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See Inside December 2011

Why Neutrinos Might Wimp Out

Particles that go beyond light speed? Not so fast, many theoretical physicists say

In case you missed the news, a team of physicists reported in September that the tiny subatomic particles known as neutrinos could violate the cosmic speed limit set by Einstein’s special theory of relativity. The researchers, working on an experiment called OPERA, beamed neutrinos through the earth’s crust, from CERN, the laboratory for particle physics near Geneva, to Gran Sasso National Laboratory in L’Aquila, Italy, an underground physics lab. According to the scientists’ estimates, the neutrinos arrived at their destination around 60 nanoseconds quicker than the speed of light.

Experts urged caution, especially because an earlier measurement of neutrino velocity had indicated, to high precision and accuracy, that neutrinos do respect the cosmic speed limit. In a terse paper posted online on September 29, Andrew Cohen and Sheldon Glashow of Boston University calculated that any neutrinos traveling faster than light would lose energy after emitting, and leaving behind, a trail of slower particles that would be absorbed by the earth’s crust. This trace would be analogous to a sonic boom left behind by a supersonic fighter jet.

Yet the neutrinos detected at Gran Sasso were just as energetic as when they left Switzerland, Cohen and Glashow point out, casting doubt on the veracity of the speed measurements. “When all particles have the same maximal attainable velocity, it is not possible for one particle to lose energy by emitting another,” Cohen explains. “But if the maximal velocities of the particles involved are not all the same,” then it can happen.

An effect of this type is well known in cases where electrons have the higher speed limit (light speed), and light itself has the lower one because it is slowed down by traveling in a medium, such as water or air. Electrons, then, can move in the medium at a speed higher than the maximum speed of photons in the same medium and can lose energy by emitting photons. This transfer of energy between particles with different speed limits is called Cherenkov radiation, and it makes the reactor pools of nuclear power stations glow with a bluish light.

In the neutrinos’ case, Cohen and Glashow calculate that the wake would mostly consist of electrons paired with their antimatter twins, positrons. Crucially, the rate of production of these electron-positron pairs is such that a typical superluminal neutrino emitted at CERN would lose most of its energy before reaching Gran Sasso. Then again, perhaps they were not superluminal to begin with.

“I think this seals the case,” says Lawrence M. Krauss, a theoretical physicist at Arizona State University. “It is a very good paper.” So was Albert Einstein right after all? Einstein’s relativity superseded Isaac Newton’s physics, and physicists will no doubt keep trying to find glitches in Einstein’s theories, too. “We never stop testing our ideas,” Cohen says. “Even those that have been establish­ed well.”

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