One of the biggest news stories of 2011 was the tentative announcement from the OPERA experimental group at CERN that they had detected neutrinos which seemed to be moving faster than the speed of light. When I first mentioned this topic back in September ("Can the Speed of Light Law Be Broken?"), I explained why the equations from the theory of relativity indicate that the speed of light set an upper limit on how fast any object in the universe should be moving.
Skepticism was certainly warranted on this claim, as on any claim that would shake the foundations of science.
In science, the way skepticism works is that efforts are made to see if the claim holds up in different circumstances. In November, it was revealed that a series of follow-up experiments were consistent with the earlier findings (see "Speed of Light Update").
It was beginning to look like maybe this might be a real result, which would have been thrilling for physicists. Contrary to what some in the media portrayed, there was no real reason for physicists to be predisposed against this idea. If anything, they would be inclined to believe it. The discovery of actual evidence of something moving faster than the speed of light would open whole new areas of investigation! Many physicists get into the field with the goal of discovering some sort of science fiction concept like this.
Alas, more recent evidence seems to be making it clear that these OPERA results very likely contained serious errors.
In February, there was some hubub over an announcement from CERN about which identified two different defects "that could have an influence on its neutrino timing measurement," according to their press release.
- An oscillator used to provide the time stamps for GPS synchronizations may have led to an overestimation of the neutrino's time of flight.
- A flaw in the optical fiber connector that brings the external GPS signal to the OPERA master clock, which would have led to an underestimation of the time of flight of the neutrinos.
Further tests would be necessary to determine the magnitude of these specific errors. Tests with shorter pulses are slated for May.
ICARUS Weighs In
There's a second experimental apparatus, ICARUS, which is also in Gran Sasso, Italy, and it also detects beams from CERN. It turns out they received some of the neutrinos from CERN back in September as well and they've just completed an independent measurement of the travel time. Their result shows that these same neutrinos traveled slower than the speed of light, as predicted by relativity.
This, together with the acknowledgement of equipment failures at OPERA, makes it highly likely that speed of light limit was not violated in the original OPERA experiment. As CERN Research Director, Sergio Bertolucci, says in the press release:
"The evidence is beginning to point towards the OPERA result being an artifact of measurement ..."
The Way Science Works
It's easy to think that this is an example of science getting something wrong, but I don't feel that way. Results were found and the scientists released them to the wider scientific community. Follow-up measurements were made, while the scientists involved in the original experiment continued to check their own results. Science doesn't guarantee that results are always correct, but only that if the scientific method is followed, then any errors will eventually be uncovered ... and that's exactly what's happening here.
One could argue that they should have found the two equipment failures earlier, but sometimes mistakes do happen and slip through the initial checks. This is why science can't be performed in a standalone manner, but requires a community of independent researchers. (One of the best explanations of this aspect of science is in this Scientific American blog post by Janet Stemwedel, "Objectivity requires teamwork, but teamwork is hard." Check it out!)
Again, I think the Bertolucci makes an excellent point in the March 23 update to the press release:
Whatever the result, the OPERA experiment has behaved with perfect scientific integrity in opening their measurement to broad scrutiny, and inviting independent measurements. This is how science works.
- CERN press release, "OPERA experiment reports anomaly in flight time of neutrinos from CERN to Gran Sasso" - Sept. 2011 (updated in Nov. 2011, Feb 2012, & March 2012)
- Time, "'Faster Than Light' Neutrino Was Product of Error" - Feb. 22, 2012
- Scientific American, "Faster-than-light Neutrinos Explained" - Feb. 22, 2012
- Cosmic Variance blog, "Neutrinos and Cables" - Feb. 22, 2012
- Scientific American, "Not So Fast: Independent Measurement Shows Neutrinos Don't Exceed the Speed of Light" - March 16, 2012
- Nature, "Neutrinos not faster than light" - March 16, 2012
- Symmetry Breaking blog, "New neutrino measurement finds particles obeying speed limit" - March 16, 2012
- New York Times, "Einstein Proved Right in Retest of Neutrinos' Speed" - March 16, 2012