domingo, 23 de octubre de 2011
Physics - "Faster-Than-Speed-of-Light" Result --Stumps CERN's Top Brains
Europe's top particle physicists have yet to reconcile faster-than-light neutrinos with the laws of physics. "For the moment, there is no explanation that works," says physicist Ignatios Antoniadis, who helped to organize a meeting of Europe's top physics minds at CERN near Geneva, Switzerland three weeks to the day after physicists in the OPERA collaboration at Gran Sasso, Italy, announced that neutrinos traveling from CERN had apparently moved faster than light. The assembled team reviewed "the situation and discuss whether it is possible [that neutrinos broke the speed of light."
The biggest challenge yet to the OPERA result comes from Nobel laureate Sheldon Glashow and his Boston University colleague Andrew Cohen in a paper posted online a few weeks ago. The Physical Review Letters has agreed to publish the paper, making it the first scientific journal to accept work on the OPERA result.
In the paper, Glashow and Cohen point out that if neutrinos can travel faster than light, they should sometimes radiate an electron paired with its antimatter equivalent – a positron – through a process called "Cerenkov radiation," which is analogous to a sonic boom, according to New Scientist.
Cohen and Glashow calculated that at the end of the experiment, the neutrinos should have had energies no higher than about 12 gigaelectronvolts. But OPERA saw plenty of neutrinos with energies upwards of 40 GeV.
"It doesn't correspond to the energies measured at all," says CERN physicist Christophe Grojean.
Another strike against the "faster-than-light" neutrinos comes from the fact that neutrinos are linked to certain other particles – electrons, muons and tau particles – via the weak nuclear force. Because of that link, neutrinos can't travel faster than light unless electrons do too – although electrons needn't travel as fast as the neutrinos.
According to New Scientist, CERN physicist Gian Giudice, who spoke at the seminar, and colleagues looked into what would happen if electrons travelled faster than light by one part in 100,000,000, a speed consistent with the OPERA neutrino measurement. Such speedy electrons should emit a cone of Cerenkov radiation in empty space – but previous experiments show that they don't.
"At the moment, there is no concrete model that really avoids all these theoretical constraints," Grojean says. "That's why it's so interesting. We cannot explain it in terms of known physics."
Another unpublished paper on the arxiv.org physics preprint server has attracted attention with its explanation. Ronald van Elburg at the University of Groningen in the Netherlands has calculated that special relativity could have messed up the synchronisation of the clocks at CERN and Gran Sasso. This would make neutrinos appear to arrive 64 nanoseconds early – almost exactly what the OPERA experiment observed.
If this argument holds up, rather than breaking Einstein's theory of special relativity, the faster-than-light neutrinos would actually end up reaffirming it. But it's unclear whether the result will hold up. "In general, the feeling of theorists is that one should repeat the experiment," Antoniadis told New Scientist.
CERN plans to provide a new neutrino beam to do this. Meanwhile, the first glimpses from another detector at the Gran Sasso laboratory don't bode well for the faster-than-light hypothesis.
An experiment there called ICARUS (Imaging Cosmic And Rare Underground Signals) has been catching neutrinos travelling from CERN since last year. The 100 or so it has seen do not seem to travel faster than light. ICARUS also doesn't see any evidence of the Cerenkov-like radiation Glashow and Cohen predicted.
"For the moment, we don't have an answer," Antoniadis says. "That doesn't mean an answer doesn't exist."
Source: The Daily Galaxy - newscientist.com