Physicists reported Thursday that sub-atomic particles called neutrinos can travel faster than light, a finding that - if verified - would blast a hole in Einstein's theory of relativity.
In experiments conducted between the European Centre for Nuclear Research (CERN) in Switzerland and a laboratory in Italy, the tiny particles were clocked at 300,006 kilometres per second, about six km/sec faster that the speed of light, the researchers said.
"This result comes as a complete surprise," said physicist Antonio Ereditato, spokesman for the experiment, known as OPERA. "We wanted to measure the speed of neutrinos, but we didn't expect to find anything special."
Scientists spent nearly six months "checking, testing, controlling and rechecking everything" before making an announcement, he said.
Researchers involved in the experiments were cautious in describing its implications, and called on physicists around the world to scrutinise their data, to be made available online overnight.
But the findings, they said, could potentially reshape our understanding of the physical world.
"If this measurement is confirmed, it might change our view of physics," said CERN research director Sergio Bertolucci, a view echoed by several independent physicist contacted by AFP.
In the experiments, scientists blasted a beam producing billions upon billions of neutrinos from CERN, which straddles the French-Swiss border near Geneva, to the Gran Sasso Laboratory 730 kilometres (453 miles) away in Italy.
Neutrinos are electrically neutral particles so small that only recently were they found to have mass.
Hugely abundant but hard to detect, these "ghost particles" are a by-product of nuclear fusion from stars, such as the Sun.
"The neutrinos arrived 60 nanoseconds earlier that the 2.3 milliseconds taken by light," Ereditato told AFP, adding that the margin or error was less than 10 nanoseconds.
Under Albert Einstein's theory of special relativity, however, a physical object cannot travel faster than the speed of light in a vacuum.
The fact that the neutrinos were moving through matter - including a slice of Earth's crust - could not have caused them to accelerate, said French physicist Pierre Binetruy, who was not involved in the experiment but has reviewed the data.
"It might have slowed them down, but it certainly didn't make them go faster than the speed of light," he told journalists in Paris Thursday night.
Binetruy described the results "altogether revolutionary," and said they will, if backed up, force physicists to go back to the blackboard.
"The theory of general relativity, the theory of special relativity - both are called into question," he said.
Alfons Weber, a neutrino expert who participated in a similar experiment in 2007 at the US Fermilab, agreed that the faster-than-light neutrinos could not be reconciled with current theories, but said the results needed to be duplicated elsewhere.
"There is still the possibility of a measurement error," he said by phone. "It would be too exciting to be true. That's why I'm cautious."
The earlier test, conducted over the same distance, also gave a slight edge to neutrinos in the race against light, but the results were within the experiment's margin of error, said Weber, a professor of particle physics at Oxford University.
The CERN announcement was likely to prompt another round of more accurate tests in the US, he added.
Even if verified, however, the new findings would not entirely invalidate Einstein's brilliant insights, which have held sway for more than a century.
"The theory of special relativity will still be a good theory if you apply it where it is valid, but there will have to be some extensions or modifications," he said.
Newton's theory of gravity, he noted, still explains the movement of planets well enough to send missions into space, even if Einstein's theories proved that it was not quite correct.
Theoretical physicists are sure to begin searching for new explanations to account for the unsuspected quickness of neutrinos.
It could be that "the particles have found a shortcut in another dimension" besides the four - three in space, plus time - we know about, Binetruy ventured.
"Or it could simply mean that the speed of light is not the speed limit we thought it was."