Earlier this year, an international team of scientists announced they had found neutrinos — tiny particles with an equally tiny but non-zero mass — traveling faster than the speed of light. Unable to find a flaw themselves, the team put out a call for physicists worldwide to check their experiment. One physicist who answered the call was Dr. Ramanath Cowsik. He found a potentially fatal flaw in the experiment that challenged the existence of faster than light neutrinos.
Superluminal (faster than light) neutrinos were the result of the OPERA experiment, a collaboration between the CERN physics laboratory in Geneva, Switzerland, and the Laboratori Nazionali del Gran Sasso in Gran Sasso, Italy.
The experiment timed neutrinos as they traveled 730 kilometres (about 450 miles) through Earth from their origin point at CERN to a detector in Gran Sasso. The team was shocked to find that the neutrinos arrived at Gran Sasso 60 nanoseconds sooner than they would have if they were traveling at the speed of light in a vacuum. In short, they appeared to be superluminal.
This result created either a problem for physics or a breakthrough. According to Einstein’s theory of special relativity, any particle with mass can come close to the speed of light but can’t reach it. Since neutrinos have mass, superluminal neutrinos shouldn’t exist. But, somehow, they did.
But Cowsik questioned the neutrinos’ genesis. The OPERA experiments generated neutrinos by slamming protons into a stationary target. This produced a pulse of pions, unstable particles that were magnetically focused into a tunnel where they decayed into neutrinos and muons (another tiny elementary particle). The muons never went further than the tunnel, but the neutrinos, which can slip through matter like a ghost passes through a wall, kept going towards Gran Sasso.
Cowsik’s and his team looked closely at this first step of the OPERA experiment. They investigated whether “pion decays would produce superluminal neutrinos, assuming energy and momentum are conserved,” he said. The OPERA neutrinos had a lot of energy but very little mass, so the question was whether they could really move faster than light.
What Cowsik and his team found was that if neutrinos produced from a pion decay were traveling faster than light, the pion lifetime would get longer and each neutrino would carry a smaller fraction of the energy it shares with the muon. Within the present framework of physics, superluminal neutrinos would be very difficult to produce. “What’s more,”Cowsik explains, “these difficulties would only increase as the pion energy increases.
There is an experimental check of Cowsik’s theoretical conclusion. CERN’s method of producing neutrinos is duplicated naturally when cosmic rays hit Earth’s atmosphere. An observatory called IceCube is set up to observe these naturally occurring neutrinos in Antarctica; as neutrinos collide with other particles, they generate muons that leave trails of light flashes as they pass through a nearly 2.5 kilometre (1.5 mile) thick block of clear ice.
IceCube has detected neutrinos with energy 10,000 times higher than any generated as part of the OPERA experiment, leading Cowsik to conclude that their parent pions must have correspondingly high energy levels. His team’s calculations based on laws of the conservation of energy and momentum revealed that the lifetimes of those pions should be too long for them to decay into superluminal neutrinos.
As Cowsik explains, IceCube’s detection of high-energy neutrinos is indicative that pions do decay according to standard ideas of physics, but the neutrinos will only approach the speed of light; they will never exceed it.