Neutrinos slower than light, but continue to befuddle physicists

When scientists at the European Organization for Nuclear Research (CERN) last year apparently measured neutrinos traveling faster than light, physicists were largely skeptical. Their skepticism now seems justified, as another recent CERN experiment has contradicted the measurements.

Ever since Einstein published the special theory of relativity in 1905, an immutable speed of light has been an integral part of the framework of theoretical physics. Scientists – including the team that measured the superluminal neutrinos – were never willing to discard Einstein's crucial idea after just one experimental result.

During the past several months, physicists have been prodding at the experimental and theoretical bases of the contentious Oscillation Project with Emulsion-Racking Apparatus (OPERA) measurements, which beamed neutrinos from the CERN lab in Geneva, Switzerland to a lab in Gran Sasso, Italy. In late February, many suspected that a poorly calibrated atomic clock, or even a recalcitrant fiber-optic cable produced an error in the timing. Earlier, in September, physicists Andy Cohen and Sheldon Glashow of Boston University published their letter on arXive that proved the measurements to be theoretically impossible.

The most recent CERN measurements are perhaps the best refutation yet. Using the underground ICARUS detector at Gran Sasso, the experiment replicated the OPERA design, with additional provisions taken to improve the accuracy of the timing measurements. The results maintained that neutrinos travel slower than the universal speed limit.

The 2011 OPERA experiment marked a peculiar moment of fame for the tiny neutrino. The measurements' possible, though very unlikely, contradiction of Einstein's special relativity dramatically overshadowed the enormous problems neutrinos had already been presenting for decades. Physicists would have been loath to make a theoretical exception, even for the ethereal neutrino. But the truth is they already have.

Little Neutral One

 Ask a physicist to define a neutrino, and she'll probably take a deep breath. Describing a neutrino as ghost-like (or ethereal) is convenient, but is also superficial. The theoretical inception of the particle was an oblique and complicated affair. In 1931 Wolfgang Pauli observed that energy was not conserved in certain radioactive decays, and hypothesized an undetectable particle that made off with the missing energy. The Italian physicist Enrico Fermi later dubbed this particle the neutrino, Italian for "little neutral one."

Physicists have since established some basic facts about neutrinos. They are chargeless, fundamental particles that interact through only two of the four physical forces – the weak force and gravity. But they do so rarely. This cannot be overstated. Interaction length in particle physics is the average distance a particle will travel through a given medium before it interacts with another particle. For instance, an electron’s interaction length is measured in centimeters of air. A neutrino’s is measured in light-years of lead (a single light-year is roughly 6 trillion miles.)

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