For keeping time, CERN uses a timing system based on synchronization by four nearest GPS satellites. CERN has something called a common view’ GPS, which synchronizes the clocks at CERN and at OPERA. Furthermore, CERN uses atomic clocks to provide a time-stamp. Since OPERA is sitting at a higher altitude than Gran Sasso (the lab to where the neutrino was beamed), you’ll have to correct for gravitational effects. Clocks run slower in a weak gravitational field than in a stronger one, according to General Relativity (GR). As the gravitational field strengths differ, the clocks lose synchronization. These clocks are regularly re-synchronized using GPS to take into account General Relativity effects.
Accuracy of measurement
They’ve measured a distance of 732 km with an accuracy of 20 cm! They’ve measured time to within 5 ns accuracy. The whole of the apparatus might contribute an error of 7.4 ns to the time of flight, well below the 60 ns discrepancy.
Why am I really going into all these gory details? It’s just to convey the impression that this is not really another crackpot claim you keep hearing about. This is really a well-done experiment. They’ve sincerely worked out all details and tried to plug in as many loopholes as possible. The effect still stays.
One might ask the following questions: what about effects external to the whole apparatus, like tides (which slow down the Earth’s rotation, albeit by a tiny amount) or atmospheric fluctuations (which would change the GPS signal speed through air, introducing an error in the time)? It is in answering these questions that one gets to know how far the CERN scientists went before publishing these results. The experimental analysis was done for 3 whole years, using as many as 16111 events! By the law of statistics, this will reduce the error of any quantity by more than 126! (The error in a quantity is suppressed by the square root of the number of statistical points. Square root of 16111 is just above 126!) Furthermore, tidal effects are periodic and would cancel out over three year periods. The same goes for atmospheric fluctuations.
The scientists even took into account tectonic shifts (graph above). They factored in the effect of the 2009 giant earthquake and tsunami, taking into account the fact that the tectonic plates had shifted by 3 cm! No wonder, Nobel Laureate Sam Ting called the experiment beautifully done’.