Physicists report 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 in Switzerland and a laboratory in Italy, the tiny particles were clocked at six kilometres per second faster than the speed of light, say the researchers.
"This result comes as a complete surprise," says 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 says.
Researchers involved in the experiments are cautious in describing its implications, and call on physicists around the world to scrutinise their data.
But they say the findings could potentially reshape our understanding of the physical world.
"If this measurement is confirmed, it might change our view of physics," says CERN research director Sergio Bertolucci.
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 away in Italy.
Neutrinos are electrically neutral particles so small that only recently were they found to have mass.
"The neutrinos arrived 60 nanoseconds earlier that the 2.3 milliseconds taken by light," says Ereditato.
Under Albert Einstein's theory of special relativity, however, a physical object cannot travel faster than the speed of light in a vacuum.
Revolutionary if true
The fact that the neutrinos were moving through matter — including a slice of Earth's crust — could not have caused them to accelerate, says 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 says.
Binetruy described the results "altogether revolutionary," and says 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 says.
Alfons Weber, a neutrino expert who participated in a similar experiment in 2007 at the US Fermilab, agrees that the faster-than-light neutrinos could not be reconciled with current theories, but says the results needed to be duplicated elsewhere.
"There is still the possibility of a measurement error," he says. "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, says Weber.
The CERN announcement was likely to prompt another round of more accurate tests in the US, he adds.
Even if verified, however, he says the new findings would not entirely invalidate Einstein's brilliant insights, which has 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."
Newton's theory of gravity, he notes, 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.\
CERN physicists break the universal speed limit
BRENDAN TREMBATH: Leading scientists in Europe say they've made a discovery that could tip our understanding of physics on its head. The scientists at the European Organization for Nuclear Research, or CERN, say sub-atomic particles called neutrinos can travel faster than light.
According to Einstein's theory of relativity that should be impossible.
Timothy McDonald reports.
TIMOTHY MCDONALD: A team of physicists at CERN made tiny particles called neutrinos travel at 300,006 per second which is just slightly faster than the speed of light.
It's a baffling finding and even the team that made the discovery can scarcely believe it. That's because it should be impossible.
Professor Geoffrey Taylor from the University of Melbourne's School of Physics explains.
GEOFFREY TAYLOR: As you try and push faster and faster the usual laws that we're used to, with putting the accelerator down on the car to make it go faster, the law has changed. That's an approximation that we see in our everyday life. And the more energy you push into a particle, it doesn't go any faster at some point. It just asymptotes to the speed of light.
TIMOTHY MCDONALD: And is it possible that we've got the speed of light wrong? Is it possibly another speed?
GEOFFREY TAYLOR: No, no the speed of light is well measured and well understood.
And there are all sorts of implications, astrophysical as well as measurements on earth. So that's pretty much not in dispute at all.
TIMOTHY MCDONALD: Now assuming for just a moment that what they've done turns out to be correct, what would it mean for physics as we know it?
GEOFFREY TAYLOR: Oh it would turn things completely upside down. That's probably why I'm so sceptical. (Laughs)
We don't have any, in the standard model of everything that we understand, particles just can't go faster than the speed of light in this universe that we're part of.
TIMOTHY MCDONALD: So do you think that they've got it wrong?
GEOFFREY TAYLOR: Yes it's a big experiment and it's very sophisticated. It's hard to imagine exactly where such a big error could have come from, 60 nanoseconds. Sounds like a tiny amount but it's well within the precision of measurement.
So we talk about systematic errors where there's some mistake being made, not in a precision but in the way the measurements have been carried out.
So my guess is that something will show up in the experimental side that has not been thought about.
TIMOTHY MCDONALD: It really would cause us to re-think just about everything, wouldn't it, being that it's the constant under which other ideas about the universe hang? This is the anchor, so to speak.
GEOFFREY TAYLOR: That's right. It really is a definition of our understanding of space time so in some sense it comes out of that. And so that relationship is absolutely key to the way we build our understanding, absolutely.
It would turn things completely upside down. But that's why also why I'm so sceptical.
TIMOTHY MCDONALD: Jon Butterworth is a physics professor at University College London. He says if the discovery turns out to be true it could have a major impact on our daily lives.
JON BUTTERWORTH: This is one of those things where it's so big if it's true that it's impossible to imagine the practical applications.
I mean when Einstein came up with relativity, I mean you wouldn't have thought of GPS and mobile phones and things.
And really with fundamental physics what we do is we're looking at how the universe works. When we find out how it works we normally find a way to use that information.
We've found Earth-like planets recently somewhere else. You know maybe this will help us get there quicker but I wouldn't bet on it.
TIMOTHY MCDONALD: Of course even the people who made the discovery are a little bit sceptical.
CERN is due to publish its results soon. That will allow other labs to try to replicate their findings, which should give some indication if the findings are a major discovery or a mere error.
BRENDAN TREMBATH: Timothy McDonald.