Basic Space

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Faster-than-light neutrinos show science in action


It may look all serene from this vantage point, but underneath the Gran Sasso mountain is a hive of neutrino-detecting activity. Credit: {link url=""}Wikipedia/w:nl:Gebruiker:Idéfix{/link}

Unless you've been hiding under a rock for the past 24 hours, you've probably heard about the neutrinos that turned up at the Gran Sasso Laboratory in Italy a few nanoseconds earlier than they were supposed to, in a feat that would have required them to travel faster than the speed of light.

The story has been covered by many news outlets already, and, while some headlines may have raised a few eyebrows, most of the coverage has been suitably cautious. Heck, even the Daily Mail has generously thrown a few "ifs" and "mights" into their take on the findings.

The results were first announced, unceremoniously, in a tweet by Reuters Science last night. The tweet linked to a story on that describes how three years worth of measurements show that some neutrinos must have travelled faster than the speed of light.

The neutrinos in question undertook a journey from CERN in Geneva, through the Earth, and finally ended up at the Gran Sasso laboratory deep underneath the mountain of the same name in Italy. The neutrinos were produced by the Super Proton Synchrotron at CERN, along with a lot of other sub-atomic particles. The trick particle physicists use to get a beam of only neutrinos, and the reason the detector for the experiment is located so far away from the source of the beam, is to send the beam of particles off to travel underground for several miles. Neutrinos are the only particles that survive the journey, because they pass through matter completely unscathed whereas the others do not. The now pure neutrino beam takes less than 3 milliseconds to travel the 730km between CERN and Gran Sasso, and the neutrinos are detected by apparatus belonging to the OPERA experiment, consisting of around 150,000 bricks of photographic film interleaved with lead plates.

The neutrinos, says the paper released by the OPERA experiment after the news was first broken, reached the detector 60 nanoseconds before they would have done had they been travelling at the speed of light. The result amounts to a statistical significance of 6-sigma. "Sigma" is shorthand for standard deviation, a statistical tool that can be used to give an estimate of the certainty of a result. Generally, the higher the number of sigma, the more trustworthy the result. A minimum of 5-sigma, equivalent to a one in 1,744,278 chance that the result is a fluke, is normally required to claim a discovery. 6-sigma, equivalent to a one in 506,797,346 chance is even more convincing.

If any other particle physics result had been shown to have a statistical significance of 6-sigma, champagne corks would be popping in laboratories all over the world. But this one is different.

The universal, unwavering, cosmic speed limit — the speed of light — is the most fundamental of constants. Without it, we can wave goodbye to relativity as we know it. Causality — the fundamental relationship between cause and effect — would have to go. As Subir Sakar, head of particle theory at Oxford University put it to the Guardian yesterday: "Cause cannot come after effect and that is absolutely fundamental to our construction of the physical universe. If we do not have causality, we are buggered."

Aside from being inconsistent with both relativity and causality, the result doesn't appear consistent with the past behaviour of neutrinos either. I've written before about supernova 1987a, whose arrival was heralded by a burst of neutrinos. In this case, the neutrinos, travelling at the speed of light, reached Earth 3 hours before the light did. The light from the supernova was delayed because it had to get through the remnants of the stellar explosion, not because of any sneaky faster-than-light travel on behalf of the neutrinos. Dr Ben Still, a particle physicist working on the T2K neutrino experiment, blogged earlier today about the Opera result and calculated that if the neutrinos from supernova 1987a had exhibited the same odd behaviour as those that are under the spotlight today, they could have arrived 4.14 years before the light did. The neutrinos and light from supernova 1987a are well documented and this does not appear to have been the case.

Of course, there could be some explanation that is able to bring all of the so far seen problems together and solve them in one fell swoop. There could also be mistakes in the paper that have been missed by physicists working for the OPERA experiment. Time will tell.

For what it's worth, I'd be prepared to put money on Einstein winning this one, but that doesn't mean that the excitement and interest these results have generated was for nothing. While, arguably, the announcement of the result could have done with a little more careful planning, the OPERA experiment has now got what it needed: lots of pairs of eyes looking over the results, and checking for something everyone else so far might have missed. Looked at in the right way, this episode is the perfect insight into how science really works. Scientists test hypotheses and current theories (yes, even ones as seemingly solid as relativity) and pipe up when they see something out of the ordinary, to allow others in the field to double and triple check their analysis. This is what is happening now.

While scepticism is necessary in situations like this — I'm sure we're all aware of the famous Carl Sagan quote, "extraordinary claims require extraordinary evidence" — progress is not made by shouting down anything that does not fit within the current status quo. You never know, perhaps this result will be the one that topples relativity. (They probably didn't, but there's a chance, however slim, that those neutrinos did travel faster than light — and that's a very interesting prospect indeed).

In science, when something out of the ordinary appears, the next step is often to repeat the experiment and try to recreate the results. So, if someone could just lend me a particle accelerator, a mountain and a deep underground mine, I'll get to it...


Now you've had my two pence, there's plenty more speedy neutrino goodness to be had:

My SciAm blogs colleague Caleb Scharf has a post up with lots of links to more coverage on this story. For a great overview of the science I'd particularly recommend Ethan Siegel's blog post at Starts with a Bang.

At Discover blogs, the Bad Astronomer has his say and Sean Carroll at Cosmic Variance also has his take on the paper.

And finally,Randal Munroe at XKCD was extremely quick of the mark and had a neutrino-themed comic up and ready earlier today.


The OPERA Collaboraton (2011). Measurement of the neutrino velocity with the OPERA detector in the CNGS

beam . arXiv: 1109.4897v1

The views expressed are those of the author and are not necessarily those of Scientific American.

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