EXTRA dimensions. Time travel. Tachyons. These ideas seemed a little more likely in the wake of claims that subatomic particles called neutrinos had moved faster than light – violating a cornerstone of physics laid down by Einstein. Now, just a few months later, the universe is back to its slightly more mundane self.
The collaboration behind the original claims has discovered two flaws during the retesting of its experiment, called OPERA. These flaws did not definitely affect the results, but offer the most concrete basis yet for doubting that the neutrinos were actually speeding. This vastly weakens the original claims.
“The evidence has, at least for now, gone away,” says of Rutgers University in Piscataway, New Jersey, a physicist and blogger who was not involved with the experiment. “Until it comes back, all bets are that Einstein will come out just fine.”
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The announcement has also opened a debate over when and how to report electrifying but tentative scientific results, and whether it is even an option to keep such findings under wraps.
The collaboration shocked the world last September when it reported that neutrinos from a particle accelerator at CERN near Geneva, Switzerland, had arrived at a detector in the Gran Sasso mountain, Italy, 60 nanoseconds earlier than if they had been travelling at the speed of light.
The result seemed to fly in the face of Einstein’s special theory of relativity, which says that nothing can accelerate beyond the speed of light. If true, it would have raised the possibility of particles called tachyons, which are born moving faster than light, or of extra dimensions beyond the three of space and one of time that we experience.
At a packed meeting at CERN in September, the team said it had spent three years chasing down every possible source of error. The researchers even reran the experiment. The result stuck, although it hasn’t been published in a peer-reviewed journal. Now it looks as if the result might have been too good to be true. On 23 February, the team that emerged after months of retesting the experimental system.
The first comes from a fibre-optic cable that sent light signals from a GPS receiver on the surface of the Gran Sasso mountain to the OPERA master clock 8.3 kilometres below. Buried in the mountain, the clock couldn’t receive GPS time data itself.
Light is used as a signal because of its speed, but must be converted into electrical signals before it can be used to time-stamp incoming neutrinos. Before the first run of the experiment in 2008, the light’s travel and conversion took 40,996 nanoseconds, give or take 1 nanosecond, which was used to calculate the neutrinos’ actual time of arrival.
But it turns out that the delay also depends on the light that the converter receives from the cable. Like a bucket that will only tip over when it is completely full of water, the converter needs a minimum amount of light to fire.
OPERA member Marcos Dracos of the University of Strasbourg in France says that the fibre-optic cable has a fault. If present when the measurements were taken, it might have allowed light to bleed out. The converter would then take longer to get the amount of light it needs, slowing the emergence of the corresponding electrical signal, and making the neutrinos appear to arrive earlier than they did (see diagram).
“A light leak may have made the neutrinos appear to arrive earlier than they did”
The team couldn’t check whether the delay had changed compared with 2008 until December, after the last experimental run had ended, says Dracos. And there is no way to tell whether the cable broke – and if so, when – or if it just degraded over time. “It may have happened by chance just recently,” says Dario Autiero of OPERA. “We have no means to reproduce the exact state of this connection. The only way we have to clarify any doubts is to take some new data.”
The second error came from a faulty clock. Touted as a highly accurate quartz crystal clock, it was supposed to oscillate 10 million times every second. Like a conductor keeping time during silences in a symphony, the clock was meant to keep the beat for 600 milliseconds between GPS transmissions, to keep OPERA’s master clock accurate. But when Autiero and his colleagues checked it against an even more accurate atomic clock, it ticked off the milliseconds ever so slightly faster than it should. “Since the frequency is higher, it points in the direction of overestimating the time of flight of the neutrinos,” Autiero says.
To lovers of extra dimensions, that may feel like a ray of hope: the neutrinos may still be super-speedy after all. But taking the two errors together just means we can’t say much about their speed. “I would say you simply cannot conclude anything right now,” says Strassler.
The OPERA team has faced ridicule since announcing the flaws, which Strassler thinks is unfair. “It’s hard to know how bone-headed this was,” he says. “It sounds like it was tricky.”
“It’s hard to know how bone-headed this was. It’s a hell of a process. All you need is one wire to be off”
Experiments depend on physical materials, which are inherently imperfect, he says, and checking “every damn wire” is “a hell of a process”. “All you need is one wire to be the tiniest bit off.”
In light of a possible mundane explanation for the startling result, was OPERA too quick to announce its result? “There’s a big debate going on about this issue in the community,” Strassler says. Some criticise OPERA and CERN for ramping up the publicity with a press release and symposium.
Others say that this is just how science works. “Even in September, we were very cautious in saying this is not a discovery, this is something that is not understood and that we want to check,” says Arnaud Marsollier of the CERN press office.
OPERA itself has been split since the initial announcement. “I was against the publicity from the beginning,” says Luca Stanco, one of 15 OPERA members who did not put their names to the result until after the second experimental run. “It’s embarrassing, of course. I think that more careful checks had to be done before the release of the paper.”
But Strassler thinks that with a result that challenged a figure as iconic as Einstein, the firestorm may have been out of OPERA’s control. “Is it even possible for an experiment to downplay things as much as would have been necessary to avoid something like this kind of scrutiny?” he says.
OPERA plans to start rerunning the experiment in May with a replaced clock and repaired cable. An experiment at Fermilab in Batavia, Illinois, meanwhile is set to re-test the result.
Even Thomas Weiler at Vanderbilt University in Nashville, Tennessee, who linked the OPERA result to extra dimensions, is circumspect: “Nature does not give up her secrets easily and superluminal neutrinos may not be one of her secrets at all.”
