Faster Than a Speeding Photon

Evidence of superluminal neutrinos is making big but baffling waves

3 min read
Faster Than a Speeding Photon

The photon should never lose a race. But on Thursday, stories started trickling in of a baffling result: neutrinos that move faster than light. News of this potential violation of special relativity is everywhere now. But despite a flurry of media coverage, it’s still hard to know what to make of the result.

As far as particle physics results go, the finding itself is fairly easy to convey. OPERA, a 1300-metric-ton detector that sits in Italy’s underground Gran Sasso National Laboratory, detected neutrinos that seem to move faster than the speed of light. The nearly massless particles made the 2.43-millisecond, 730-kilometer trip from CERN, where they were created, to OPERA about 60 nanoseconds faster than a photon would.

After analyzing three years of data, OPERA spokesperson Antonio Ereditato told Science that it was time to spread the news and put the question to the community. "We are forced to say something," Ereditato said. "We could not sweep it under the carpet because that would be dishonest." And the experiment seems carefully done. The OPERA team estimates it measured the 60-nanosecond delay with a precision of about 10 nanoseconds. Yesterday, Nature News reported the team’s result has a certainty of about six sigma, “the physicists' way of saying it is certainly correct.”

Reactions to this tricky time-of-flight experiment have been mixed. Words like “flabbergasted” and “extraordinary” are circulating, but often with a strong note of caution. Physicist Jim Al-Khalili of the University of Surrey was so convinced the finding is the result of measurement error, he told the BBC’s Jason Palmer that “if the CERN experiment proves to be correct and neutrinos have broken the speed of light, I will eat my boxer shorts on live TV."  Others say it’s just too early to call. When approached by Reuters, renowned physicist Stephen Hawking declined to comment on the result. "It is premature to comment on this," he said. "Further experiments and clarifications are needed."

For now, no one's speculating too wildly about what the result might mean if it holds up: there has been some talk of time travel and extra dimensions. And on the whole, media coverage of the OPERA findings, especially given the fast-breaking nature of the news cycle (the team’s preprint posted last night) has been pretty careful. But there is one key issue few have tackled head on: the conflict with long-standing astrophysical results.

One of the key neutrino speed measurements comes from observations of supernova 1987A. Photons and neutrinos from this explosion reached Earth just hours apart in February 1987. But, as Nature News and other outlets noted, if OPERA’s measurement of neutrino speed is correct, neutrinos created in the explosion should have arrived at Earth years before the light from the supernova was finally picked up by astronomers.

New Scientist’s Lisa Grossman found a few potential explanations for the conflicting results. She quotes theorist Mark Sher of the College of William and Mary in Williamsburg, Virginia, who speculates that maybe—just maybe—the speed difference between the OPERA and supernova results could be chalked up to differences in the energy or type of neutrinos.

That said, no one is arguing that the OPERA results are in immediate need of a theoretical explanation, because there could be errors the team hasn't accounted for. The experiment relies on very precise timing and careful measurement of the distance between the neutrino source at CERN and the detector. John Timmer of Ars Technica does a good job of explaining how the OPERA team used fastidious accounting, GPS signals, and atomic clocks to reduce the uncertainty. But he notes that there are other potential sources of error that could add up, not to mention those pesky "unknown unknowns."

Many physicists seem to be looking forward to independent tests using two other neutrino experiments—the MINOS experiment in Minnesota, which captures neutrinos created at Fermilab, and another neutrino beam experiment in Japan called T2K.

But for now, we can only wait. And, perhaps, come up with explanations of our own. There's been a lot of activity on Twitter devising “mundane” explanations for the OPERA result. I've excerpted a few of my favorites below.

(Image: Istituto Nazionale di Fisica Nuclear)

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