Tomorrow, researchers from CERN will be releasing experiment results that suggest neutrinos, the lightest particles we're aware of, may be moving slightly faster than the speed of light. Although the results have not yet been made public (UPDATE: the paper has now been released), rumors of the finding have spread far and wide, leading to coverage by the BBC and the AP. Still, because the findings would seem to violate relativity, the authors are being very cautious about their results, and many in the physics community are expressing skepticism.

Neutrinos have generally made the news because they engage in what are called flavor oscillations, in which (to give one example) an experiment that creates only muon neutrinos will see some of them behave as electron neutrinos when they hit a detector. These oscillations confirmed that neutrinos must have mass, although they are orders of magnitude lighter than any of the other fundamental particles. That extremely low mass means that it doesn't take much energy to get them moving very quickly, which allows physicists who work on neutrino detectors to simply treat them as if they are moving at the speed of light when calculating their expected behavior.

As far as we could tell, this was a pretty good approximation. Neutrinos are produced in copious quantities in supernovae, and despite the enormous distances, they arrive at about the same time as the photons generated in the explosion. (The neutrinos actually get here a bit more quickly because they so rarely interact with matter, meaning they can escape the core of the star without incident. The photons, in contrast, get held up by interacting with the outer layers of the star, and typically appear a short time afterwards.)

Fermilab's MINOS experiment has also measured the speed of neutrinos that are produced on-site near Chicago, then sent to a detector in a mine in Minnesota. "MINOS also measured a faster-than-light value of the neutrino speed," MINOS co-spokesperson Jenny Thomas told Ars, "but the errors were so big we dismissed it."

Thomas said there are three potential sources of error. You have to measure both the distance travelled and time of flight very accurately, and then you have to account for what she called the "time structure of the accelerator," which is related to how you handle the protons that are used to produce neutrinos in these experiments. Errors in any one of these factors can introduce errors into the results.

CERN has a similar, higher-energy version of the Fermi experiment called OPERA, which sends neutrinos from a source in Switzerland to a detector at Gran Sasso in Italy. After accounting for all the sources of error, the people running the OPERA experiment expect that their measurements may be off by as much as 10 nanoseconds. The neutrinos got there 60 nanoseconds ahead of when we'd expect them to arrive if they were moving at the speed of light.

There's definitely the chance that the error estimates are off, and several physicists are expecting that will be the case. That's in part because this would seem to violate relativity, which is a very-well-supported theory, and in part because we don't have a good theoretical basis for expecting neutrinos to do anything like this. One physics blogger who commented on the results wrote, "If you are wondering about theories that allow tachyonic [faster than light] neutrinos the least wacky one I can find is that neutrinos can take 'shortcuts off the brane through large extra dimensions.'"

The final reason to be skeptical is the fact that this effect hasn't shown up in previous measurements. Thomas noted that it might be a matter of energy. Neutrinos from supernovae are relatively low energy; MINOS' were much higher, at which point a weak effect turned up. The OPERA studies are at higher energy still. So the results don't appear to be exactly comparable.

For now, however, those results are being kept under wraps until the webcast tomorrow. Once they're released, you can bet the physics community will be overloading the server, looking for the chance to poke holes in the results.