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Dec. 13, 2011

Neutrinos May Be Able to Exceed the Cosmic Speed Limit

by Zach Lynn

Zach Lynn, Carleton College

The OPERA detectorThis September, the OPERA experiment (Oscillation Project with Emulsion-tRacking Apparatus) announced that they had observed a pulse of neutrinos moving faster than the speed of light. The difference wasn't large. The neutrinos arrived at the detector in Gran Sasso, Italy about 60ns (60 billionths of a second) faster than a photon in a vacuum would have. The neutrinos appeared to be moving about 1/40,000 times (about 17,500mph) faster than the speed of light.

According to special relativity, particles with mass cannot exceed the speed of light in a vacuum (about 186,000 miles per second). Neutrinos have a small mass even on the atomic scale. The upper limit for their mass is billions of times less than that of a hydrogen atom. If neutrinos can move faster than light, then changes will need to be made to special relativity and the myriad theories that stem from it. There are also a few theories that explain these results without violating special relativity. You can read about them here.

This experiment was not the first to suggest that neutrinos may travel faster than the speed of light. In March of 2007, physicists shot a beam of neutrinos from Fermilab, which lies outside Chicago, at a detector 450 miles away in Minnesota. They found with 99% confidence that the neutrinos were traveling between 0.999976 and 1.000126 times the speed of light. Due to this miniscule uncertainty in their results, they were not able to definitively state whether the neutrinos had exceeded the speed of light.

Due to the consequences these results have on physics as we know it, many in the field are not yet willing to accept them. A paper posted one week after OPERA published their results discredited the data. They argued that a particle moving faster than the speed of light would emit low-energy particles as it traveled through space. In addition to creating new low energy particles, this process would deplete the kinetic energy of the neutrinos. The OPERA experiment tested two levels of neutrino kinetic energy, 17.5 and 28GeV (gigaelectron volts). Upon arriving at the detector, both groups of neutrinos had the same kinetic energies with which they were created. The neutrinos in OPERA's experiment did not leave behind the expected "wake" of low energy particles.

Even the scientists at CERN and LNGS (Laboratori Nazionali del Gran Sasso), who conducted the experiment, were skeptical. When OPERA published their report, they immediately began making preparations to repeat the experiment and invited other high-energy physics labs to do the same. On November 22, the OPERA team published similar results and eliminated a potential source of error. In the initial experiment, the neutrinos were created in bunches over a long period of time (much longer than 60ns). If they had assumed that a neutrino came from the start of the bunch rather than the end, it would increase their estimated speed. In the second experiment, the bunches were created in less than 3ns, which greatly reduced the uncertainty. It is possible that there are other sources of error in OPERA's procedure, apparatus or analysis that are still unaccounted for. Many scientists in the field will only be convinced when another group is able to replicate OPERA's results. They might not have to wait too long. Fermilab plans to have results checking OPERA's data by Spring 2012.

You can download and read OPERA's report here.

_____________________
Zach first discovered his passion for science as a high school student at Trinity School in New York City. He now attends Carleton College, where he plans on majoring in Physics. His interests in science include high energy physics, medicine, and technology.

About Zach Lynn

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