Finding that contradicts Einstein's theory of special relativity is repeated with fine-tuned procedures and equipment
The scientists who appeared to have found in September that certain particles that can travel faster than light have ruled out one potential source of error in their measurements after completing a second, fine-tuned version of their experiment.
Their results, posted on the arvix preprint server on friday morning and submitted for peer review in the Journal of High Energy Physics, confirmed earlier measurements that neutrinos, sent through the ground from Cern near Geneva to the Gran Sasso lab in Italy 450 miles (720km) away seemed to travel faster than light.
The finding that neutrinos might break one of the most fundamental laws of physics sent scientists into a frenzy when it was first reported in September. Not only because it appeared to go against Albert Einstein's theory of special relativity but, if correct, the finding opened up the troubling possibility of being able to send information back in time, blurring the line between past and present and wreaking havoc with the fundamental principle of cause and effect.
Around 20 neutrino events have been measured at the Gran Sasso lab in the fine-tuned version of the experiment in the past few weeks, each one precisely associated with a pulse leaving Cern. The scientists concluded from the new measurements that the neutrinos still appeared to be arriving earlier than they should.
"With the new type of beam produced by Cern's accelerators we've been able to to measure with accuracy the time of flight of neutrinos one by one," said Dario Autiero of the French National Centre for Scientific Research (CNRS). "The 20 neutrinos we recorded provide comparable accuracy to the 15,000 on which our original measurement was based. In addition their analysis is simpler and less dependent on the measurement of the time structure of the proton pulses and its relation to the neutrinos' production mechanism."
In a statement released on Friday, Fernando Ferroni, president of the Italian Institute for Nuclear Physics, said: "A measurement so delicate and carrying a profound implication on physics requires an extraordinary level of scrutiny. The experiment at Opera, thanks to a specially adapted Cern beam, has made an important test of consistency of its result. The positive outcome of the test makes us more confident in the result, although a final word can only be said by analogous measurements performed elsewhere in the world."
Since the Opera (Oscillation Project with Emulsion-tRacking Apparatus) team at Gran Sasso announced its results, physicists around the world have published scores of online papers trying to explain the strange finding as either the result of a trivial mistake or evidence for new physics.
Dr Carlo Contaldi of Imperial College London suggested that different gravitational effects at Cern and Gran Sasso could have affected the clocks used to measure the neutrinos. Others have come up with ideas about new physics that modify special relativity by taking the unexpected effects of higher dimensions into account.
Despite the latest result, said Autiero, the observed faster-than-light anomaly in the neutrinos' speed from Cern to Gran Sasso needed further scrutiny and independent tests before it could be refuted or confirmed definitively. The Opera experiment will continue to take data with a new muon detector well into next year, to improve the accuracy of the results.
The search for errors is not yet over, according to Jacques Martino, director of the National Institute of Nuclear and Particle Physics at CNRS. He said that more checks would be under way in future, including ensuring that the clocks at Cern and Gran Sasso were properly synchronised, perhaps by using an optical fibre as opposed to the GPS system used at the moment.
This would remove any potential errors that might occur due to the effects of Einstein's theory of general relativity, which says that clocks tick at different rates depending on the amount of gravitational force they experience – clocks closer to the surface of the Earth tick slower than those further away.
Even a tiny discrepancy between the clocks at Cern and Gran Sasso could be at the root of the faster-than-light results seen in September.
The team also rechecked their statistical analysis, confirming that the error on their measurements was indeed 10 nanoseconds. Some team members, including Stanco, had worried that the true error was larger. What they found was "absolutely compatible" with the original announcement, he says.
That was enough for Stanco to put his name to the paper, although he says six or seven team members are still holding out. The team was planning to submit the paper to a European physics journal on Thursday.
They are still running other tests, including measuring the length of a fibre-optic cable that carries information from the underground lab at Gran Sasso to a data-collection centre on the surface. The team is also trying to do the same test using another detector at the lab called RPC. That test will take another several months.
Even though he agreed to sign the paper, Stanco says: "I'm not so happy. From a theoretical point of view, it is not so appealing. I still feel that another experiment should make the measurement before I will say that I believe this result."
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