Can general relativity explain the OPERA neutrino result?

Imperial College physicist looks at gravitational time effects

CERN’s decision to release data about its “superluminal neutrino” experiments at an early stage is providing the world with a rare insight into the process of scientific peer review. Another small step in that process in relation to the fascinating OPERA results asks whether general relativity can be called in to help explain the results.

The CERN experiment appeared to measure neutrinos moving faster than the speed of light between the LHC and a receiving station in Italy. That result would define the limit of Einstein’s special relativity, since that theory considers lightspeed to be the cosmological constant, the same everywhere (in our universe at least) and unbreakable.

This letter, posted at, examines possible errors in the experimental setup. In particular, since the transmitting and receiving stations are in different places, it calls for an analysis of the gravitational effects on time synchronization in those two stations. The way gravity affects time is part of the Einsteinian heritage expounded in his General Theory of Relativity.

Author Carlo Contaldi, a reader in Theoretical Physics at Imperial College, London, is particularly interested in how the OPERA setup accounts for correcting GPS timing to provide a universal time coordinate (UTC) that’s the same for CERN and Gran Sasso, where the neutrinos originated and were detected, respectively.

In most experiments measuring the speed of light, a round-trip design is used: since the transmitting and receiving stations are in the same place, there’s no risk that relativistic time differences (such as a marginally different experience of gravity in two places) could distort the results.

Since GPS clocks are only managed to 100 nanosecond accuracy, the CERN experiment needed to devise some way to increase the accuracy threshold. “the OPERA experiment employed a travelling Time-Transfer Device (TTD) to calibrate the difference in time signals at each receiver. We assume this device to be a transportable atomic clock of sufficient accuracy [15]. The TTD constitutes a classic moving clock synchronisation conundrum in relativity,” the letter states.

He notes that the experimental setup introduces three relativistic time distortions that need to be corrected in analyzing the apparent time-of-flight of the neutrinos: time dilation resulting from “moving the TTD through a non-uniform gravitational potential”; a “Doppler-type effect” resulting from the TTD’s velocity with respect to Earth’s “rotating frame of reference”; and finally, errors due to “the rotation of the Earth as the TTD travels to its destination”.

The most important of these, Contaldi writes, is the first – the effect of non-uniformity of gravity on the TTD. Since “the time differences the result hinges on are extremely small”, even trivial details such as whether the TTD was transported by car or by air could potentially change the synchronization between the two ends of the experiment.

It’s not a “debunking” of the OPERA experiment – although The Register fully expects simplistic headlines to that effect to start appearing quickly – but rather a public window into what’s usually not a public process.

Usually, peer-review looks opaque to non-scientists. The general public often learns of a research result after a paper has been accepted by a journal – and therefore after the peer-review process is completed (and most often, only because the journal decides to throw some bones at the general media).

As examination and analysis of the OPERA experiment proceeds, however, the public is getting a fly-on-the-wall view of peer-review at work. Condaldi may be right or wrong; OPERA may survive this examination, but fall at some other hurdle; a new physics might emerge, or not.

Whatever the result, giving the public a ringside seat as academics rake over the OPERA results is already looking like a win for science. By the time OPERA is either settled or falsified, we’ll have had our most detailed demonstration of why science works. ®

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