From Geneva to Italy Faster Than a Speeding Photon?

Adrian Cho When news spread last week that physicists in Europe had spotted subatomic particles called neutrinos traveling faster than light, some of their colleagues reacted with incredulity. After all, the observation would contradict Einstein’s special theory of relativity, which…

When news spread last week that physicists in Europe had spotted subatomic particles called neutrinos traveling faster than light, some of their colleagues reacted with incredulity. After all, the observation would contradict Einstein’s special theory of relativity, which says that nothing can travel faster than light. Jim Al-Khalili, a theorist at the University of Surrey in the United Kingdom, even vowed to eat his boxer shorts on live television if the result holds up. But if it does, physicists won’t be quite as bewildered as such reactions imply. Some have already developed a theoretical framework that can handle faster-than-light neutrinos and all other potential breaches of special relativity.

Known as the standard model extension (SME), that theory generalizes the standard model of particle physics (Science, 11 February 2005, p. 866). “The standard model extension is a pretty robust, reliable framework, and it’s internally consistent,” says Neil Russell, a theorist at Northern Michigan University in Marquette. If faster-than-light neutrinos exist, “there’d be lots of work to do, but the SME would provide a starting point.” And if neutrinos do travel faster than light, the SME predicts no absurd consequences such as violations of causality.

Of course, it may be the spectacular result itself that won’t hold up. 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 elusive, nearly massless neutrinos that are fired through Earth from the European particle physics laboratory, CERN, near Geneva, Switzerland.

Over 3 years, OPERA researchers clocked roughly 16,000 neutrinos. On average, the particles made the 730-kilometer, 2.43-millisecond trip roughly 60 nano seconds faster than expected if they were traveling at light speed, 299,792,458 meters per second. The uncertainty in the measurement is 10 nanoseconds, the experimenters reported in a paper posted to the arXiv preprint server and in a seminar at CERN on 23 September.

OPERA researchers are not declaring relativity wrong; they are merely presenting a curious result they cannot explain so that others can scrutinize it, says Antonio Ereditato of the University of Bern, spokesperson for the 160-member OPERA team. “We could not sweep it under the carpet because that would be dishonest,” he says.

The big question is whether an unidentified “systematic error” in the experiment is making the travel time look artificially short. Chang Kee Jung of Stony Brook University in New York state, who works on a similar experiment in Japan called T2K, says he’d wager that the result is the product of a systematic error in the complex timing, which involves the Global Positioning System, atomic clocks, and masses of electronics. “I wouldn’t bet my wife and kids because they’d get mad,” Jung says. “But I’d bet my house.”

All agree that the result must be confirmed by other experiments. But even if it is, theoretical physics won’t collapse, says V. Alan Kostelecky, a theorist at Indiana University, Bloomington, who 15 years ago invented the SME, which is now a touchstone for researchers interested in testing relativity. People get the notion that any violation of relativity will lead to absurdities, such as effects that precede their causes and time travel, because they try to inject into relativity the idea of something traveling faster than light while keeping all the rest of the theory the same. That’s not logical, Kostelecky says.

So what would change if relativity isn’t exactly right? Einstein derived his theory in part from the assumption that there is no “preferred frame” of reference in which something is or is not stationary relative to the universe. That tenet goes out the window if relativity is violated, Kostelecky says. The SME violates relativity by allowing for “background fields” a bit like an electric field that would be woven into the fabric of space to give it a preferred frame or direction, as it would be possible to tell if something is moving relative to the field. If such a field exists that interacts only with neutrinos, it could make them fly slightly faster than light, Russell says: “If the observation is true, then it would seem that there is some sort of background field but that only neutrinos can feel it.”

Other effects would likely be subtle, says Matthew Mewes, a physicist at Swarthmore College in Pennsylvania. For example, neutrinos come in three types or “flavors,” and as they zing along, one type can morph or “mix” into another. In the conventional model, the amount that different flavors mix does not depend on the neutrinos’ energy. If relativity is violated, it might, Mewes says.

Tests of the OPERA results could come soon. Physicists at Fermi National Accelerator Laboratory in Batavia, Illinois, shoot neutrinos to a detector called the Main Injector Neutrino Oscillation Search (MINOS), 735 kilometers away in the Soudan mine in Minnesota. MINOS researchers should weigh in within 6 months, says collaboration co-spokesperson Jennifer Thomas of University College London. Researchers with T2K, who shoot neutrinos from the Japan Proton Accelerator Research Complex in Tokai to the Super-Kamiokande detector in the Kamioka mine 295 kilometers away, may also try to reproduce the result. Stay tuned.