Tuesday, 11 October 2011

xkcd on neutrinos

The Faster than Light Fiasco

If you haven’t been marooned on a desert island over the past few weeks since 23 September, you will have no doubt heard the media reports of neutrinos breaking the speed of light.
This has got everyone quite excited, as it would mean that Einstein’s theory of relativity is wrong, and therefore a large part of physics has been brought into question.

The European Organization for Nuclear Research (CERN) sent out a press release on 23 September about the OPERA1 experiment, which observes a neutrino beam from CERN2 730 km away at Italy’s INFN Gran Sasso Laboratory. They stated:

“The OPERA result is based on the observation of over 15000 neutrino events measured at Gran Sasso, and appears to indicate that the neutrinos travel at a velocity 20 parts per million above the speed of light, nature’s cosmic speed limit. Given the potential far-reaching consequences of such a result, independent measurements are needed before the effect can either be refuted or firmly established. This is why the OPERA collaboration has decided to open the result to broader scrutiny.”

Given the wording, it is a bit disappointing that the media has seized on it in the way it has. But CERN is probably equally to blame for sending out a press release when it is almost certain that the results are wrong. At no time have the researchers stated that neutrinos move faster than light. All they have said is that their measurement shows a reading greater than the speed of light, and further confirmation is required.

Although there are various assurances regarding the accuracy of the measurement, a measurement error or systematic effect still seems a likely explanation. As neutrino experiments are well known for being very hard to perform, the chances of a measurement error would appear to be far more likely than relativity theory being wrong. A claim of such extraordinary magnitude requires extraordinary evidence.

However, theoretical physicist Heinrich Päs of the Technical University Dortmund in Germany says it is possible that spacetime could be warped in such a way that allows neutrinos to take a shortcut without actually travelling faster than the speed of light. He goes on to state that, “if something moves faster than the speed of light, causality violations—aka time travel—may be a possibility”.

Another theory offered relates to quantum tunnelling after a neutrino’s properties change. If neutrinos that did not change flavour travel at the speed of light, and the neutrinos that did change flavour arrived an extremely small amount faster, then tunnelling may have occurred. If the distance is the Planck length or less, then perhaps the new flavour of neutrino left the Planck length area of the transition at the same time the old flavour of neutrino arrived. A virtual particle allows the neutrino to jump a Planck length. This apparently happens all the time when electrons tunnel through semiconductors (like the ones in your computer).
So far relativity theory and the cosmic speed limit set by light have survived all the challenges thrown at them, so such a challenge to them needs to be taken with a great many grains of salt until it has been properly tested.

The fact is that the OPERA finding doesn’t even fit with known facts. We observed the neutrinos from supernova 1987A arrive at the time the explosion was seen in the sky. The revised speed would mean that we would have observed them three and a half years earlier.

It seems likely that the observation will be explained as a simple goof (e.g. instrument error), or as an interesting phenomenon in which nothing goes faster than the speed of light but sometimes something appears to, due to some reason we are yet to discover.
To have a look at the scientific paper with the results, go to http://arxiv.org/abs/1109.4897
This article (and the one prior) were published in the Newsletter for the Phoenix Astronomical Society. (link to right).