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When particle physicists at CERN, the underground particle physics laboratory located on the Franco-Swiss border, thought they detected a neutrino that traveled faster than light, Einstein’s theory of relativity, which states that particles that have mass can approach the speed of light but cannot match or exceed it, was challenged in September 2011.
Finding no flaws with their experiment, and despite knowing that something must have been wrong, they published their findings. An uproar in the science community resonated. Neutrinos contain little very mass – but they have mass nonetheless. Hence, they cannot possibly be superluminal. Furthermore, if these neutrinos really were superluminal, they would not only go against the theory of relativity, but also upset the laws of energy conservation and momentum.
Ramanath Cowsik, Utpal Sarkar, and Shmuel Nussino, three of the many physicists who were skeptical of CERN’s findings, undertook the challenge of performing the same experiment. Three months later, in late December, they found the fault.
One of the major experiments at CERN, OPERA (Oscillation Project with Emulsion-Racking Apparatus), conducted this particular experiment, collaborating with the Laboratori Nazionali del Gran Sasso (LNGS) in Gran Sasso, Italy. OPERA and LNGS created neutrinos by smashing protons into a stationary target. Pions (a light type of meson, which is an elementary particle) were produced and focused into a tunnel, where they decayed into muons and neutrinos. The muons stopped at the end of the tunnel, blocked by a barrier, but the neutrinos were able to pass through and arrived at the laboratory in Gran Sasso – 60 nanoseconds earlier than particle physicists anticipated. It was then that OPERA tentatively concluded that they produced superluminal neutrinos.
Keeping in mind the theory of relativity and the laws of conservation of energy and momentum, and taking a second look at how particles decay, Cowsik and his colleagues formed their own calculations.
“Simple calculations, based on the conservation of energy and momentum, dictate that the lifetimes of those pions should be too long for them ever to decay into superluminal neutrinos,” Cowsik says in the press release provided by Washington University, where he works as a professor of physics in Arts & Sciences and the director of the McDonnell Center for the Space Sciences.
Performing the experiment at IceCube, a neutrino observatory in Antarctica below ice that detects neutrinos formed by cosmic rays that collide with the atmosphere, Cowsik, Sarkar, and Nussino mimicked the experiment conducted at CERN, observing the pions decay by letting protons collide with the ice. They observed that the neutrinos at IceCube had much higher energy than the neutrinos at OPERA.
“We’ve shown…that if the neutrino that comes out of a pion decay were going faster than the speed of light, the pion lifetime would get longer, and the neutrino would carry a smaller fraction of the energy shared by the neutrino and the muon,” explains Cowsik. “But the observation of high-energy neutrinos by IceCube indicates that these high-energy pions do decay according to the standard ideas of physics, generating neutrinos whose speed approaches that of light but never exceeds it.”
He and his colleagues deduced that the pions must have also had high energy. It turns out that pions with a lot of energy would last longer and take more time to spontaneously decay. They then would not be able to decay to create superluminal neutrinos. It was then concluded that it would have been impossible for the neutrinos to travel faster than or meet the speed of light during OPERA’s experiment, and that such will always be impossible.