Dimension-hop may allow neutrinos to cheat light speed

New Scientist, 23 September 2011 by Lisa Grossman 

http://www.newscientist.com/article/dn20957-dimensionhop-may-allow-neutrinos-to-cheat-light-speed.html?DCMP=NLC-nletter&nsref=dn20957

A CERN experiment claims to have caught neutrinos breaking the universe's most fundamental speed limit. The ghostly subatomic particles seem to have zipped faster than light from the particle physics laboratory near Geneva, Switzerland, to a detector in Italy.

Fish that physics textbook back out of the wastebasket, though: the new result contradicts previous measurements of neutrino speed that were based on a supernova explosion. What's more, there is still room for error in the departure time of the supposed speedsters. And even if the result is correct, thanks to theories that posit extra dimensions, it does not necessarily mean that the speed of light has been beaten.

"If it's true, it's fantastic. It will rock the foundation of physics," says Stephen Parke of Fermilab in Batavia, Illinois. "But we still have to confirm it."

Neutrinos are nearly massless subatomic particles that are notoriously shy of interacting with other forms of matter. An experiment called OPERA (Oscillation Project with Emusion tRacking Apparatus) sent beams of neutrinos from a particle accelerator at CERN to a detector in the Gran Sasso cavern in Italy, 730 kilometres away.

The neutrinos arrived 60 nanoseconds sooner than they would have if they had been travelling at the speed of light, the team says.

Supernova contradiction

If real, the finding will force a rewrite of Einstein's theory of special relativity, one of the cornerstones of modern physics (and a theory whose predictions are incorporated into the design of the accelerators at CERN). "It's not reasonable," says theorist Marc Sher of the College of William and Mary in Williamsburg, Virginia.

One problem is that the CERN result busts the apparent speed limit of neutrinos seen when radiation from a supernova explosion reached Earth in February 1987.

Supernovae are exploding stars that are so bright they can briefly outshine their host galaxies. However, most of their energy actually streams out as neutrinos. Because neutrinos scarcely interact with matter, they should escape an exploding star almost immediately, while photons of light will take about 3 hours to get out. And in 1987, trillions of neutrinos arrived 3 hours before the dying star's light caught up, just as physicists would have expected.

The recent claim of a much higher neutrino speed just doesn't fit with this earlier measurement. "If neutrinos were that much faster than light, they would have arrived [from the supernova] five years sooner, which is crazy," says Sher. "They didn't. The supernova contradicts this [new finding] by huge factors."

Fuzzy departure

It's possible that the neutrinos that sped to the Italian mine were a different type of neutrino from the ones streaming from the supernova, or had a different energy. Either of those could explain the difference, Sher admits. "But it's quite unlikely."

A measurement error in the recent neutrino experiment could also explain the contradiction.

"In principle it's a very easy experiment: you know the distance between A and B, you know how long it takes the neutrinos to get there, so you can calculate their speed," Parke says. "However, things are more complicated than that. There are subtle effects that make it much more difficult."

For instance, although the detectors in Italy can pinpoint the neutrinos' time of arrival to within nanoseconds, it's less clear when they left the accelerator at CERN. The neutrinos are produced by slamming protons into a bar-shaped target, sparking a cascade of subatomic particles. If the neutrinos were produced at one end of the bar rather than the other, it could obscure their time of flight.

Sher also mentions a third option: that the measurement is correct. Some theories posit that there are extra, hidden dimensions beyond the familiar four (three of space, one of time). It's possible that the speedy neutrinos tunnel through these extra dimensions, reducing the distance they have to travel to get to the target. This would explain the measurement without requiring the speed of light to be broken.

Antonio Ereditato with the OPERA collaboration declined to comment until after a seminar to be held at CERN today at 4 pm Geneva time.

Extraordinary evidence wanted

In the meantime, Parke is reserving judgement until the result can be confirmed by other experiments such as the MINOS experiment at Fermilab or the T2K experiment in Japan.

"There are a number of experiments that are online or coming online that could be upgraded to do this measurement," he says. "These are the kind of things that we have to follow through, and make sure that our prejudices don't get in the way of discovering something truly fantastic."

In 2007, the MINOS experiment searched for faster-than-light neutrinos but didn't see anything statistically significant.

Although sceptical, he is willing to give their colleagues at OPERA the benefit of the doubt. "They certainly didn't do anything that's obviously stupid, or they would have caught that," Parke says.

"They're smart people, these are not crackpots," Sher agrees. "But as the old saying goes, extraordinary claims require extraordinary evidence. This is about as extraordinary as you get."

Reference: arxiv.org/abs/1109.4897