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Neutrinos win but Einstein hasn't lost yet

News analysis Many physicists still reject claims the speed of light has been smashed despite a second set of experiments producing identical results.

In late September, scientists from Gran Sasso and CERN posted results of an experiment claiming to have timed particles travelling six kilometres per second faster than the speed of light.

The paper on the prepress website ArXiv.org has sparked the most important debate in physics today.

If the results are confirmed it would overturn Einstein's Special Theory of Relativity and rock the foundations of our understanding of how the cosmos works.

Professor Victor Flambaum, head of the Department of Theoretical Physics at the University of New South Wales, says the results contradict all we know about special relativity.

"It's not very easy to overrule Einstein's theory. There have been thousands of tests of special relativity, people have confirmed it, there's never been a confirmed deviation," he says.

Measuring neutrinos

The experiment known as OPERA shoots protons at a graphite block producing bursts of neutrinos that travel from CERN in Switzerland to San Grasso in Italy.

The first experiment found the neutrinos covered the distance 60 nanoseconds faster than the 2.3 milliseconds it would take a photon travelling at the speed of light (299,792,458 metres per second in a vacuum) to cover the same distance.

The findings were greeted with shock and scepticism by the scientific community and a multitude of papers questioning the original findings have been published in response.

One potential area of concern was properly identifying which neutrinos were being measured.

In the original experiment, pulses of neutrinos were each 10 microseconds long making measuring their exact arrival time at Gran Sasso open to errors.

OPERA repeated the experiment altering the beams making them three nanoseconds shorter, with longer 524 nanosecond gaps between them, allowing the departure and arrival times of individual particles to be identified.

The second experiment, announced late last week, confirmed the results of the first experiment.

Theoretical physicist, Professor Craig Savage from the Australian National University says repeating the experiment cleared up some questions surrounding the original paper.

"This is a really hard experiment, almost all elementary particle experiments are difficult, but neutrino ones are doubly hard because neutrinos are so hard to detect," says Savage.

"The OPERA team only detected 20 individual neutrinos over the two weeks they re-ran the experiment."

While the second test cleared up concerns over identifying the neutrinos, critics say it still fails to address several concerns including relativistic effects, time calibration and distance measurements.

Independent studies needed

It's not the first time that scientists have clocked neutrinos travelling faster than light.

"People seem to forget it's not the first result on faster than light neutrinos, it's the second," says Savage.

"Fermilab 's MINOS experiment achieved the same result in 2007. However that wasn't statistically significant. There was some reasonable probability that what they were seeing was just a statistical fluctuation."

What is needed, says Savage, is for the new results to be replicated by an independent group.

Another group working in San Grasso, known as ICARUS, measured the energy output from the same neutrinos OPERA were working with and found no changes in energy, says Professor John Webb, from the University of New South Wales.

"If the neutrinos were travelling faster than light they should lose energy by creating electron-positron pairs of sub-atomic particles," says Webb.

"They concluded no such energy was being lost and therefore suggest the OPERA team may have a calibration error."

"Still it's early days, it's all still work in progress."

Flambaum says there's clear observational data contradicting the OPERA results.

He points to Supernova 1987A which exploded in a neighbouring galaxy in 1987 called the Large Magellanic cloud.

Flambaum says the OPERA team found the speed of the neutrinos to be just fractionally over the speed of light but the supernova observations showed that neutrinos and photons generated at the same time by the supernova, 160 thousand light years away, arrived here at the same time as well.

He points out that if the OPERA observations were correct, the neutrinos should have arrived here four years before the photons and that didn't happen.

"Outstanding statements require outstanding proof," he says.