The neutrino mystery

A group of physicists from Italy claimed they had observed the subatomic particles called neutrinos travelling faster than the speed of light. That,of course,is the cosmic speed limit declared in Albert Einstein’s theory of special relativity in 1905.

If they are right—and the jury is still out—Einstein might have some explaining to do. Among other things,a neutrino or anything else that went faster than the speed of light could go backward in time.

Physicists,who are quite sure that in fact E does still equal MC squared,have expressed skepticism. But that has not stopped the ghostly neutrinos,which can sail through miles of solid lead with impunity,from achieving a sort of pop culture fame.

The neutrino news came from a group of physicists based at the Gran Sasso underground laboratory in Italy and doing business under the apt acronym OPERA. The neutrinos,they reported on September 23 in a paper and at a special symposium at CERN,the European Center for Nuclear Research,had beaten a metaphorical light beam from CERN to Gran Sasso,a distance of 457 miles,by 60 nanoseconds.

The initial response of physicists assembled at CERN and around the world was that there was probably a mistake in the experiment. Einstein’s theory is the basis of all modern physics and has been tested a zillion times.

Technically,relativity does allow some particles,known as tachyons,to go faster than light—in fact it forbids them to slow down to light speed. The hitch is that they would have imaginary masses,whatever that means. And there is also the possibility,in some versions of string theory,of particles taking a shortcut through another dimension. But allowing anything to travel faster than light would open up the possibility of all kinds of problems with cause and effect and even time travel. “It looks too big to be true,” Alvaro de Rujala,a CERN theorist,said at the time.

Physicists have been flooding arXiv.org,the physics Internet archive,with papers debunking the OPERA experiment and defending Einstein. In one paper,two professors from Boston University,Andrew G. Cohen and the Nobelist Sheldon L. Glashow,showed that if the neutrinos had been going faster than light en route to Gran Sasso,they would have lost energy at a fearsome rate by emitting other particles,causing distortions in the beam that were not seen by OPERA.

Another paper—by Gian Giudice of CERN,Sergei Sibiryakov of the Institute for Nuclear Research in Moscow and Alessandro Strumia of the University of Pisa in Italy and the National Institute of Chemical Physics and Biophysics in Tallinn,Estonia—argued that according to the Standard Model,the reigning theory in particle physics,if neutrinos could violate relativity,electrons should violate it also,something that has also not been observed.

Recently,Ronald A.J. van Elburg,an artificial intelligence researcher at the University of Groningen in the Netherlands,suggested that the OPERA group had failed to make a relativistic correction for the motions of the GPS satellites used in timing the neutrino beams. The resulting error,he said,amounted to 64 nanoseconds,almost exactly the universe-shaking discrepancy the OPERA researchers were hoping to explain.

That paper got wide attention. It was mentioned on a physics blog of the magazine Technology Review and was published by the Massachusetts Institute of Technology and other news sites around the Internet as a possible explanation of the neutrino mystery.

The OPERA collaborators and other outside physicists now say van Elburg’s analysis is wrong and reflects confusion about how GPS systems work. In an email,Antonio Ereditato,a spokesman for OPERA,said the paper did have some errors,but he declined to go into details.

John Learned,a neutrino physicist at the University of Hawaii,wrote in an email that while the OPERA results might not be right,“they are still not easily dismissed.”