Why has Cern’s discovery of subatomic particles travelling faster than light caused such a scientific sensation?
If verified, it would overturn a scientific dogma that has stood since Albert Einstein published his theory of relativity early in the 20th century: that nothing can move faster than light – 299,792,458 metres per second.
A huge superstructure of theoretical physics rests on the assumption that the speed of light (c in the famous equation of mass energy equivalence, E=mc2) is a fundamental constant.
Some scientific discoveries turn up when expected; the discovery of the much-publicised Higgs particle at Cern’s atom smasher, the Large Hadron Collider, would come into this category. Others, like neutrinos apparently breaking nature’s ultimate speed limit, come out of the blue – and invite the response: “Extraordinary claims require extraordinary evidence.”
How good is the evidence so far?
The first thing is that it comes from an impeccable source: a large international group of scientists at Cern, the European Organisation for Nuclear Research, which is the world’s largest and most respected physics laboratory. Their scientific paper appears on the preprint server arXiv.
The experiment, known as Opera, beams neutrinos – ghostly subatomic particles with infinitesimal mass and no electric charge – from an accelerator at Cern headquarters outside Geneva to underground detectors at Gran Sasso in central Italy. Analysis of 15,000 neutrino detections shows that they complete the 730km journey 60 nanoseconds sooner than they would travelling at the speed of light.
Can such an important discovery be based on such a tiny difference in arrival times?
That is the question being asked by the experiment’s critics. The difference is very small. It would mean that the neutrinos travelled less than 0.01 per cent faster than light.
However, Cern maintains that, using the latest high-precision instruments such as GPS and atomic clocks, they can measure the neutrino’s travel distance to within 20cm and their time of flight within 10 nanoseconds. If so, the conclusion of “superluminal” speed is statistically sound – but there may be still undetected errors in the procedure that would invalidate it.
What are the prospects of other researchers either confirming or disproving the discovery?
There are two similar neutrino beam experiments elsewhere, in the Japan and the US, and both are moving to re-examine their existing data and obtain more results, to check the Opera findings.
Most intriguing is the Minos experiment at Fermilab outside Chicago, which also seemed to find neutrinos breaking the speed of light barrier by a small margin. Unlike the Cern team, the Fermilab scientists decided that their results were not statistically significant and they did not draw attention to them.
On the other hand, one natural neutrino experiment has also taken place and given clearly negative results. In 1987 neutrinos and light from a supernova – exploding star – 168,000 light years away reached Earth. If the neutrinos had been travelling at the speed calculated from the Opera experiment they would have arrived about five years before the light. In fact they arrived about three hours earlier, which means that they had moved at the same speed, because supernova theory shows that neutrinos escape from the explosion hours before the light.
How are independent physicists reacting to the results?
Of course they are intrigued. But most are also deeply sceptical. Jim Al-Khalili, professor of physics at the University of Surrey, said it was possible that neutrinos break Einstein’s speed limit “but it’s far more likely that that there is an error in the data. If the Cern experiment proves to be correct and neutrinos have broken the speed of light, I will eat my boxer shorts on live TV.”
But if the result is right what might explain it?
Speculation is already under way. For example Stefan Soldner-Rembold, professor of particle physics at the University of Manchester, said: “Neutrinos might propagate in a warped space-time, whereas light propagates in ordinary space-time.” If so neutrinos could short-cut space and seem faster than the speed of light.