Published online 5 October 2011 | Nature | doi:10.1038/news.2011.575

Corrected online:

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Did gravity mess with the clocks that measured particles breaking cosmic speed limit?

The OPERA experiment's stopwatch is coming under scrutiny. CERN

Less than two weeks after the revelation that ghostly particles called neutrinos had been spotted travelling faster than the speed of light, physicists are saying they have found flaws in the analysis that would stop the claim in its tracks.

The extraordinary result came from the OPERA experiment (Oscillation Project with Emulsion-tRacking Apparatus), situated 1,400 metres underground in the Gran Sasso National Laboratory in Italy (see 'Particles break light-speed limit'). There, scientists timed muon neutrinos arriving from CERN, Europe's particle-physics facility near Geneva, Switzerland, some 731 kilometres away.

They were astonished to find the neutrinos arriving 60 nanoseconds earlier than a light beam travelling through a vacuum would have done — breaking what physicists had thought was an immutable cosmic speed limit1.

Since the OPERA group's 22 September announcement, more than 30 papers attempting to explain the result using various exotic theoretical models have been posted to the physics preprint server at arXiv.org. But one paper2, posted on 28 September by theorist Carlo Contaldi of Imperial College London, bears the distinction of being the first to challenge the experimental calculations.

The OPERA team timed the neutrinos using clocks at each location that were synchronized using GPS (Global Positioning System) signals from a single satellite. Contaldi's paper says the group's calculations do not take into account one aspect of Albert Einstein's general theory of relativity: that slight differences in the force of gravity at the two sites would cause the clocks to tick at different rates.

Because of its location relative to the centre of Earth, the CERN site feels a slightly stronger gravitational pull than Gran Sasso. Consequently, a clock at the beginning of the neutrinos' journey would actually run at a slower rate than a clock at the end. "It would reduce the significance of the result," Contaldi says.

Dario Autiero of the Institute of Nuclear Physics in Lyons (IPNL), France, and physics coordinator for OPERA, counters that Contaldi's challenge is a result of a misunderstanding of how the clocks were synchronized. He says the group will be revising its paper to try to make its method clearer. Autiero notes that OPERA has been careful to present its startling observations without concluding that the laws of physics have been upended. His e-mail discussion with Contaldi — being followed by dozens of other physicists — is ongoing.

Going deep

Because two clocks are needed to time the neutrinos' journey — one at the beginning, and one at the end — they must be synchronized to within nanoseconds to get an accurate measurement, explains Toby Wiseman, a theoretical physicist also at Imperial College London. Measuring the speed of light on this journey would be much easier, because the beam can be reflected back to its origin, and the round trip timed with just one clock. "Whether they have or haven't synchronized their clocks correctly is the crucial question," says Wiseman.

Contaldi admits that his original analysis posted at arXiv wrongly assumed that OPERA's timings relied on a clock being moved from one end of the beam to the other. But even synchronizing the clocks using GPS does not remove the difference in the time dilation effect, which Contaldi says could amount to tens of nanoseconds.

That effect would reduce the statistical significance — which OPERA claimed was six standard deviations — of the group's result (five is enough to count as strong evidence in the field of particle physics). Contaldi says the additional error would reduce that number to two or three standard deviations, enough to make only a tentative claim of a faster-than-light effect.

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Wiseman says that the difficulty of the experiment and a lack of detail about clock synchronization in the initial OPERA paper may explain why so few critiques of the experiment methodology have been published so far, although more are probably on the way. "Carlo is pointing out how difficult it is to critique what has been done unless you're in the collaboration," he says.

Contaldi already has some company. On 2 October, Gilles Henri of the Institute of Planetary Science and Astrophysics in Grenoble, France, posted his own critique3. He argues that fluctuations in the beam of neutrinos could change the probability that Gran Sasso would detect them, and increase the uncertainty in their time of flight enough to produce the faster-than-light result. OPERA did not immediately respond to Nature 's request for comment on that paper.

Corrected: This article originally stated that Earth's gravitational pull at CERN is smaller than at Gran Sasso. It is, in fact, slightly greater. The text has been amended to reflect that fact.