Does Einstein's cosmic speed limit not apply to neutrinos after all?

Credit: Veer Images

Related articles

• Physicists capture image of elusive neutrinos
• Tests needed for faster-than-light neutrino
• 'Faster-than-light' particles fail cross-check
• Scientists revisit 'faster-than-light' experiment
• 'Faster-than-light' experiment exposed as mistake

In September 2011, neutrinos were handed a traffic ticket for exceeding the cosmic speed limit: they were caught travelling faster than the speed of light. The result came from a physics experiment called OPERA, a neutrino detection experiment based in the core of the mountain of Gran Sasso in central Italy. Worldwide, physicists' initial response was, "No way! What did they do wrong?"

In the experiment, beams of neutrinos were sent from CERN, in Geneva, Switzerland, to the Gran Sasso Laboratory, a 730 km journey through the Earth. After travelling for about 2,500,000 billionths of a second, the neutrinos seemed to arrive earlier than light by 60 billionths of a second - with an overall uncertainty of 10 billionths of a second.

This means Einstein's cosmic speed limit might not apply to neutrinos, elementary particles of historic notoriety, which have an uncanny ability to switch between 'flavours', and are extremely difficult to detect as they interact very weakly with other matter.

Upon hearing the news, most physicists stopped work immediately to follow the 'crisis'. Where is the paper? What, no paper? No seminar? Oh, it started with blog post... no surprise there.

In science, big news - especially unexpected big news - circumnavigates the globe at least at light speed. Normally, a detailed paper written by the collaboration with all the gory details is expected when any result is made public. In this case, it turned out the world would have to wait a few days for OPERA to write, edit, and post it. Anticipation was high. Physicists kept asking themselves, "How could this result be true?"

Donald Rumsfeld's classic retort would advise me to consider the "known unknowns" and the "unknown unknowns". As a particle physicist, I am supposed to know that everything obeys the universal speed limit: 'c' (the speed of light) is the fastest anything is allowed to go, whether it's a muon, proton or electron.

But these particles are all electrically charged and the neutrino isn't. Charged elementary particles interact through the weak force - electromagnetic force - and in some cases the strong force too. Neutrinos only interact through the weak force.

So, perhaps that could explain the dilemma? As Sherlock Holmes said, sometimes even the highly improbable turns out to be true. This is an experimental result after all, and nature has surprised us before. And neutrinos are the masters of surprise!

Just after the announcement, I checked the emails from my colleagues: Anyone blowing a gasket? Yes. So, what do they see as the issue? The GPS measurement of distance has to be wrong... that must the weak link. After all, as the neutrinos travel from CERN, they travel through the Earth, underground the whole way. Maybe the GPS clock synchronisation was done incorrectly?

Then the Canadian emails come storming in. What about the neutrinos that were detected from Supernova 1987a? A well-publicised paper evaluating the transit times for neutrinos and light beaming from this 'star death' event were recorded and everything was in order. The particles travelled 160,000 light years - and no anomaly. Both light and neutrinos arrived on time and on schedule.

Then the theories roll in. There are three types of neutrinos detected in experiments. Let's call them lab neutrinos. Each lab neutrino (named electron-like, muon-like and tau-like neutrino) is measured (reliably) to be a different mixture of three 'elementary' neutrinos (named 1, 2, 3). As neutrinos travel through space, the mixture for any given type of neutrino changes.

Think of three guys wearing three shirts coloured with three dyes, red, blue, and yellow. Guy number one's shirt looks mostly red, guy 2 mostly blue, and guy 3 mostly yellow. As they travel along the street, the hues shift. The red one looks more blue, the blue more red, and so on. Later along the street they look like they did at the beginning, mostly red, blue or yellow. Neutrinos similarly switch 'flavours' from tau to muon or electron-like. Now, if neutrinos of one type travelled at a different speed than another type, could the supernova result be circumvented? Yes. Neutrinos from CERN are muon-like and those from 1987a are electron-like.

Readers' comments