60.7 nanoseconds, a novel by Gianfranco D'Anna
It all began when Luisa Giacometti, a young researcher just turned twenty-five, intent on her doctoral work at the Gran Sasso National Laboratory, saw the unwelcome figure “+60.7 ns” flash up on her computer screen.
“It’s not possible”, she told herself, as she fed the data into the computer for the third time and the processor, making the calculation with lightning speed, again came up with the answer “+60.7 ns”. She seemed to hear its synthetic voice saying: “Plus sixty point seven nanoseconds”. There must be an error somewhere.
Her job was to use the data gathered by the OPERA particle detector to determine the velocity of the neutrino, an as yet little-known quantity. In practice, Luisa’s task was to measure the time taken by neutrinos produced at CERN in Geneva to travel the distance from CERN to the Gran Sasso, not far from Rome, and compare it with the time that light would take to cover the same distance. In this way, she could obtain the so-called “difference in time of flight between light and neutrino” and establish which of the two arrived first.
This might seem a simple task, but technically it was anything but. When a neutrino arrived, the OPERA detector measured the exact time of the event to the nanosecond. What was known at the other end was the departure time of long bursts of neutrino-projectiles, or “packets”, separated by periods of “silence”: the departure time of the neutrino in question was therefore approximate. However, it was possible to deduce, by statistical calculation, an average time of flight, its degree of uncertainty diminishing as the number of events observed increased. For this reason, Luisa’s task was to study all the thousands of events that had occurred in the last three years, sixteen thousand in all; hardly a cushy job.
Nor was it simply a matter of using the raw data. For each and every neutrino, she had to calculate the specific flight distance, i.e. the 730,534.61 metres (roughly 730 kilometres) from CERN to the reference point on the eastern side of OPERA, plus the distance to the exact position of the collision that had taken place in the detector, which was more than 20 metres in length. In addition, she had to estimate with precision a number of corrective parameters for events occurring during the first three months, during the next seven months, and so on, due to changes in the equipment between the time when they had begun to gather data, in mid-2008, and the present. Finally, all this work yielded only an approximate difference in time of flight, because various delays in the measurement line still needed to be subtracted, such as the time required for the signals to pass through the optical fibres and other connecting cables, values accurately calibrated beforehand and kept in the archives.
Hers, then, was a task demanding the patience of Job: she had to catalogue and order the data for all the neutrinos observed since the start of the OPERA experiment, carefully correlate the dates on which observations were made with the correct experimental parameters, and so on. After months of this painstaking work, it all ended with that brief moment in which the computer performed its calculation, the few seconds in which it produced the result.
Imagining she had made a mistake was the most obvious response and, when the electronic brain flashed up its “+60.7 ns”, she began the entire process again from scratch. Because the result must, must be zero. Neutrinos travel at the speed of light, according to the theory: full stop. It was impossible that there should be a difference in the flight time. Of course, if she had obtained a negative difference, a delay, indicating that neutrinos travelled slower than the speed of light, some rational explanation might be found. Some theoretical scientists had said that they would not be surprised. But a positive difference, an advance, like “+60.7 ns”, was just not possible and Luisa, after yet again checking the log books, the information sheets, the reports, the tables, the data fed into the computer, decided to ask her line manager for help.