It was a pretty big deal when scientists at CERN announced that they had recorded neutrinos traveling faster than the speed of light in experiments conducted at a neutrino observatory in Italy which measured a neutrino beam generated some distance away at the main CERN facility.   The difference was tiny, only a few billionths of a second.  However, if true, it could turn science on its head.   The experiment was repeated several times and the results were clearly too great to be random statistical error.

At the time, many scientists expressed skepticism, and rightfully so.  All data to this point has indicated that nothing travels faster than the established speed of light.  Neutrinos have been observed from distant stellar supernovas, and they arrived at the same time as light from the supernova, indicating they did not travel faster.  However, it was suggested that the high energy levels of the neutrinos generated by accelerators may have pushed them a little faster.  Still, if true, this could undermine the foundation of relativity, a well tested and universally accepted fact in science.

Many things were proposed as an explanation for the discrepancy.  It could have been that the measurements of distances were not accurate, despite extreme steps being taken to confirm them.   It was suggested that there could have been relativistic factors involving the rotation of the earth or local gravity coming into play and causing distortions in time.

Now, however, we have a much simpler explanation.  While it has not been proven to be the case, suspicion has turned to a loose cable that was part of the time synchronization system.

Via the CS Monitor:

Loose cable could explain ‘faster-than-light’ neutrinos
Those famous neutrinos that appeared to travel faster than light in an Italian experiment last September probably did not do so after all. A faulty connection between a GPS receiver and a computer may be to blame for the mistake.

In September, and again in a repeat run in November, scientists on the OPERA team had detected neutrinos travelling from the CERN laboratory in Geneva to the Gran Sasso Laboratory near Rome at what appeared to be a light-speed-shattering pace. The neutrinos completed the trip about 60 nanoseconds faster than a beam of light would have done.

Though the physicists felt confident in their experimental setup, they and the rest of the scientific community suspected that the shocking result was probably due to some error, considering that light as the universe’s speed limit is a central tenet of Einstein’s theory of special relativity.

And indeed, in November, another group of physicists also working at Gran Sasso Laboratory demonstrated that the neutrinos in question could not possibly have been traveling faster than light, because if they had, they would have given off a telltale type of radiation, which was not detected.

Further complicating matters, even the OPERA scientists couldn’t yet explain why the neutrinos clocked in as fast as they did. Now, according to Science Insider, sources familiar with the OPERA experiment say a fiber optic cable connecting a GPS receiver and an electronic card in one of the lab computers was discovered to be loose. (The GPS was used to synchronize the start and arrival times of the neutrinos).

Tightening the connection changed the time it took for data to travel the length of the fiber by 60 nanoseconds. Because this data processing time was subtracted from the overall time-of-flight in the neutrino experiment, the correction may explain the seemingly early arrival of the neutrinos. To confirm this hypothesis, the OPERA team will have to repeat their experiment with the fiber optic cable secured.

The fiber optic cable was loose but was still connected to the system and signals were able to be passed. In many circumstances, this would not have made much difference, but since the measurements in this case had to be precise to within billionths of a second, it does matter. When measuring events like neutrino detection, it’s important to remember that the detectors and their signals are never truly instantaneous. When a neutrino strikes the detection medium, it produces light. That light takes a few picoseconds to reach the photomultipliers that detect it. The photomultiplier takes a few picoseconds to respond and the signal then goes through amplifiers and wiring before it is registered, taking nanoseconds or even microseconds. This all has to be accounted for. Lengths of cable must be measured precisely and the time for the signals to propagate calculated. Timing circuits must be equally precise and compensated.

The image above and to the right shows some of the stacks of detection medium at the OPERA neutrino observatory, where these observations were made.

Because the fiberoptic cable was loose and did not have as solid a connection as expected, the light that was being transmitted to the instrument can bounce around a little bit before it is detected. The fine calibration of the system that registers each pulse of light may be upset or the pulses of light can be distorted, causing them to trigger a timer differently.

It should be noted that we do not have proof positive that this caused the discrepancy. The experiment will need to be repeated with the loose connector fixed in order to establish more reliable data. However, it now appears that this was likely the culprit. This is indeed how science should work. Extraordinary claims should be met with skepticism, regardless of how well established those making the claims are. Ultimately, judgement must be held until the experimental setup and data have been very thoroughly audited and reviewed and the results confirmed by other researchers repeating the experiment independently.

I’d be willing to bet money that the results will not show neutrinos traveling faster than light after the experiment is repeated with all the timing systems double-checked. If that is the case, those involved will certainly have some egg on their faces, but hopefully they will not receive too much professional rebuke. In such a complex scientific experiment, mistakes can happen and in this case, the scientists did the right thing by opening their data to the interpretation of others and inviting them to find flaws. No scientist should ever be penalized for saying “It turns out I was wrong. I thought I had good data I could be confident in, but it was pointed out that there was a flaw I was unaware of.”

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This entry was posted on Thursday, February 23rd, 2012 at 11:26 am and is filed under Good Science, Misc. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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