EVER since scientists in Italy announced in September that they appear to be seeing particles called neutrinos travelling faster than light people have been trying to poke holes in their findings. Perhaps none more so than the boffins from the OPERA collaboration responsible for the furore. On February 23rd they fessed up to unearthing two potential sources of experimental error. Both have to do with the Global Positioning System (GPS) signals used to synchronise atomic clocks at either end of OPERA's neutrino beam.

The first would lead to an underestimate of the time it took neutrinos overtook light on their journey. If confirmed, then, it would show the neutrinos' antics to have been an illusion, after all. This would please the many physicists understandably reluctant to part with Albert Einstein's theory of relativity, which forbids such behaviour (at least for particles that, like neutrinos, have mass). The second discovered flaw, though, might have had the opposite effect. It would, in other words, reinforce OPERA's original claim.

The Einstein-saving niggle concerns the optical-fibre connector that brings the GPS signal to the OPERA master clock. This, the OPERA team now says, may not have been functioning properly when the measurements were taken. According to an anonymous leak published on the website of Science magazine, after tightening the connection and then measuring the time it takes data to travel the length of the fibre, the researchers found that the data arrive 60 nanoseconds earlier than assumed. That is precisely the time by which neutrinos appeared to have overtaken light on their trip from CERN, Europe's main particle-physics laboratory near Geneva, 730km through the Earth's crust to a detector underneath the mountain of Gran Sasso in the Apennines.

The error which would prop up OPERA's result has to do with a device called an oscillator, used to provide the time stamps needed to synchronise the clocks at CERN and Gran Sasso. Both bits of kit will now come under close scrutiny.

As we have written previously, the team has already eliminated one potential source of error. Originally, the bunches of neutrinos were several orders of magnitude longer than 60 nanoseconds. This meant that if a neutrino struck the OPERA detector and was thought to come from the tail of the proton beam, but actually came from its head, it would not be travelling faster than light even though the measurement would suggest that it was. So the OPERA researchers tweaked their beam in such a way that it consisted of shorter bunches, just three nanoseconds long, or 20 times shorter than the apparent discrepancy. The intervals between bunches, meanwhile, were longer, allowing for a more precise measurement of speed.

To check whether the errors did, indeed, creep in, OPERA plans to perform further tests with short particle pulses in May. Meanwhile, scientists at the MINOS experiment in Fermilab, America's main particle-physics facility outside Chicago, are attempting to replicate OPERA's claim, with results expected later this year. The saga of supraluminal neutrinos isn't over yet.