Neutrinos clocked at light-speed in new Icarus test

An experiment to repeat a test of the speed of subatomic particles known as neutrinos has found that they do not travel faster than light. Results announced in September suggested that neutrinos can exceed light speed, but were met with scepticism as that would upend Einstein's theory of relativity. A test run by a different group at the same laboratory has now clocked them travelling at precisely light speed. 

The results in September, from the Opera group at the Gran Sasso underground laboratory in Italy, shocked the world, threatening to upend a century of physics as well as relativity - which holds the speed of light to be the Universe's absolute speed limit. Now the Icarus group, based at the same laboratory, has weighed in again, having already cast some doubt on the original Opera claim.

Shortly after that claim, Nobel laureate Sheldon Glashow co-authored a Physical Review Letters paper that modelled how faster-than-light neutrinos would behave as they travelled. In November, the Icarus group showed in a paper that the neutrinos displayed no such behaviour. However, they have now supplemented that indirect result with a test just like that carried out by the Opera team. 

The Icarus experiment uses 600 tonnes - 430,000 litres - of liquid argon to detect the arrival of neutrinos sent through 730km of rock from the Cern laboratory in Switzerland. Since their November result, the Icarus team have adjusted their experiment to do a speed measurement. What was missing was information from Cern about the departure time of the neutrinos, which the team recently received to complete their analysis.

They found that the neutrinos do travel at the same speed as light, within a small error range. "We are completely compatible with the speed of light that we learn at school," said Sandro Centro, co-spokesman for the Icarus collaboration.

Dr Centro said that he was not surprised by the result. "In fact I was a little sceptical since the beginning," he said. "Now we are 100% sure that the speed of light is the speed of neutrinos."

Most recently, the Opera team conceded that their initial result may have been compromised by problems with their equipment.

Rumours have circulated since the Opera result was first announced that the team was not unified in its decision to announce their findings so quickly, and Dr Centro suggested that researchers outside the team were also suspicious. "I didn't trust the result right from the beginning - the way it was produced, the way it was managed," he said. "I think they were a little bit in a hurry to publish something that was astonishing, and at the end of the day it was a wrong measurement."

Four different experiments at Italy's Gran Sasso lab make use of the same beam of neutrinos from Cern. Later this month, they will all be undertaking independent measurements to finally put an end to speculation about neutrino speeds.The Minos experiment in the US and the T2K experiment in Japan may also weigh in on the matter in due course - if any doubt is left about the neutrinos' ability to beat the universal speed limit.