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The British astrophysicist Arthur S. Eddington once wrote, “No experiment should be believed until it has been confirmed by theory.”
So when a group of physicists going by the acronym OPERA announced in September that a batch of the strange subatomic particles known as neutrinos had traveled faster than the speed of light in a 457-mile trip through the earth, the first response among many physicists was to wonder what had gone wrong with the experiment.
After all, Albert Einstein’s theory of relativity, which proclaimed the speed of light as the cosmic speed limit, is the foundation of modern science and has been shown to work to exquisite precision zillions of times. Knock it down and you potentially open the door to all kinds of things, like the ability to go back in time and kill your grandfather.
That, of course, did not stop the rest of us in the physics bleachers from dragging the old guru of space-time by his frizzy coronal hair into the media version of the public square and crowing that, perhaps this time at last, Einstein was finally going to be proved wrong. Neutrino jokes proliferated on the Internet, as well as this rousing song by the Corrigan Brothers and Pete Creighton:
Tooraloo, tooraloo, tooraloo, tooralino,
Is light now slower than a neutrino?
Now it seems that Einstein’s six-month nightmare may be over.
Last week another team of physicists whose apparatus lives right next door to the OPERA group — under Gran Sasso mountain in Italy — reported that they had clocked neutrinos, produced in an accelerator at CERN, outside Geneva, racing over the same path to Gran Sasso at the speed of light and not a whit faster. Which is exactly how fast scientists had always thought the enigmatic particles, with barely zilch for mass, should go.
The second group, which goes by the acronym ICARUS, was led by Carlo Rubbia, a former director of CERN and a Nobel-winning physicist, who called the results “very convincing.”
Physicists swung into line with great sighs of relief.
“The evidence is beginning to point toward the OPERA result being an artifact of the measurement,” said CERN’s research director, Sergio Bertolucci.
Cue the famous picture of Einstein sticking out his tongue. As it happened, the ICARUS result was announced March 16, two days after his 133rd birthday — almost in time for the cake.
Adding to the sense of finality was the simple fact — as Eddington might have pointed out — that faster-than-light neutrinos had never been confirmed by theory. Or as John G. Learned, a neutrino physicist at the University of Hawaii, put it in an email, “An interesting result of all this fracas is that no new model I have seen (or heard of from my friends) really is credible to explain the faster-than-light neutrinos.”
During a panel discussion recently at the American Museum of Natural History, Sheldon L. Glashow, a physics professor and Nobel laureate from Boston University, said the best theory he had heard was that the neutrinos had behaved lawfully in Switzerland and speeded up when they crossed the border into Italy.
Eddington’s dictum is not as radical as it might sound. He made it after early measurements of the rate of expansion of the universe made it appear that our planet was older than the cosmos in which it resides — an untenable notion. “It means that science is not just a book of facts, it is understanding as well,” explained Michael S. Turner, a cosmologist at the University of Chicago, who says the Eddington saying is one of his favorites. If a “fact” cannot be understood, fitted into a conceptual framework that we have reason to believe in, or confirmed independently some other way, it risks becoming what journalists like to call a “permanent exclusive” — wrong.
Not that scientists don’t like a surprise. At the museum discussion, Glashow said that at first he was very excited when he heard about the faster-than-light neutrinos, thinking they could be a window into new physics.
But if neutrinos are going faster than light, he quickly realized, they would be accompanied by a whole host of other effects, like a shower of particles called Cherenkov radiation, which were not being seen. His paper to this effect, with Andrew G. Cohen, was one of the first to let the air out of the neutrino balloon.
Luckily, the charms of neutrinos go far beyond their possibly being able to outrace light. They can also waltz through walls and planets like moonlight through a screen door, which is why they can go underground from CERN to Gran Sasso so easily.
This month, a group of Fermilab-based physicists operating under the acronym MINERVA used a beam of neutrinos to send a message over a distance of about a kilometer, spelling out the word “neutrino” with the ASCII code used for computer keyboards.
Moreover, neutrinos come in three varieties and can morph from one form to another as they travel. One variation of this shape-shifting was measured for the first time by an international team working in Daya Bay, China, in March. The OPERA experiment was designed to record another mode.
Dethroning Einstein was never on the OPERA team’s agenda, and to be fair, scientists in the group never claimed that they overturned relativity — only that they had a very puzzling anomaly. They diligently continued to troubleshoot their experiment, and in February they found a couple of flaws. One of them was a loose wire that had the effect of making the neutrinos appear to move faster, the other an improperly set clock that made them look slower.
“That means they have no result,” Glashow said. “We’re back to square one.”
Whether these effects are enough to explain the anomalous neutrino speeds would not be known until the experiment was repeated sometime this spring, the OPERA group said.
At the museum discussion, Laura Patrizii, an OPERA member from the University of Bologna, said it was not unlikely that her group’s work would wind up in agreement with the ICARUS team’s result.
Whether this is a happy ending depends on you. Some physicists were unhappy that the OPERA group had managed to commandeer CERN’s auditorium, the biggest stage in physics, to turn it into a circus. OPERA is not even part of CERN; the group only buys its neutrinos there. Gran Sasso, rather, is part of the Italian National Institute for Nuclear Physics, based in Rome.
Displaying a quaint faith in the ability of the Twitterverse to keep secrets, the critics have suggested that it would have been more responsible for the OPERA group to have shut up and kept checking the results.
Patrizii disagreed, saying the neutrino affair had produced a wonderful discussion and a great lesson in how science is done. The whole world was watching; the editors of great newspapers were waking up thinking about subatomic particles.
Anyway, she added, scientists — even Einstein — proceed by trial and error.
“We are allowed to be wrong,” she said.
