Neutrinos have become a buzzword this week since the OPERA collaboration released a pre-print last Thursday suggesting some neutrinos had been found travelling faster than the speed of light. If the OPERA collaboration results are confirmed, this means the beginning of a new era in our understanding of the cosmos, no less.
So even though I work only a few kilometres from CERN, I decided to catch the OPERA team's seminar at CERN on the webcast along with 15000 other enthusiasts, since events like this are bound to be full. For the event CERN prepared three different webcasts and so I watched, along with the packed main auditorium, Dario Autiero present the results for the OPERA team on Friday afternoon.
Neutrinos are ultra-light neutral particles that hardly interact with anything, meaning they can travel long distances before bumping into something, like a detector. The OPERA experiment uses a beam of neutrinos created at CERN in Geneva and detects them 730km away at Gran Sasso in Italy 2.4 milliseconds later. This fact alone should be enough to get you excited about particle physics.
Neutrinos come in different types and naturally switch or "oscillate" from one to the other as they travel long distances. The original aim of the OPERA experiment was to detect these neutrino oscillations. The experiment also allowed a measurement of the speed at which the neutrinos were travelling, but this was not at all the original goal. To their great surprise, the team found that the neutrinos were travelling faster than they should have. In fact, faster than the speed of light, making them "superluminal".
Their measurements were so incredible - and carried so many implications about theoretical physics - that the team spent the last three years re-doing their calculations and calibrations in search of some mistake.
Two main issues are that to measure a speed, you need to measure the precise distance travelled, as well as the exact time it took to travel that distance. This is no easy feat, and this is where the largest possible blunders can lie.
Even if you measure the distance properly once, it is possible that the earth dynamics (think of continental drift or earthquakes) can distort the path somewhat and falsify your results over time. But the team's results showed that whether the data was taken in 2009, 2010 or 2011, whether it was taken during the day or the night, during summer or autumn, it always showed this: the neutrinos travelling from Geneva to Gran Sasso arrived too early by 60 nanoseconds. That's 0.00248% faster than the speed of light.
As amazing as this sounds, you should still temper your enthusiasm. The hour-long seminar took us through a long list of possible problems in the distance measurement, and more so in the time measurement. In fact, Dario Autiero seemed almost apologetic when he announced the final results, as if, in spite of the team's best efforts, they just couldn't get rid of their fluke measurements.
His conclusion was to speak with "words of caution" and not to attempt any "theoretical or phenomenological interpretation of the results". In fact, as the Q&A session started, the chair explicitly asked that questions be related to the possible errors in the time and distance measurements, rather than talk about the reason everyone was here: was this a major discovery in physics? Detractors were promptly reminded that this was a technical seminar about calibration issues, not about wormholes.
The particle physics community will be working hard to reproduce or disprove these results in the next few years, and the world is watching. And though we all want to witness a scientific revolution, it is overly optimistic to start re-writing physics textbooks.
Note: The exact results of the OPERA team show that the neutrinos arrived at Gran Sasso early by 60ns ± 6.9ns (stat.) ± 7.4ns (sys.), where the first error bar corresponds to statistical errors, and the second to systematics. The main challenge is to ensure the systematic errors are correctly accounted for.