September 23, 2011

expert reaction to measurements of neutrino speed

Consternation reigned at CERN and the OPERA particle physics facility in Italy as experiments appeared to show neutrinos travelling faster than the speed of light.

Prof Stefan Soldner-Rembold, Professor of Particle Physics at the University of Manchester, said:

“Neutrinos are elusive particles that have fascinated physicists for decades, ever since they first appeared in the beta decay of nuclei. There are three types of neutrinos, often called “flavours” and one of the unique properties of neutrinos is that they can change their flavour when travelling over long distances. Many experiments are being performed to measure these oscillations. One of them, called OPERA, detects neutrinos produced at the European Laboratory for Nuclear Research (CERN) in an underground laboratory in the Gran Sasso tunnel in central Italy. The neutrinos travel through the Earth on their way to Gran Sasso since they hardly ever interact with the matter in their way.

“Neutrinos only have a tiny mass, much lower than the mass of electrons, and they should therefore propagate with nearly the speed of light. From Einstein’s theory of relativity we know that the mass-less photons travel exactly with the speed of light and any particle that has mass can only come close to the speed of light, but never fully reach it.

“The distance between CERN and the Gran Sasso tunnel is 730 km and has been measured very precisely. The time of flight of the neutrinos between the two laboratories can be determined using precise atomic clocks and GPS systems. From this, the OPERA scientists measured the speed of the neutrinos and found that the neutrinos travel slightly faster than the speed of light. This result is completely unexpected. A previous experiment at Fermilab close to Chicago (MINOS) had published a similar result in 2007, where they measured a similar speed difference as OPERA but an uncertainty much larger than OPERA. In 1987, a much smaller upper limit on the speed of neutrinos reaching Earth from a Supernova was found. These neutrinos have much smaller energies than the ones from accelerators which could be an explanation for the difference.

“Particle physicists are still struggling to find theories explaining the effect. Neutrinos might propagate in a warped space-time, whereas light propagates in ordinary space-time. In this way neutrinos can shortcut space, unlike light, make them appear faster than the speed of light. Other explanations are more mundane, related to the measurement process, and all these have to be understood first before any claim for a discovery can be made. At this point, the OPERA scientists say they have found no such explanation and they therefore present the result for the wider science community to scrutinise. If the results are confirmed, though, it would be revolutionary for physics and science.”

Prof Jim Al-Khalili, Professor of Physics at the University of Surrey, said:

“The scientists are right to be extremely cautious about interpreting these findings. If the neutrinos have broken the speed of light, it would overturn a keystone theory from the last century of physics. That’s possible, but it’s far more likely that that there is an error in the data. So let me put my money where my mouth is: 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.”

Prof Dave Wark, Chair in High Energy Physics at Imperial College London and STFC Director of Particle Physics, said:

“If this result is right, it calls into question the laws of Special Relativity that provide the basis for most of modern physics. We will require a very high standard of proof and confirmation from other neutrino experiments around the world.”

Prof Jenny Thomas, UCL and co-spokesperson for the MINOS neutrino experiment at Fermilab in the USA, said:

“The impact of this measurement, were it to be correct, would be huge. In fact it would overturn everything we thought we understood about relativity and the speed of light.”