FTL Neutrinos? Not so fast….

Rumors have been circulating for a few days that the OPERA Collaboration would today announce the detection of what appear to have been neutrinos from CERN arriving at the Gran Sasso detector in Italy (a distance of 730 kilometers) a full 60 nanoseconds faster than photons would have. If true, this would of course have major ramifications for the foundations of physics. Naturally, the experiment would have to be replicated by other researchers in order for the startling results to gain validity.

Here are some links to a pre-print of the relevant article, as well as some of the buzz generated by it:

• OPERA, “Measurement of the neutrino velocity with the OPERA detector in the CNGS beam” (arxiv.org preprint)
• Video of CERN briefing
• Uncertain Principles, ‘Faster Than a Speeding Photon: “Measurement of the neutrino velocity with the OPERA detector in the CNGS beam”‘
• Jennifer Ouellette, “NAUGHTY ‘FASTER THAN LIGHT’ NEUTRINOS A REALITY?”
• Tommaso Dorigo, “New Results From Opera Due Tomorrow”
• Tommaso Dorigo, “A Six-Sigma Signal Of Superluminal Neutrinos From Opera!”
• Phil Plait, “Faster-than-light travel discovered? Slow down, folks”
• Adrian Cho, “Neutrinos Travel Faster Than Light, According to One Experiment”, Science Now
• Ethan Siegel, “This Extraordinary Claim Requires Extraordinary Evidence!”
• Luboš Motl, “Italian out-of-tune superluminal neutrino opera”
• Sean Carrol, “Faster-Than-Light Neutrinos?”
• Ben Radford, “Reaction to Faster Than Light Claims Expose Anti-Skeptic Myth”
• BBC News, “Brian Cox on Cern’s baffling light-speed find”
• BBC News, “Speed-of-light results under scrutiny at Cern”
• Resonaances, “The Phantom of OPERA”
• The Guardian, “Those faster-than-light neutrinos. Four things to think about”
• The Hammock Physicist, “What Wrong With Those Neutrinos?”
• Cosmic Variance, “Can Neutrinos Kill Their Own Grandfathers?
• The Gauge Connection, “A box seat at OPERA”
• Starts With A Bang!, “Are we fooling ourselves with faster-than-light-neutrinos?”
• A Quantum Diaries Survivor, “Tamburnini: Neutrinos Are Majorana Particles, Relativity Is OK”
• Nature News, “Speedy neutrinos challenge physicists”

Needless to say, this has been greeted by a mixture of excitement and skepticism in the scientific community. Excitement because scientists LOVE having the opportunity to overturn the conventional wisdom and delve into areas of new physics, and skepticism because, as Ethan mentions in quoting Sagan’s Standard, “extraordinary claims require extraordinary evidence.” It should be noted that, if neutrinos actually traveled at such a large velocity beyond c, a neutrino burst from supernova 1987a would have been detected a full 4 years before light from that event reached earth. It wasn’t.

As usual, Randall hit the nail on the head with today’s xkcd comic:

So, what is the deal with neutrinos, anyway? They have long been a source of surprises for physicists since they were first discovered. Because they couple so weakly with other particles, they are almost impossible to detect. The cosmos absolutely teems with them. In any given moment, billions are passing through our bodies, yet few actually interact with whatever they are passing through. This difficulty in detecting them is what makes it so bloody difficult to measure and characterize them.

The existence of neutrinos was first postulated in 1930 by Wolfgang Pauli. In an open letter to his colleagues, Pauli proposed the existence of these leptons, which he referred to as “neutrons” (a name which later ended up being applied to the neutral hadron which currently bears that name), as a mechanism for explaining apparent violations of the Laws of Conservation of Energy and Momentum observed in cloud chamber experiments involving the weak nuclear interaction. It was not until 1956 that the existence of neutrinos was experimentally verified. The November 25, 1997 issue of Los Alamos Science magazine has an excellent article about the experiments which resulted in the first detection of neutrinos.

So, if neutrinos are so difficult to detect due to their weak coupling with other particles, how does one go about detecting them? The answer is to build colossal neutrino detectors consisting of huge volumes of water located deep underground to screen out other types of radiation, usually in abandoned mines or caverns (such as the Gran Sasso cavern mentioned above). These huge tanks of ultra-pure water are lined with photomultiplier tubes, photodetectors so sensitive that they can be triggered by a single photon, or with highly sensitive photographic emulsions sandwiched between lead plates. (Even the rods and cones of the human retina require at least a handful of photons in order to detect light.)

On the rare occasion that a neutrino DOES interact with another particle, the resulting reactions produce particles moving at a velocity higher than the speed of light in that medium. (While the speed of light in vacuum is theoretically regarded as an absolute speed limit, it should be noted that light traveling through a medium is slower, since the photons are constantly being absorbed and re-emitted by the medium, a process which introduces propagation lag.) The result of this is Cherenkov radiation, a tell-tale blue light emitted in a conical pattern from the reaction site. This cone of light gets detected as a circle on the detectors lining the chamber walls, which can then be used to calculate the geometry of the interaction.

Since the days when Pauli first speculated about their existence, it had generally been assumed that neutrinos were massless. However, a long-standing problem resulted in a re-thinking of that assumption. It had been noted since the mid-60’s, when the first large-scale neutrino detection experiments went into operation, that the flux of solar neutrinos striking the Earth was lower than what was predicted by theory. One plausible explanation for this was if the solar neutrinos were experiencing oscillation, which is to say that they were changing “flavors” among the three neutrino types: electron neutrinos, muon neutrinos, and tau neutrinos. However, according to the Standard Model, neutrinos could only change flavor if they had mass, even if only a tiny bit. The idea of neutrinos having a tiny amount of mass also correlated well with neutrino observations made in conjunction with supernova 1987a, but it was not until the late 90’s and early 2000’s that the concept of neutrino mass gained traction due to experimental results from the Super-Kamiokande collaboration in Japan and the Sudbury Neutrino Observatory (SNO) in Canada.

Those little neutrinos keep springing surprises on us. There is no telling what the next one might be. Perhaps superluminal travel is it. Perhaps not.

Update: Oct. 14

There have been a flurry of articles on arxiv.org suggesting explanations for his results. Here are a few of the more prominent ones:

• Andrew G. Cohen, Sheldon L. Glashow, “New Constraints on Neutrino Velocities” (arxiv.org/abs/1109.6562)
• Carlo R. Contaldi, “The OPERA neutrino velocity result and the synchronisation of clocks” (arxiv.org/abs/1109.6160)
• Gian F. Guidice, Sergey Sibiryakov, Alessandro Strumia, “Interpreting OPERA results on superluminal neutrino” (arxiv.org/abs/1109.5682)
• Alessandro Drago et al., “The Hypothesis of Superluminal Neutrinos: comparing OPERA with other Data” (arxiv.org/abs/1109.5917)
• Eugenie Samuel Reich, “Faster-than-light neutrinos face time trial”, Nature News
• Henri, G., “A simple explanation of OPERA results without strange physics” (arxiv.org/abs/1110.0239)
• The Physics arXiv Blog, “Faster-Than-Light Neutrino Puzzle Claimed Solved by Special Relativity”
• Ronald A.J. van Elburg.  Times Of Flight Between A Source And A Detector Observed From A GPS Satellite.  (arxiv.org/abs/1110.2685)

Update: Oct. 27

The OPERA experiment is in the process of being repeated, this time with 1-2 ns neutrino pulses rather than the original 10,500 ns pulses. This will help rule out a few categories of experimental error, as described here.