This is what one of my professors told us way back then at the end of a very serious lecture on relativity. And rumors have certainly been going fast and wild at CERN this past week. The news of the possible spotting of particles travelling faster than the speed of light is just as exciting as it is unexpected. This could be the most earth-shattering discovery we’ve had in decades. Hard to get any wilder than this! The whole question now is to determine if this is really the case.
I remember being a teenager and hearing about tachyons, these hypothetical particles that could travel faster than the speed of light. They were supposed to have a speed at rest not of zero, but equal to the speed of light. Not only that, but they would lose mass (that is energy) as their speed increased! I remember the shiver I got then. I got it again when I heard about this measurement earlier this week. So here are more details.
The OPERA experiment is located in an underground laboratory under the Gran Sasso mountain in central Italy 730 km away from CERN. Neutrinos produced at CERN travel through the Earth’s crust to reach the OPERA detector. That’s what so great and so hard with neutrinos: they can zip through matter nearly unaffected, making them also terribly difficult to detect.
OPERA was built mostly to detect the appearance of tau neutrinos from a beam of muon neutrinos, a phenomenon called “oscillation” that enables one type of neutrinos to mutate into a completely different type when travelling over a large distance. This is something that had never been observed directly before but OPERA spotted one such event last year. To do so, they need to correlate the appearance of tau neutrinos in their detector with the arrival of muon neutrinos sent from CERN. Obviously, getting the timing right was essential, hence their careful checks on the arrival time of the muon neutrinos in Gran Sasso.
And there, surprise! The neutrinos reached the Gran Sasso laboratory 60 nanoseconds (i.e. 60 billionth of a second) faster than light travelling over the same distance, even though neutrinos are expected to travel slightly below the speed of light.
Both CERN and OPERA upgraded their equipment to use the most sophisticated timing devices and the same GPS satellite to synchronize their atomic clocks to within one nanosecond. The timing equipment was calibrated by the Swiss Metrology Institute and independently verified by the German Metrology Institute PTB (Physikalisch-Technische Bundesanstalt).
The distance between the point of emission at CERN and the point of detection in Gran Sasso was determined by CERN surveyors to be 731278.0 ± 0.2 meters, that is a 20 cm uncertainty over a distance of 731 km! All experimental errors amount to ten nanoseconds, six times smaller than the effect observed, a clear evidence if (and only if…) nothing has been omitted. This is precisely what further scrutiny from the scientific community at large and other experiments will try to determine in the near future.
So after months of very thorough crosschecks, the OPERA collaboration is finally going public with it. They are looking for independent measurements to refute or confirm what they observe. Anything is possible given the complexity of the measurement: a technical flaw, a small calibration problem, an experimental bias. Nothing can be neglected.
But if it turns out to be real, much work will remain to get the proper theoretical interpretation and its implications on relativity. We will still have to figure out if these particles are travelling faster than light or if they are just sneaky little neutrinos taking a shortcut through some extra dimension…
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