European scientists believe they've observed subatomic particles traveling faster than the speed of light - something that Einstein's theory of relativity says should be impossible.
"The potential impact on science is too large to draw immediate conclusions or attempt physics interpretations," says OPERA spokesperson Antonio Ereditato of the University of Bern.
The OPERA experiment, housed at Gran Sasso in Italy, observes the passage of neutrinos from CERN, about 500 miles away. And, based on the observation of over 15,000 neutrino events, the team says the neutrinos are arriving sooner than they should - about 20 parts per million faster than the speed of light.
The team is very, very cautious - after all, Einstein's theories have been challenged many times before, and never overturned.
"This result comes as a complete surprise," says Ereditato. "After many months of studies and cross checks we have not found any instrumental effect that could explain the result of the measurement."
The scientists say they're confident their measurements are accurate. The 730km travel path has been measured with an uncertainlty of just 20cm, and the time of flight of the neutrinos should be accurate to within less than 10 nanoseconds, thanks to advanced GPS systems and atomic clocks.
The time response of all elements of the CNGS beam line and of the OPERA detector has also been measured with great precision, they say.
While the result's been replicated many times at Gran Sasso itself, it needs repeating elsewhere before scientists can be certain it's the real thing.
"If this measurement is confirmed, it might change our view of physics, but we need to be sure that there are no other, more mundane, explanations," says CERN Research Director Sergio Bertolucci. "That will require independent measurements."
It's possible that the US's Fermilab could carry out the experiments, although its measuring systems aren't as accurate as CERN's.