Key figures: OPERA collaboration (Gran Sasso National Laboratory, Italy); CERN (Geneva, Switzerland); Antonio Ereditato (OPERA spokesperson)
Summary
In September 2011, the OPERA collaboration announced that it had measured beams of neutrinos apparently travelling faster than the speed of light — a result that, if correct, would have overturned Albert Einstein’s special theory of relativity. The particles seemed to cover the 730-kilometre journey from CERN near Geneva to the Gran Sasso laboratory in Italy about 60 nanoseconds sooner than light would. The claim drew intense global attention and skepticism, and within months the anomaly was traced to instrumental errors — most consequentially a loose fibre-optic cable — rather than any new physics. The episode became a widely cited case study in scientific self-correction.
The Measurement
OPERA (Oscillation Project with Emulsion-tRacking Apparatus) was an experiment designed primarily to study neutrino oscillations, using a beam of muon neutrinos produced at CERN and fired through the Earth’s crust to a detector 1,400 metres underground at the Gran Sasso National Laboratory. Precisely measuring the beam’s time of flight required synchronising clocks at both ends via GPS and accurately determining the 730-kilometre baseline distance.
On September 23, 2011, the collaboration released a preprint reporting that the neutrinos arrived approximately 60 nanoseconds earlier than expected for light-speed travel — a discrepancy of roughly one part in 40,000, with a stated statistical significance of about six standard deviations. Aware that the result appeared to violate a cornerstone of modern physics, the OPERA team explicitly presented it not as a discovery but as an anomaly they could not explain, inviting independent scrutiny.
Skepticism and Scrutiny
The announcement was met with widespread caution from the physics community. A frequently raised objection was that observations of neutrinos from Supernova 1987A had arrived in step with the event’s light, implying that any faster-than-light effect, had it been real, should have produced a far larger timing difference over interstellar distances. Theorists also noted that superluminal charged byproducts should have radiated energy in ways not observed.
In November 2011, OPERA repeated the measurement with a redesigned, tightly bunched beam that reduced certain systematic uncertainties, and again reported the early arrival — deepening the puzzle without resolving it.
Resolution
In February 2012, OPERA identified two instrumental problems affecting the timing. The decisive one was a fibre-optic cable connecting an external GPS receiver to the experiment’s master clock that had not been fully screwed in. The loose connection altered the timing of the signal by about 60 nanoseconds — closely matching the anomalous “early” arrival — meaning the neutrinos had not, in fact, exceeded light speed. A second fault, an oscillator used to time-stamp events, worked in the opposite direction. Independent cross-checks, including a May 2012 measurement by the neighbouring ICARUS experiment, confirmed that neutrinos travel at essentially the speed of light, consistent with relativity.
The controversy carried an institutional cost. In late March 2012, amid internal disagreement over how the result had been handled, spokesperson Antonio Ereditato resigned.
Significance
The OPERA episode is frequently held up as an illustration of how experimental science polices itself. Rather than suppressing an implausible result, the collaboration published it transparently as an unexplained anomaly, and the broader community subjected it to intense theoretical and experimental scrutiny that exposed the underlying error within months. Special relativity emerged unchallenged.
The affair also became a widely discussed example of the tension between open scientific communication and media sensationalism: a carefully hedged request for scrutiny was widely reported as a claim to have “broken the speed of light.” Alongside 2011’s genuine milestones in physics — including the Nobel Prize for the accelerating expansion of the universe and the intensifying search for the Higgs boson at CERN’s Large Hadron Collider — the neutrino anomaly stands as the year’s most instructive scientific non-discovery.
Key Dates
| Date | Event |
|---|---|
| September 23, 2011 | OPERA releases a preprint reporting apparently faster-than-light neutrinos |
| November 2011 | A repeated measurement with a refined beam confirms the same anomaly |
| February 2012 | A loose fibre-optic cable and a faulty oscillator identified as sources of the timing error |
| March 2012 | Spokesperson Antonio Ereditato resigns amid the fallout |
| May 2012 | The ICARUS experiment confirms neutrinos travel at light speed |