Faster-than-light neutrinos mean Einstein is wrong! At least, that’s what some popular press articles have suggested since researchers with the OPERA experiment in Italy presented evidence of neutrinos arriving 60 nanoseconds earlier than thought possible.

But scientists, quite intrigued by the anomalous results, have since been busy generating more measured responses. In the three weeks after the announcement, more than 80 explanations have been posted to the preprint server arxiv. While some suggest the possibility of new physics, such as neutrinos that are traveling through extra dimensions or neutrinos at particular energies traveling faster than light, many offer less revolutionary explanations for the OPERA experiment.

One of the earliest objections to the faster-than-light interpretation came from an astrophysical observation. In 1987, a powerful supernova showered Earth with light and neutrinos. While neutrino detectors observed neutrinos arriving about three hours before the light, this was due to the lightweight particles getting a head start. Neutrinos, which hardly interact with matter, escaped the exploding stellar core with relative ease while photons, absorbed and re-emitted by the various elements, took longer to flee. If the effect from OPERA were as large as observed, scientists have calculated that the neutrinos should have arrived more than four years in advance of the light.

Other scientists have taken the faster-than-light results to task using the Standard Model of physics, which describes all known subatomic particles and their interactions. According to the Standard Model, neutrinos at sufficiently high energies should produce a virtual electron-positron pair through a process known at Cohen-Glashow emission. As explained in a paper by Nobel laureate Sheldon Glashow and his colleagues, these emanations would have sapped energy from the faster-than-light neutrinos, causing them to slow down.

Theoretical physicist Matt Strassler also noted on his blog that the Standard Model's properties suggest that making neutrinos go faster than light requires electrons to do the same. But if electron neutrinos moved at the speed suggested by the OPERA experiment, then electrons should also travel faster than the speed of light by at least one part in 1,000,000,000, or one billionth. Experiments have established theoretical limits that electrons remain subluminal at a precision down to more than 5 part in a thousand trillion, effectively ruling this scenario out.

Among the most recent ideas is a paper invoking Einstein's supposedly challenged theory of relativity. The OPERA team used GPS satellites to accurately measure the 730-km distance between their detector and the CERN beam where the neutrinos were produced. Yet, according to special relativity, calculations will be slightly different when two observers are moving relative to one another.

Since the satellites were zipping around the Earth, the positions of the neutrino source and the detector changed. According to the paper, the movement would account for a 64 nanoseconds discrepancy, nearly exactly what the OPERA team observes.

Ultimately, it will take a great deal more time and scholarship before the physics community settles on the true explanation for the OPERA results. Until then, vigorous debate is likely to continue.

Image: OPERA experiment

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