Reprinted from sciencebits.com with author's permission
Three weeks ago I visited the underground laboratory of Gran Sasso near l'Aquila. Little did I know that it would make headline news so soon, for "discovering" particles moving faster than the speed of light. Since a few people asked me what did I think of it, I decided to write something about it here.
Before I get into the details, here's a recap of the news item.
The OPERA experiment, which is one of a dozen or so running at the underground laboratory at Gran Sasso in Italy (the largest underground laboratory), came out with sensational results: they measured neutrinos traveling faster than the speed of light. Now the details.
The OPERA experiment was built to detect 17 GeV or so neutrinos which are produced at CERN, and aimed towards Gran Sasso 700 km's away. The idea is to try and find muon neutrinos that "oscillated" and changed into tau neutrinos. This can be expected because neutrinos appear to have a small mass. (For the physicists of you, this is a consequence of the fact that the "flavor" eigenstates are not the mass eigenstates – this is akin to two pendula coupled with a very week spring, as a consequence, if one perturbs one pendulum, eventually the oscillation passes to the other pendulum and back). Anyway, they have thus far detected one(!) such event (see arxiv.org/abs/1006.1623).
Besides measuring the neutrinos and their flavor (using strong magnets, to see how massive is the particle that they form [whether it is a muon or a tau particle, both of which are heavier brothers to the electron]), they also measure the timing of the events. This they can compare to the expected time it takes the neutrinos to travel between CERN at Geneva and Gran Sasso near l'Aquila. Under standard theory of special relativity, because the neutrinos are extremely relativistic, this time should be very nearly the time it takes light to cross this distance. However, they found that it took the neutrinos less time to cover it, 60 nanoseconds less (which is equivalent to cover 18 meters more). This implies, if the measurement is right, that the neutrinos have travelled faster than the speed of light. Is this correct?
The gut feeling I have when I hear something like that, is that there is something wrong in the experiment. The reason is that the speed of light is so deeply engrained in physics, that giving it up would require very convincing results. This result, on the other hand, is not. And why is it so?
To begin with, a good experiment to measure anything which is not "zero" (e.g., the difference of something from the speed speed of light, a non-vanishing mass) is to plan an experiment that gives a signal if the measured quantity does not vanish, and no signal if there is no difference. One classic example is the Michelson-Morley experiment. The set up was such that there would be no change in the diffraction fringes if there is no compressed aether, and a changing pattern if there is (all this, without having to accurately know the length of the arms in the experiment or their difference).
The present experiment simply measures a very large number, which is the time it takes the neutrinos to travel from CERN to Gran Sasso, and then this large number should be compared with another large number, which is the time it takes light to travel this distance. One should then hope that both are calculated/measured accurately enough such that their difference is calculated with a high enough accuracy.
The second reason why my guts tell me that the measurement is not due to superluminal neutrinos is because (anti-)neutrinos at a lower energy (of several MeVs) were measured to have a speed which is very nearly that of the speed of light, at one part in a billion. This means that this superluminal effect so happens to kick in at the right energy for OPERA.
The third reason is that particles at this energy ballpark were already extensively measured. It would have been a different story if there would have been a claim that cosmic rays at 1021eV are superluminal, because that is a regime which wasn't studied on Earth.
Bottom line, I find it hard to fathom that neutrinos are superluminal. But I don't have a good explanation for the measurements either. (Frankly, I have other things to think about.) So, what can the source be? Look at the list of possibilities summarized in Luboš Motl's blog, perhaps one of them is the reason.
The author standing in front of the best OPERA in town, 1400 m below the surface, in the Gran Sasso Laboratory. Click to zoom in.
Let me end with the first super-luminal neutrino joke I have heard:
"We don't allow faster than light neutrinos in here," said the bartender. A neutrino walks into a bar.
LM: You may also listen to a fresh radio interview with Lisa Randall, on neutrinos, Tevatron, LHC, and her book, on Ira Flatow's NPR radio show (plus transcript).
LM: You may also check a paper by my fellow birthdaymate Sheldon Glashow and Andrew Cohen, an ex-co-author of mine, who argue that superluminal neutrinos are impossible also because they would emit eine Bremsstrahlung.
This is a cute argument but I guess that those who are eager to abandon relativity will have no problem to claim that the formulae to quantify the intensity of der Bremsstrahlung near the speed of light should be modified, too. After all, the breakdown of relativity would lead to many other pathologies. For example, one could construct perpetuum mobile devices employing black holes which would have different horizons for different particle species (neutrinos vs photons etc.).
LM: In another preprint, Carlo Contaldi elaborated upon one of the mistakes suggested by your humble correspondent (search for "simultaneity"), namely that the experimenters could have been sloppy about the definition of "simultaneity of two events" which is important for synchronization and which, according to special relativity, depends on the speed of your inertial system.
My numbers indicated that they would have to incoherently mix the static Earth's frame with the satellites' frame (i.e. to synchronize the time from the satellites' viewpoint while using the Earth's frame in other contexts) – the speed of the satellites is enough to produce 30 nanoseconds in this way (one half of the OPERA speed excess). However, in my opinion, the speed of the Earth's rotation itself (and the time difference produced by this speed out of 730 km via the Lorentz transformation) isn't enough to explain away most of the 60 nanoseconds: the speed is just too low.
Contaldi seems to disagree but one probably has to study his paper in more detail to see what is the exact effect he claims to have been neglected.