OPERA reports particles arriving about 60 nanoseconds early over a 730-kilometre journey. The apparent challenge to physics turns out to be a problem with the measurement.
The universe’s speed limit appeared to have a measurement problem.
On Friday, 23 September 2011, the OPERA collaboration presented a startling result at CERN: its neutrinos appeared to have travelled faster than light.
The experiment studied a beam sent from CERN to the Gran Sasso laboratory in Italy, approximately 730 kilometres away. Neutrinos pass through matter so readily that the journey required no tunnel connecting the laboratories.
The original analysis used more than 15,000 detected events. Compared with the expected journey at the speed of light, the particles appeared to arrive roughly 60 nanoseconds early—60 billionths of a second.
Small discrepancy. Enormous implications.
Special relativity does not permit massive particles to accelerate through the speed of light in a vacuum. If this result survived scrutiny, the explanation would require more than adjusting a timetable.
The researchers did not announce that Einstein had been defeated. Having failed to identify the cause after extensive checks, they invited independent investigation.
Reporting the anomaly was part of the scientific process, not an exemption from it.
In November, a test using shorter beam pulses reproduced the apparent anomaly. That eliminated one possible explanation; it did not establish that every component of the timing system was correct.
By February 2012, attention had turned to two equipment effects: an oscillator associated with GPS timestamps and the optical-fibre connector carrying the external GPS signal to OPERA’s master clock.
They could distort the measured journey time in opposite directions. The oscillator could make it too long; the faulty connection could make it too short.
Neither required the universe to change its rules.
Independent measurements followed. In June, results from Borexino, ICARUS, LVD and OPERA were consistent with the established speed-of-light limit. CERN attributed the original anomalous measurement to a faulty element of the fibre-optic timing system.
The particles had not supplied a verified exception to relativity. The investigation had supplied a memorable demonstration of why extraordinary measurements need independent checks.
Before rewriting the universe’s speed limit, check the equipment keeping time.
A tiny timing discrepancy appeared to challenge a fundamental limit of physics. Scrutiny brought the explanation back from the laws of the universe to the experiment’s timing equipment.
The measurement needed correcting. The universe did not.
Evidence status
Sources
Original announcement and correction record
CERN — OPERA announcement · 23 September 2011 · Updates through 8 June 2012
Original language: EnglishContemporary Italian institutional announcement
INFN — OPERA reports anomalous neutrino flight time · 23 September 2011
Original language: Italian