Less massive than an electron, faster than a speeding photon, able to fly through thick rock without hitting a single quark. It's a lepton. It's uncharged. It's... it's... a neutrino!
Neutrinos are tiny subatomic particles without any charge, which makes it possible for them to speed through the earth without interacting with the atoms along the way. Neutrinos are so tiny and finicky to work with, they aren't even assigned a quantified mass. For example an electron has a mass of 9.11×10^−31 kg (which is exceedingly small), but a neutrino's mass is described simply as "non-zero." Just barely there.
The Opera Experiment:
Italy's Gran Sasso National Laboratory is buried deep under the ground, out of the way of the Sun's cosmic radiation. There a group of 160 physicists from 11 countries have been collaborating in an experiment called Oscillation Project with Emulsion-Tracking Apparatus, or "Opera" for short. The Opera experiment does not begin in Italy, though. Just over 450 miles away at the CERN Large Hadron Collider on the border of France and Switzerland, naked protons, stripped of electrons, are fired into a graphite target. There they turn into a pulse of smaller particles called mesons that decay into neutrinos and speed through the earth toward the Gran Sasso lab.
Much to their dismay, the Opera physicists found the neutrinos traveled a distance of 454 miles from the CERN particle accelerator to the underground lab in Italy about 60 nanoseconds faster than a beam of light. That's significant, because the margin of error was only 10 nanoseconds. In 2007, scientists at Fermilab in Chicago clocked neutrinos exceeding light speed, but their margin of error was higher - high enough to render the results inconclusive - high enough to protect the conceptual order and the sanity of the world's physicists.
The Opera scientists are not trying to prove a point, nor do they wish to upset Einstein. They were not even initially focusing on neutrino speed. Neutrinos can morph from one shape to another, and the scientists performing the Opera experiment have been trying to study the transformation of muon neutrinos to tau neutrinos. In the process, they measured the speeds of thousands of neutrinos, and came to the uncomfortable discovery that they were going too fast. After 16,000 neutrinos detected over three years, the physicists only detected one tau neutrino. While the speed of neutrinos is becoming the big issue, OPERA spokesperson, Antonio Ereditato of the University of Bern, said, "We would like to see some tau neutrinos."
Skeptical physicists abound. Few are ready to believe that any particle, however small, surpasses the speed of light. Astrophysicist Martin Rees of the University of Cambridge reminds Scientific American readers that neutrinos and photos arrived about the same time from SN1987A, a supernova first observed in February of 1987. If neutrinos indeed travelled faster than light, then the neutrinos from the supernova should have beat the photons in their space race from the explosion by a good four years. The SN1987A neutrinos had lower energies than the particle accelerator neutrinos, but still. Theoretical physicist Lawrence Krauss of Arizona State University voiced the same sentiments, also noting that the experiment appears to violate Lorentz invariance, "which is at the heart of so much known physics."
That's the struggle. So much known physics, tested over and over, depends on the speed of light as the max of the universe. The equations work. The numbers fit over and over again. So, if the speed of light can be surpassed, what does that really mean? Does it mean that neutrinos actually leapfrog forward in time? Or, is there something that physicists have missed?
Most theoretical physicists assume there must be a mistake somewhere. Is there a systematic error that the Gran Sasso team missed? Did they measure the underground distance with the necessary precision, accounting for the tidal bulges in the Earth's crust due to the location of the Moon?
Dr. Dario Autiero, of the Institut de Physique Nucleaire de Lyon in France declared that his group has been trying to explain the results for six months without success.
"We cannot explain the observed effect in terms of systematic uncertainties," Dr. Autiero told the physicists at CERN. "Therefore, the measurement indicates a neutrino velocity higher than the speed of light."
"This result comes as a complete surprise," confirmed Ereditato. "After many months of studies and cross checks we have not found any instrumental effect that could explain the result of the measurement. While OPERA researchers will continue their studies, we are also looking forward to independent measurements to fully assess the nature of this observation."
Physicists in other labs are jumping to see if they can repeat the Italian lab's results. Until others can repeat the experiment and get the same data, it's unwise to take a stubborn position on the issue. At the same time, an editorial in The Guardian warns against rejecting discoveries just because they're uncomfortable:
"[B]ut the history of science cautions against branding it unthinkable...Recall, too, that it was the then inexplicable Michelson-Morley experiment which encouraged the spread of Einstein's early ideas, and the baffling perihelion precession of Mercury which lent support to his general theory. The first thing in science is to face the facts; making sense of them has to come second."