Nov 182011
Albert Einstein

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Now, relativity is looking even shakier than before

IN SEPTEMBER a furore erupted among physicists after it emerged that neutrinos—diaphanous particles which pervade the universe but rarely interact with anything—appear to be travelling faster than light. Since neutrinos are thought to have mass, and since Albert Einstein’s special theory of relativity posits that accelerating any non-zero mass to the speed of light requires infinite energy, this implied that Einstein was not quite right. Either that, or the researchers who sent their neutrinos from CERN (Europe’s, and the world’s, main particle-physics facility, outside Geneva) 730km through the Earth’s crust, to a huge detector sitting under the mountain of Gran Sasso in Italy’s Apennines, made a mistake.

Now, relativity is looking even shakier than before. The Italian Institute for Nuclear Physics (INFN), which runs the Gran Sasso lab, has just confirmed the earlier result based on a re-examination of the old data. Crucially, it also replicated the finding with an all-new batch of neutrinos. The paper outlining the latest research has been posted on arXiv, an online database, and submitted to the Journal of High Energy Physics.

By tweaking the CERN neutrino beam, the OPERA collaboration behind the experiment has been able to remove one possible source of error raised by physicists after the initial announcement. The old beam was created by smashing long bunches of protons into a target to produce neutrinos, which would then travel on to Gran Sasso. The problem was that these bunches were serveral orders of magnitude longer than the 60 nanoseconds by which the neutrinos overtook light on their journey. In other words, if a neutrino struck the OPERA detector and was thought to come from the head of the proton beam, but actually came from its tail, it would not be travelling faster than light even though the measurement would suggest that it was.

The new beam consisted of shorter bunches, just three nanoseconds long, ie, 20 times shorter than the apparent discrepancy. The intervals between bunches, meanwhile, are longer, allowing for a more precise measurement of speed. (Since the new beam was less intense than the original one, that came at the expense of the number of recorded events—just 20, compared with a whopping 15,000 in the earlier analysis.)

The latest result is consistent with the one presented in September, but both could still be wrong. Precision measurements of this sort are fraught with difficulty and other sources of error may have crept in. For instance, when a neutrino strikes the detector in Gran Sasso, that signal has to travel 8km through a waveguide before a timing measurement can be made. The properties of the waveguide were last measured three years ago. Any changes to those properties—caused by ageing of the plastic of which it is made, say—could lead to an overestimate of the time it took the signal to run the length of it, and thus to an underestimate of neutrinos’ actual time of flight.

The INFN freely admits that many niggles remain and urges physicists to replicate OPERA’s result using different experiments. One, called MINOS and based at Fermilab (America’s answer to CERN, near Chicago) is trying to do precisely that. After OPERA published its result in September, MINOS, which had observed a similar, albeit less statistically significant effect in 2007 but dismissed it as probably a mistake, redoubled its efforts.

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