Leading Light: What Would Faster-Than-Light Neutrinos Mean for Physics?

Most physicists are betting against the idea that neutrinos can pierce the cosmic speed limit, but that has not stopped some researchers from exploring the implications
SN 1987A

NASA, ESA, P. Challis and R. Kirshner (Harvard-Smithsonian Center for Astrophysics)

The stunning recent announcement of neutrinos apparently exceeding the speed of light was greeted with startled wonderment followed by widespread disbelief. Although virtually every scientist on record expects this discovery to vanish once more detailed analysis takes place, dozens of researchers are exploring the question whose answer could shake the foundations of physics: What if this anomaly is real?

Neutrinos are ghostly particles that only weakly interact with normal matter; trillions of neutrinos stream through our bodies every second. Last month researchers from the European OPERA (Oscillation Project with Emulsion-tRacking Apparatus) collaboration reported clocking pulses of neutrinos moving at speeds that appeared to be a smidgen faster than light-speed. That might seem impossible, given the universal speed limit set by Albert Einstein's long-standing and well-tested special theory of relativity, but neutrinos have proved chock full of surprises over the years. For instance, in the late 1990s they were found to have mass after decades of thought to the contrary.

The credibility of the OPERA scientists who made the supposed discovery of superluminal neutrinos is not in doubt. "This is legitimate, professionally done science—these are not cranks," says neutrino physicist John Learned of the University of Hawaii at Manoa. Astroparticle physicist Steen Hannestad of Aarhus University in Denmark agrees that "there are no obvious problems with the experiment." Still, Hannestad adds, "on any given year, one or more such anomalies show up in experiments—this is particularly true for neutrino physics, where measurements are notoriously hard."

Virtually every physicist interviewed strongly doubts the results will hold up, including the experimenters themselves. Recent calculations also suggest that any neutrinos traveling faster than light would have radiated most of their energy away before reaching detectors, something the researchers did not see.

Nevertheless, if scientists really have discovered superluminal neutrinos, "it probably presents the biggest revolution to fundamental physics in about a century, and almost every physicist loves this challenge," Learned says. As such, a multitude of studies have already popped up to address the OPERA results, including some that suggest new physics to explain the findings.

One explanation that a number of researchers have proposed is that the neutrinos might be traversing extra dimensions of reality beyond the familiar three of space and one of time. As such, they would only appear to be traveling faster than light from our perspective. This idea would keep Einstein's theory of relativity intact and help end decades of debate over whether extra dimensions actually exist.

"However, these models have other potential problems," says Hannestad, a researcher with a potential extradimensional explanation for the neutrino results. For instance, he notes, the neutrino pulses predicted to emerge from a trip through extra dimensions may differ in their attributes from the pulses the OPERA researchers actually detected.

Another notion is that neutrinos may travel faster through Earth than through empty space, with Earth essentially acting like a lens, says theoretical physicist Dmitri Semikoz of University of Paris Diderot in France. "Several checks of our proposal can be performed," he adds. "In particular, it predicts that the effect is proportional to the intensity of the neutrino wave. This can be checked experimentally by the OPERA and MINOS collaborations with data they already have." (MINOS, the Main Injector Neutrino Oscillation Search, is a competing neutrino experiment in the U.S.)

Dark energy, the mysterious force apparently driving the accelerating expansion of the universe, or dark matter, the as-yet unidentified substance making up about 85 percent of all matter in the universe, might be the explanation instead, suggests theoretical physicist Susan Gardner at the University of Kentucky in Lexington. The idea is that photons of light might interact more strongly with the dark universe than neutrinos do, such that the speed of light measured on Earth might be lower than its theoretical speed due to dark matter and dark energy found throughout the universe slowing it down. As such, neutrinos on Earth might only go faster than the highest speed of light we have so far discovered.

A challenge to Gardner's model comes from analysis of neutrinos from supernova SN 1987A, which strongly indicated the ethereal particles obey the cosmic speed limit. Perhaps, however, the light and neutrinos did not emanate from the explosion simultaneously, as assumed, and instead the star actually emitted faster-than-light neutrinos after it did the light. "Prior to SN 1987A theory told us that blue supergiant stars did not explode—SN 1987A came as something of a surprise," Gardner says. Perhaps this explosion behaved differently from more familiar supernovae, with its neutrino and light bursts occurring farther apart in time, and only by a fantastic coincidence did astronomers detect them close together.

To help settle the neutrino mystery, OPERA and MINOS will continue to make ever-more precise measurements of neutrino speeds.

"I think all we have to do is sit back and cheer our OPERA [and] MINOS colleagues onwards, and in two years or so we will know whether to get really excited," Learned says. "This is a great example of science in action, a great drama unfolding. It is either a fantastic discovery which seemingly cannot but have huge and as yet unknown consequences or it is a mistake. One would be a fool not to bet against the results, but yet they are not obviously wrong. Stay tuned!"

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