Experiment hints at possible violation of Einstein’s speed limit

By Devin Powell

Web edition: September 23, 2011
Print edition: October 22, 2011; Vol.180 #9 (p. 18)

An experiment that creates particles called neutrinos has called into question Einstein’s theory of special relativity. Even though few believe that these results will ultimately hold up, their implications have stirred up quite a fuss.

After painstakingly checking and rechecking their data, physicists working on Italy’s OPERA experiment say they have clocked neutrinos traveling faster than the speed of light. According their calculations, there’s only a one in a billion chance that what they’re seeing is a statistical fluke.

But that doesn’t make it real.

The official announcement of the result, on September 23 at the European physics laboratory CERN near Geneva, was met with cheering — but also with a barrage of questions from those scrutinizing the experiment for unknown sources of error that may be misleading the physicists. 

“This will be a tremendous revolutionary finding if it is true,” says Chang Kee Jung, a particle physicist at Stony Brook University in New York and a spokesperson for the T2K neutrino experiment in Japan. Ask him to bet against the new results, though, and he says he’d be willing to bet his house.

After all, this isn’t the first report of improbably speedy neutrinos. In 2007 the MINOS experiment turned up hints of neutrinos traveling impossibly fast between the Fermi National Accelerator Laboratory in Batavia, Ill., and a mine in Minnesota. But the uncertainties in those measurements were too large to justify calling it a discovery.

OPERA’s neutrinos were born from protons smashed into a chunk of graphite at CERN. They then traveled underground to Italy’s National Gran Sasso Laboratory beneath the Apennines Mountains. A detector spotted the arrival of a small fraction of the particles — about 16,000 in total between 2009 and 2011.

Thanks to GPS devices, the distance of this trip, about 730 kilometers, is known to within 20 centimeters — a feat of accuracy that required closing a lane of traffic for a week in a tunnel above the detector in Italy.

“We could have done an even better job if we stopped all the traffic,” says Dario Autiero, an OPERA team member and a physicist at the Institute of Nuclear Physics of Lyon in France.

Light traveling in a vacuum would have made this trip in 2.43 milliseconds. The neutrinos shaved about 60 nanoseconds off that time, according to atomic clocks at either end synchronized by a satellite. Divide distance by time, and the particles must have been traveling 0.0025 percent faster than the speed of light in a vacuum.

Still, Autiero and his colleagues may have missed some unknown systematic uncertainties built into their equipment, says Kevin McFarland, a particle physicist at the University of Rochester in New York and a spokesperson for Fermilab’s MINERvA neutrino experiment.

“It’s just odd,” says McFarland. “Everybody’s bias in responding to this is going to be that this is some sort of systematic uncertainty that they didn’t figure out.”

Other neutrino experiments plan to double-check the results. At Japan’s T2K experiment, where particles travel only 295 kilometers, the speed discrepancy would be smaller and more difficult to observe. Fermilab might have a better shot. Neutrinos in the MINOS experiment cover 735 kilometers, about the same distance as CERN’s experiment.

MINOS will soon upgrade its equipment with snazzy new atomic clocks, says Rob Plunkett, a Fermilab physicist working on a MINOS experiment. The upgraded experiment, which will start in 2013 and last for a year or so, should have uncertainties comparable to OPERA’s.

Confirmation of the results would be exciting news for theoretical physicists such as Matthew Mewes of Swarthmore College in Pennsylvania, who have long played around with ways to modify relativity.

“This may mean that there’s much more going on in particle physics than we thought possible,” says Mewes. “We could be seeing signs of exotic theories like string theories.”

In 2004 Mewes and Alan Kostelecky of Indiana University in Bloomington published a paper in Physical Review D describing one such theory. They discard one of the basic assumptions of relativity, a symmetry that makes the laws of physics look the same when viewed from different reference frames. By filling spacetime with a field that has a preferred direction, the physicists create a universe that still has an ultimate speed limit — just not one that’s necessarily set by light.

Other proposals could accommodate faster-than-light travel with violating this principle of relativity, says Lee Smolin, a theoretical physicist at the Perimeter Institute for Theoretical Physics in Waterloo, Canada. But they would also need to explain why previous experiments with particles of light have already ruled out effects that could explain the new neutrino results.

“This is a serious experiment, and these are serious people,” says Smolin. “But at this point nobody sober would be willing to say that this is right.”