If you want a fight, just challenge Einstein. That's what a team of Italian physicists found out over the last six months.
To recap: Last September, the physicists announced that they had detected neutrinos, fundamental subatomic particles, traveling faster than light. The speed of light is considered to be the speed limit of the universe, and these speedy neutrinos appeared to be breaking the rules—rules that had been shown to be true over and over again by physicists. As a result, skepticism abounded, and in February, details exposing two technical problems with the experiment came to light. For a moment, these finding appeared to kill the controversy, and any chance that faster-than-light neutrinos would rewrite the physics textbooks.
But the battle is not over yet—at least not the sparring between physicists. On Tuesday night, when the American Museum of Natural History held its annual Isaac Asimov Memorial Debate, it pulled back the curtain on the internal strife still burning within the scientific community.
Moderated by Hayden Planetarium director Neil deGrasse Tyson, the panel was a mix of experimental physicists—David Cline, astronomy and physics professor at UCLA; Laura Patrizii, member of the research team responsible for the superluminal neutrinos; and Gabriella Gonzalez, Louisiana State University professor of physics and astronomy—and their theoretical colleagues—Gian Giudice, researcher at CERN's theoretical physics division; and Sheldon Glashow, Metcalf Professor of Mathematics and Physics at Boston University. "It's as though you were eavesdropping on our conversation at the bar," Tyson said to the audience.
Faster-Than-Light Neutrinos and the Fallout
The OPERA experiment with which the Italians thought they saw faster-than-light neutrinos is officially named the Operation Project with Emulsion-Racking Apparatus (OPERA), and it was not designed to measure the speed of the particles, but rather to offer further proof that they transform. "Neutrinos exist in three families—electron neutrino, muon neutrino, and tau neutrino—and they can change their nature," Patrizii said. "It's a peculiar property of those properties, which proves that they have mass."
When Patrizii and her colleagues released in September the findings that neutrinos had arrived 60 nanoseconds earlier than a beam of light, the physics community did not collectively scoff and dismiss them, Giudice said. "Immediately, they tried to make sense of it. For several months many physicists were trying to understand if it was possible to modify the properties of neutrinos or the properties of space time to reconcile the OPERA result with our knowledge of special relativity." Yet, after months of trying to modify the current understanding of special relativity, physicists realized that there was no way to account for superluminal neutrinos, he said.
"There was only one plausible theoretical explanation that I heard for this result. When the neutrinos are produced in Switzerland, they travel at the speed of light. Then, as soon as they pass the border into Italy, they no longer respect the speed limits," Giudice joked.
But while some theoretical physicists like Giudice were indeed trying to reconcile the OPERA results, others such as Glashow were actively trying to disprove them. Glashow said that if neutrinos could travel faster than light, then they would give off a specific kind of energy known as Cherenkov radiation, only present in charged particles traveling faster than light. "That radiation has been looked for and looked for, and other people have looked for it. It ain't there," he said. "This unambiguously, beyond a shadow of a doubt tells me that this charming young lady [referring to Patrizii] is absolutely wrong."
"I think that this story has a good moral for theoretical physics," Giuduce said. "Yes, we are eager to chew every bone that experimentalists throw at us, but we don't swallow anything."
Doing the "Right" Science?
As Giudice's quote suggests, experimental and theoretical physicists have a long-standing tendency to be combative toward one another. Often it's all in good fun or friendly rivalry. But the researchers on hand last night brought up another important point: This dialog shapes what experiments are conducted and thus the direction of physics.
Cline, who helped replicate the OPERA results in a separate experiment known as ICARUS, measured neutrinos traveling very close to—but not exceeding—the speed of light. Based on his research and the technical faults found in the OPERA experiment, he questions if it's even necessary to continue testing for faster-than-light neutrinos, saying it might be more productive for physicists to chart a different path.
"OPERA is going against the standard tradition and has found an error in their terminal," Cline says. To him, that technical error justifies the decision to halt any further studies on neutrino velocity. "These experiments can take a decade, so you don't go after some wild crazy idea, and 20 years later you regret you did it," he said.
But Patrizii disagrees. "We're allowed to be wrong," she said. "Physicists, they can be wrong. Physics is not. Eventually we will see what is true." To Glashow, an experiment that turns up a surprise—even if it turns out to be untrue—is a good thing that can push physics further. "We need surprises, we depend on results that contradict our own theories. We want to be contradicted."