OPERA was measured with an uncertainty of 20 cm over the 730-km travel path, or about 7.8 inches over a 454-mile distance.

The neutrinos' time of flight was determined with an accuracy of less than 10 nanoseconds by using sophisticated instruments including advanced GPS systems and atomic clocks. The time response of all elements of the CNGS beam line and of the OPERA detector has also been measured with great precision.

"The potential impact on science is too large to draw immediate conclusions or attempt physics interpretations. My first reaction is that the neutrino is still surprising us with its mysteries," Ereditato said.

“When an experiment finds an apparently unbelievable result and can find no artifact of the measurement to account for it, it’s normal procedure to invite broader scrutiny, and this is exactly what the OPERA collaboration is doing; it’s good scientific practice,” Bertolucci said.

One such experiment that will scrutinize the results is MINOS, Main Injector Neutrino Oscillation Search, at Fermilab, outside Chicago.

The OPERA experiment was inaugurated in 2006, with the main goal of studying the rare transformation of muon neutrinos into tau neutrinos. One first such event was observed in 2010, proving the unique ability of the experiment in the detection of the elusive signal of tau neutrinos.

For more information, visit CERN or the OPERA project or CNRS.