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 have been posted online.
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 posted on the online server Arxiv 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.
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I didn’t trust the result right from the beginning”
Sandro Centro Icarus collaboration
What was missing was information from Cern about the departure time of the neutrinos, which the team recently received to complete their analysis.
The result: they find that the neutrinos do travel at the same speed as light, within a small error range.