In 2011, in the context of the OPERA experiment, after collecting observational evidence for the speed of tau neutrinos, scientists came to conclude that there indeed could be found a “faster-than-light” neutrino. A neutrino is a subatomic particle that is extremely similar to an electron, but that has no electric charge as well as a tiny mass. As such, even though tens of thousands of neutrinos cross our hand every second, they interact very little with matter and thus, neutrinos are incredibly hard to detect. These particles permeate the universe and are to be found everywhere in space and around us.

OPERA Experiment

What the OPERA experiment was exactly, was a collaboration between CERN and the Laboratori Nazionali del Gran Sasso at Gran Sasso (LNGS) in Italy. OPERA stands for Oscillation Project with Emulsion-tRacking Apparatus and was the instrument that the scientists used to pursue their aim: detect tau neutrinos the last of the leptons (third generation) in the standard model whose existence was implied after the discovery of the tau particles (similar to electrons).

A Quantum Experiment That Once Reported Particles Travelling Faster Than Light. 2
The standard model of particle physics

These were to be created by muon neutrino (second generation of leptons) oscillations produced by the decay of pions and kaons which would be generated by firing proton pulses at a carbon target thanks to the Super Proton Synchrotron at CERN. Then was used the CERN neutrinos to Gran Sasso neutrino beam, firing neutrinos from Geneva to Gran Sasso where the OPERA apparatus was located.

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Origins of the Discrepancy

These results were immediately considered anomalous because of the direct violation of special relativity. Although neutrino speeds were expected to be approaching the speed of light, estimated to be almost ultra-relativistic, their mass nonetheless remains nonzero and as such, is bound by the upper limit of the speed of light. Therefore, such results would be impossible or Earth-shattering towards the established frame of reference governing physics.

OPERA Experiment

The team was using common-view GPS, an apparatus derived from the standard everyday GPS to measure the time and position of the respective creation and then detection of a said neutrino. After computation, the average time of flight between CERN and Gran Sasso would manifestly less than the time light would take to travel from one laboratory to the other in a vacuum. Scientists were facing a flagrant anomaly.

Nonetheless, after six months of pondering the results, the following contradicting result was published: neutrinos were calculated to have traveled the same distance as light in about 60.7 nanoseconds less. Scientists from the OPERA experiment had not necessarily published these results by certainty but rather by lack of contradicting evidence, inviting the wider scientific community to repeat the experiment and challenge the results.

The flaw of these results was later reported by team members as being two equipment set-up errors. The first one was a fibre optic cable which was improperly attached and the second one, a clock oscillator clicking too fast.

Reactions Within the Physics Community

Although the results published by the OPERA experiment were received with stupor, most physicists around the globe adopted an open-minded attitude of “wait and see”, accounting for the possibility of errors as well as the innate complexity of the experimental setup. Nonetheless, most remained skeptical but at the time no had been able to prove the results of the OPERA experiment otherwise. 

Further Experiments

In order to attempt to provide an explanation for these abnormal results, in March 2012, the ICARUS experiment, operating on the same facilities as the OPERA experiment, reported velocities constant with the relativistic frame of reference, later confirmed by LVD, BOREXINO (other experiments), and OPERA itself.

A Quantum Experiment That Once Reported Particles Travelling Faster Than Light. 3
Artwork by Sandbox Studio, Chicago with Ana Kova

Finally, on the 8th of June 2012, after the MINOS experiment at Fermilab confirmed the results held by many others, the director of research at CERN, Sergio Bertolucci, withdrew the initial claims made by the OPERA experiment and stated that indeed, the speed of neutrinos was consistent with that of light. The research paper was eventually updated, and the sources of error were explained and accounted for the last adjustments: the speed of neutrinos agreed with our current understanding of physics.

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