Relativity-7: What could be other reasons for the CERN-Gran Sasso results?

(For previous posts in this series, see here.)

The reactions to the reports of the CERN-Gran Sasso discovery of possibly faster-than-light neutrinos open a window into how science operates, and the differences in the way that the scientific community and the media and the general public react whenever a result emerges that contradicts the firmly held conclusions of a major theory.

The initial reaction within the scientific community is almost always one of skepticism, that some hitherto unknown and undetected effect has skewed the results, while the media and public are much more likely to think that a major revolution has occurred. There are sound reasons for this skepticism. Science would not have been able to advance as much if the community veered off in a new direction every time an unusual event was reported.

What usually happens is that most of the community goes on as before as if nothing had occurred while a relatively small number who are experts in that area examine the new results closely. Some will try to identify possible sources of systematic errors that the original experimenters did not consider. The experimenters who reported the possibility of faster-than-light neutrinos are reportedly careful people and if any errors occurred, we can be sure that they are not trivial ones that will be uncovered easily or quickly. Others will examine if any of the side effects that would accompany faster than light travel are also seen. If those two efforts fail to turn up any problems, other groups will try to repeat the basic experiment with different experimental set-ups, measuring the time and distance using different techniques so that the likelihood of systematic biases pushing the results in the same direction is reduced. The last option is very expensive and time-consuming, since these experiments are very difficult to do, which is why it is usually the last resort. During this period, there will often be claims and counter-claims and some confusion until the dust settles and a consensus emerges. But it is this painstaking investigation seeking replicability and consistency that characterizes science and enables it to be confident that once a consensus emerges, that it has produced reliable knowledge.

In this case, recall that the original experiment (which has the acronym OPERA) that aroused such interest involved sending neutrinos over a distance of 730 km and measuring their speed, where the distance and time measurements used GPS satellite technology. Assuming that 730 km was the exact distance, if the neutrinos travelled at exactly the speed of light, it should take them 2.435 milliseconds to make the trip. What was observed was that the neutrinos arrived 60 nanoseconds earlier than expected, thus violating Einstein causality, though not overthrowing the theory of relativity. This effect would go away if there were a 60 nanosecond error in the time measurement and/or an 18 meter error in the distance measurement of the journey, and searching for hitherto unconsidered factors that could produce effects of that size has been the initial focus.

There have already been some developments. When it comes to looking at sources of systematic errors, Lubos Motl has a long discussion on possible errors and has compiled a partial list of potential sources that need to be examined closely.

  • inconsistencies in the whole GPS methodology of measuring space and time coordinates
  • inconsistencies of units (meters, second) used at various places: the errors would have to be huge, indeed, so this is unlikely
  • subtle old-fashioned physics issues neglected by GPS measurements: the index of refraction of the troposphere and (even more importantly) ionosphere that slows down and distorts the path of GPS signals; confusing spherical Earth and geoid; neglecting gravitational effects of the Alps; neglecting magnetic fields at CERN that distort things; and so on
  • forgetting that 2 milliseconds isn’t zero and things change (e.g. satellites move) during this short period, too
  • subtle special relativistic effects neglected in the GPS calculations
  • subtle general relativistic effects neglected in the GPS calculations
  • wrong model of where and when the neutrinos are actually created on the Swiss side
    more radical: wrong model of the wave equation for the neutrinos (regardless of oscillations etc., neutrinos should never move information faster than light in the vacuum, but maybe we’re doing some mistake about the group vs phase velocity and entanglement of the two places: recall that the difference between the phase and group velocity for these neutrinos should be negligible, around 10

Notice that a lot of the suggested errors focus on the GPS or the Global Positioning System. This currently consists of 31 orbiting satellites that are continuously emitting signals that include the time the signal was sent as well as the orbital information of the satellite. Receivers on the ground (such as in your car) take that information and calculate the position of the receivers. The OPERA experiment used such signals to pinpoint the locations of the detectors at CERN and Grand Sasso and the time of travel. Most everyday situations do not require very high levels of accuracy. But since time interval errors of just 60 nanoseconds or distance errors of 18 meters could nullify the results, people have been looking into the possible sources of subtle errors, especially those associated with Einstein’s general theory of relativity.

Next: General relativity effects.


  1. Paul Jarc says

    It seems to me that Einstein causality depends on there being some maximum speed for information transmission, but I don’t see how it would require that light in particular must travel at that speed. Could you elaborate on that?

  2. says


    You are right, it doesn’t, and I will write about it more later in the series. Some people have speculated that if the results hold up, then we may have to identify a new maximum speed that is decoupled from the speed of light.

    But doing so introduces other problems that will need to be addressed.

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