Was there an error in the CERN team's timing of faster-than-light neutrinos?

By: Wesley Fenlon

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In September 2011, scientists at CERN, the European Organization for Nuclear Research, discovered something bewildering. They clocked a group of neutrinos fired from a particle accelerator traveling a little bit too fast. The neutrinos arrived at their destination, 450 miles (724 kilometers) away, 60 nanoseconds sooner than they should have. Sixty nanoseconds doesn't sound like much, but it's a big deal in this case -- the neutrinos should have been traveling at the speed of light.

And you know what travels faster than the speed of light? Nothing. Or so we think. That's been a cornerstone of physics since 1905, when Albert Einstein wrote the special theory of relativity. If those neutrinos were actually capable of exceeding the speed of light, well, the special theory of relativity would go out the window, and we would have to re-examine the laws of physics.


The scientists released their findings and encouraged the scientific community to scrutinize, challenge and prove or disprove the results. In February 2012, they figured out what went wrong: The GPS system used to measure the speed of the particles wasn't at the top of its game, and was malfunctioning. A loose fiber optic cable caused the measurement error, but the process CERN and the scientific community went through to verify its research is more interesting than the anticlimactic ending of a loose connection.

Carrying Out the Faster-than-light Neutrino Test

CERN's experiment accelerated neutrinos to the speed of light and fired them from Geneva to a detector at Italy's Gran Sasso National Laboratory, located 450 miles (724 kilometers) away. According to the project Web site, the goal of OPERA -- Oscillation Project with Emulsion-tRacking Apparatus -- was to detect the appearance of tau neutrinos from the oscillation of muon neutrinos. When the accelerated particles seemingly traveled faster than the speed of light, the OPERA project received a great deal of attention in the media and scientific community.

Most scientists were appropriately skeptical. After all, if these neutrinos were actually traveling faster than the speed of light, a principle theory of physics would no longer be reliable. According to a CERN press release from September 2011, the OPERA scientists spent several months cross-checking their experiments and found no instrumental anomalies that could explain the results.


Using GPS systems and atomic clocks, the OPERA project could determine the speed of its neutrinos down to an accuracy of 10 nanoseconds or less. The science community naturally decided to perform their own tests. The February 2012 meeting of the AAAS, or the American Association for the Advancement of Science, indicated that five similar experiments were gearing up to test out OPERA's claims, but they weren't completely necessary: On Feb. 23, 2012, CERN announced that two elements of their system could've caused the faster-than-light readings.

Diagnosing the False Faster-than-light Reading

Considering the size and number of highly-specialized components that make up CERN's research arsenal, it's not really surprising that it would be difficult to identify a malfunctioning fiber optics circuit.
Considering the size and number of highly-specialized components that make up CERN's research arsenal, it's not really surprising that it would be difficult to identify a malfunctioning fiber optics circuit.
©Luis Davilla/Getty Images

Despite poring over their results for months before publishing the faster-than-light readings of the OPERA project in September 2011, the CERN scientists missed the cause of the inaccurate readings. They kept working to verify their experiment, and in November announced that a new test confirmed the accuracy of its timing measurement. The results still showed faster-than-light travel.

They didn't hit on the problem until February 2012, when OPERA discovered two potential issues with its measurements. An oscillator used to provide GPS time stamps could have been overestimating the neutrino travel time. If that was the issue, it could mean the neutrinos were actually traveling even faster. The other potential issue meant the opposite: An optical cable bringing GPS signals to the OPERA master clock may have been malfunctioning, which would cause the system to underestimate the neutrino travel time.


With the optical cable fixed, the 60 nanosecond advantage over light speed disappeared, and all subsequent neutrino tests at CERN have matched up with the expected speed of light. Mistakes happen all the time in the scientific community, and they're treated exactly as this one was: Testing, re-testing and external studies seek to confirm a result. Faster-than-light travel just happened to be a big enough deal to attract a whole lot of attention.

Author's Note

The Large Hadron Collider is so cool, it's easy to forget CERN is a huge scientific organization undertaking all sorts of experiments that don't involve slamming atoms together! The faster-than-light neutrino experiment is a great example: The results got tons of publicity because you don't need a brain for physics to understand nothing travels faster than the speed of light. You also don't need a mind for physics to start dreaming up the sci-fi potential of faster-than-light travel! I almost wish OPERA's findings had borne out. How cool would that have been?

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