Has The Speed of Light Been Broken?

In any discussion about neutrinos, take anything with a very large grain of salt. These particles, long thought to have zero mass, have an incredibly low interaction rate with anything: for example, the Sudbury Neutrino Observatory, designed to detect neutrinos arriving from the sun, counted them at a rate of about one per hour out of a total flux in the millions or even billions.

That being said, the recently published results are significant because they start with, and detect, varieties of neutrinos which should not ordinarily be present in the background (or at worst, should be exceedingly rare). The experiment itself is designed to detect changes in neutrino “flavour”, in this case a muon neutrino becoming a tau neutrino (the other neutrino flavour is associated with the electron). The neutrino beam is produced at CERN in Geneva, and detected at the Gran Sasso Laboratory of the Istituto Nazionale di Fisica Nucleare in northern Italy, a distance of 732 km (measured through the earth) away.

The huge flux of solar neutrinos (and indeed most other neutrinos of “cosmic” origin) mentioned above is not a problem due to the engineering design of the detector (OPERA at Gran Sasso) and the beam source (a highly focused proton beam crashing into a graphite block at CERN). The resultant neutrino beam is very narrow, and has a relatively narrow range of neutrino energies, making it fairly easy to make sure the neutrino one counts at OPERA came from the right direction, and (from momentum calculations) from the right source.

(I sure hope I haven’t lost everyone already :-D)

Today’s result (as reported by CBC): the neutrinos at Gran Sasso arrived 60 nanoseconds too soon (uncertainty ±10 ns), with a total travel time from CERN of 2.435018 milliseconds at the speed of light. Translated into distance, this means that the neutrino seems to travel approximately 17.9 meters farther than it should, in the given time. In terms of speed, the neutrinos are apparently travelling approximately 828 km/s faster than light (in vacuum).

The observed travel times are a result (see the BBC article jdog cites) of some 15,000 measurements taken, I assume, over the lifetime of the experiment to date (it has been online since August 2006). The accuracy of the speed of light is not at issue; that has been defined exactly since 1983 (see also the SI definition of the second).

The total distance travelled is more problematic, as this is highly dependent upon the shape of the earth, the locations (latitude/longitude/altitude) of both Geneva and Gran Sasso, and also the possible presence of any local mass anomalies in the earth’s crust and mantle (this is a mountain range, after all; it’s the subduction zone between the African and Eurasian plates). Leaving all topographical questions to the geographers, I will simply state that the stated result can be accounted for simply by an error of 24 parts per million in the length of the neutrino path from CERN to Gran Sasso.

I hope I’ve answered jdog’s questions to your satisfaction (I’ve certainly answered them to my own :cool2:); however, one comment he makes is telling, because it points to another nagging question, one he does not ask: ” It’s not like you can easily label subatomic particles.”

The question is simply this: How does CERN tell Gran Sasso that it has switched on its beam (or altered its nature, eg. polarization)? Anyone who has ever used the “ping” or “traceroute” commands over the Internet should know already where I am going with this. For Gran Sasso to know the neutrinos are coming, CERN must send a signal of some sort, and this presumably travels along an optical fiber path between the two locations (optimal method, in my opinion).

Now questions abound in great numbers. What is the total length of the fiber chain? How many times have fibers been joined together, either by cleaving/splicing, or through a mechanical connector? What strain and/or temperature variations are there along the way? All these factors will have an effect on the fiber’s index of refraction, hence on the signal travel time, and 24 ppm does not seem to be too outlandish an error to expect. All of this must be added to any uncertainty in transmitting the signal in the first place.

The final nagging question has been asked probably by every physicist alive today, namely: Why have we never seen this before? There are perhaps only two other facilities on earth today that can replicate the experiment, these being Fermilab in the USA and Super-Kamiokande in Japan, the latter unfortunately being currently offline since the recent earthquake and tsunami.

All we can do now is await any results these two institutions may publish in the future. Personally, I am on the side of relativity, and fully expect the current result to be primarily a result of systematic errors in both the neutrino path length and the “beam on” signalling apparatus.

Final footnote: since I am no longer doing active research in physics, I am in a sense glad to see this news in the mass media, as it allows me to keep abreast of current developments. In terms of both cosmology and elementary particle physics, this is a far more exciting time than nearly 40 years ago, when I was just beginning my post-graduate work. I am definitely looking forward to my next life (I am, after all, a Buddhist ;-)), when, if there is any justice at all in the universe, I will be able to continue what I have already begun. I figure that, 30 or 40 years from now, what we are seeing today will seem like chicken scratches in the ground, that in that future time, the truly important work will only have just begun.

In another sense, I am also disappointed to see it published so soon, and with so little clarification. The CBC and BBC reports are written with a certain degree of sensationalism and certainty, as if the issue is now settled, but that is far from the truth. To discern the true present situation, one must read far into both articles, something which the average reader will never do (journalistic rule number 2: say everything of importance in the first 2 paragraphs, no one will ever read the rest).

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