Neutrinos slower than light, but continue to befuddle physicists

A recent experiment has demonstrated that neutrinos do not, in fact, travel faster than light. But this ethereal subatomic particle continues to undermine established physical models in other ways. 

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.)

The neutrino was originally thought to be massless. If that was true then, according to special relativity, it would have to travel at the speed of light. In 1987, however, a supernova originating in the Large Magellanic Cloud (LMC) produced a burst of neutrinos that registered at three separate detectors with different amounts of energy. This has a pretty cut-and-dried explanation. Neutrinos would only have different energies if they traveled at different speeds. This implied that they had mass, albeit an extremely small one. [Editor's note: An earlier version mischaracterized the data from the supernova.]

There's also different types of neutrinos, which physicists call flavors. Pauli’s indirect inference that led to the first observation of the electron neutrino was also used in 1962 at CERN and Brookhaven labs to infer the existence of another type of neutrino. Except this time the energy imbalance involved a muon – a type of subatomic particle similar to an electron  – and this way the existence of the muon neutrino was inferred.  It happened again in 1978 at the Stanford Linear Accelerator, resulting in the inference of the tau neutrino.

Our own sun helped scientists answer another mystery of the neutrino. The Solar Neutrino Problem was a late 20th century discrepancy in the expected amount of electron neutrinos emitted from our sun. If the solar model was trusted, the expected number of detections could be computed with some pretty straightforward math. The problem arose when various detection facilities received only a rough third of what that calculation would have them expect. This phenomenon was inexplicable until about 2002. The theory that accounted for the discrepancy did so by stipulating that neutrinos aren’t invariably locked into their respective flavors. They switch between them. This phenomenon is called neutrino oscillation.

A few problems for the Standard Model

When the Standard Model of particle physics was established in the mid 1970s, it made some basic assumptions about the neutrino. Yet the evidence physicists have recently discovered seems to suggest otherwise.

The original model assumes that neutrinos are massless. Yet the accepted solution to the Solar Neutrino Problem and the observations from the 1987 Large Magellanic Cloud supernova, indicate that the particles have mass.

The Standard Model also assumes that mirror images of particles look and behave the same way. This is intimately related to the conservation of energy. Yet many particles that interact through the weak force, such as neutrinos, have long been known to violate this.

To preserve the Standard Model, physicists have proposed a new symmetry, called CP symmetry (a product of charge and parity symmetry). CP symmetry states that the laws of physics should be the same for a particle that is made into its mirror image, and interchanged with its antiparticle. The addition of the antiparticle clause allowed the conservation of energy to hold. But neutrinos were soon discovered to violate CP symmetry, as well.

All the recent evidence seems to suggest that the neutrino will have to settle for light speed. After all, it has other ways of creating hassles for physicists.