Tag Archives: sources of error

No Faster-Than-Light Neutrinos Yet

At the end of September, a paper was published with the provocative conclusion that neutrinos had been measured to travel faster than the speed of light. It was big news, and was widely reported in the media as a reasonably established finding (when in reality it had been released on arXiv, an open-access physics portal, but was not yet peer reviewed, and was subject to much eyebrow raising from physicists at large). In the past week, news has come out that there were some systematic sources of error, and the results are not necessarily accurate.

Here’s a quick recap of what the original experiment entailed:

  • a beam of neutrinos was generated at the Large Hadron Collider (LHC), and the beam was aimed through the Alps (ie, underground) to a detector in Italy called OPERA about 730 km away
  • the signal leaving the LHC is timestamped, using a highly accurate (and very carefully calibrated) GPS system
  • as the neutrinos leave the LHC, a light signal is sent to the same location in Italy via a fibre optic cable. This provides a time-of-flight for a light signal to compare to the time-of-flight for the neutrino signal.
  • when the light and neutrino signals are detected in Italy, they are timestamped using the GPS system, and the times of flight cane be compared
  • this repeated over and over again to reduce statistical errors in the measurements
Schematic diagram for the experiment.

The scientists reported that there was a difference in the times-of-flight of about 60 ns, with the neutrino beam reaching the detector before the light signal. Now, the scientists involved have found two sources of error, which, indicentally, mirror my hunch from when this first hit the news. First, the GPS equipment is operating far outside of its normal operating range, and so my not behave exactly as expected; this error is thought to produce a faster neutrino speed. But the second source of error is a loose connection for the fibre optic cable that carries the light signal, introducing a delay that may account for the difference in time elapsed for the two signals (light and neutrino).

I’m not at all surprised at this — when the paper was first put out, I thought it was only a matter of time before a systematic source of error was found. General relativity has held up spectacularly in every experimental test undertaken, and it would take a lot to upend all of that.

I was surprised that the group released their paper as early as they did, and without initial peer review, and I think that speaks to the authority that the scientific community confers on the CERN collaboration. If this exact paper had been written be a group at a small, less renowned institution (assuming they had all the equipment to do the experiment), would it have ever seen the light of day? Would a smaller group release very controversial results which naturally invite a huge amount of attention from popular media, without even subjecting them to peer review first? Would they ask for scrutiny from all and sundry, not just their peers?

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