Physicists have found yet another reason to doubt recent reports of neutrinos traveling faster than light. The existence of such speedy particles would screw up not only Einstein’s theory of special relativity, but also the laws of conservation of energy and momentum.
In September, the OPERA experiment reported clocking neutrinos traveling faster than the speed of light, arriving 60 nanoseconds early on their 730-kilometer journey between the European laboratory CERN, near Geneva, and the Gran Sasso National Laboratory in Italy. To try to explain the result, two new studies examined the particles that give birth to neutrinos. Both found that these particles, called pions, could not possibly have had enough energy to give rise to the faster-than-light, or superluminal, speeds indicated by OPERA.
“We give a clear constraint on the superluminality of neutrinos,” says Xiaojun Bi, a particle astrophysicist at the Chinese Academy of Sciences’ Institute of High Energy Physics in Beijing. His team reported its findings in the Dec. 6 Physical Review Letters.
If neutrinos can travel faster than light, they should get heavier as their energy increases. So there’s a limit to how fast the particles can zip along, dictated by the energy of their unstable pion parents.
OPERA’s pions, made at CERN, have about 3.5 times as much energy as their neutrino progeny. That sets a neutrino speed limit that’s lower than the speed measured by OPERA, physicist Ramanath Cowsik of Washington University in St. Louis and colleagues reported in the Dec. 16 Physical Review Letters. Bi suggests that OPERA’s highest-energy neutrinos push this speed limit even lower.
Achieving the mind-boggling velocities measured by OPERA would have required pions with energies 20 times greater than their offspring, Cowsik’s team calculates. At such energies, though, the lifetimes of pions would be six times longer, which has been ruled out by measurements from OPERA and other experiments.
The most stringent limit yet on neutrino speed comes from high-energy neutrinos born when cosmic rays strike the atmosphere. The IceCube detector at the South Pole has measured these neutrinos to energies more than 10,000 times as high as OPERA’s neutrinos. If the speeds measured by OPERA are correct, the cosmic-ray neutrinos should be far too heavy to have been produced by pions. IceCube thus throttles back the potential speed of any superluminal neutrinos to be a few ten billionths of a percent above the speed of light — a far cry from the few thousandths of a percent reported by OPERA.
For Cowsik and other researchers, these problems and contradictions suggest that the laws of physics as currently understood are correct. But physicists will still be watching other neutrino experiments that can check OPERA’s result, which may be clouded by some unknown source of error.
“No one is saying that the OPERA result is impossible, even though it would require extreme revisions to what we know about physics,” says Sheldon Glashow, a Nobel Prize-winning theoretical physicist at Boston University. “But if it turns out to be true, I would say to Nature, ‘You win.’ Then I’d give up, and I’d retire.”