In the last six weeks the physic world may have witnessed the overturn of Einstein’s theory of special relativity or we may have only witnessed an interesting example of the effect of Internet dynamics and sociology on scientific discourse. As of this posting we do not know which alternative is correct.
As has been publicized everywhere, in newspapers, in blogs and via scientific electronic publishing, on September 23, 2011 researchers from the OPERA experiment announced the astonishing possibility that muon neutrinos may acquire velocities larger than c, the velocity of light. [arXiv:1109.4897v1]
The layout of the OPERA experiment.
The production of the muon neutrino beam at CERN [arXiv:1109.4897v1]
The OPERA detector in the underground Gran Sasso Laboratory.
Their abstract summarizes the experiment and result simply.“The OPERA neutrino experiment at the underground Gran Sasso Laboratory has measured the velocity of neutrinos from the CERN CNGS beam over a baseline of about 730 km with much higher accuracy than previous studies conducted with accelerator neutrinos. The measurement is based on high statistics data taken by OPERA in the years 2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the OPERA detector, as well as a high precision geodesy campaign for the measurement of the neutrino baseline, allowed reaching comparable systematic and statistical accuracies. An early arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (60.7 ± 6.9 (stat.) ± 7.4 (sys.)) ns was measured. This anomaly corresponds to a relative difference of the muon neutrino velocity with respect to the speed of light (v-c)/c = (2.48 ± 0.28 (stat.) ±0.30 (sys.)) ×10-5.”
Incidentally, I have a soft spot in my heart for this experiment because its first purpose was to detect the oscillation of muon neutrinos into tau neutrinos. This has been accomplished. [arXiv:1107.2594]
The OPERA paper also summarizes previous measurements of neutrino velocity. “With a baseline analogous to that of OPERA but at lower neutrino energies (Eν peaking at ~3 GeV with a tail extending above 100 GeV), the MINOS experiment reported a measurement of (v-c)/c = 5.1 ± 2.9×10-5 [MINOS Collaboration, P. Adamson at al., Phys. Rev. D 76 (2007) 072005.]. At much lower energy, in the 10 MeV range, a stringent limit of |v-c|/c < 2×10-9 was set by the observation of (anti) neutrinos emitted by the SN1987A supernova [K. Hirata et al., Phys. Rev. Lett. 58 (1987) 1490; R. M. Bionta et al., Phys. Rev. Lett. 58 (1987) 1494; M. J. Longo, Phys. Rev. D 36 (1987) 3276].
Since then there have been a great number of papers and blog comments on the result. Some recent papers take this result as new physics and connect it with known or speculative phenomena such as : violation of Lorentz invariance, dark energy, dark sector neutrinos and so forth. Other papers present arguments against the possibility of the measurement being correct, for example “New Constraints on Neutrino Velocities” by Cohen and Glashow, arXiv:1109.6562v1.
Some blog comments present analysis showing where the OPERA experimenters may have made a mistake. But the result is statistically strong and if the conclusion is wrong it must be due to an error or errors in the experimental method: perhaps in the timing measurement method or in determining the 730 km baseline. My experience is that it is often difficult for outside researchers to find errors in a well designed experiment exhaustively studied by the inside experimenters. One example of a possible error pointed out by an outsider is a special relativity correction to the GPS timing, discussed by van Elburg [arXiv:1110.2685v2]. Another problem in finding errors in the research results of others is that sometimes the suspicious result comes from a number of errors.
My piecemeal and anecdotal survey of opinion in the physics community is that the consensus of interested physicists is that the OPERA result is mistaken, there is no consensus as to where the mistake or mistakes lie. But as my Ph.D. advisor, Isadore Rabi, said “Physics is an experimental science”. The great number of papers on the internet do not replace more experiments.
However as of this posting, October 29, 2011, we have only one additional experimental result, that from the ICARUS experiment. [arXiv:1110.3763]. The ICARUS experiment is a large liquid argon Time Projection Chamber in the Gran Sasso Laboratories that sits in the same muon neutrino beam as OPERA. The ICARUS experiment measures the energy of neutrinos that interact in the liquid argon and also records a ‘picture’ of the interaction. It does not measure the travel time of the neutrino from CERN. If one applies the theoretical deductions of Cohen and Glashow to the ICARUS results there are no faster-than-light neutrinos in the beam. A strong but indirect argument against the OPERA result.
We would all like to see a speedy settlement of the question- is the OPERA result correct? How rapidly can the question be settled? What about the OPERA experimenters repeating their experiment. Since the statistical situation is so strong, a repeat would have to involve physical changes in the apparatus. For example a large change in the mean neutrino energy or a different neutrino detector timing system or a major change in the neutrino beam. It will take time for the OPERA experimenters to make such changes and study the
results, even if the actual new running time is short. Probably a month or more. We should be patient.
An obvious settlement path is repeating the experiment using the other long baseline neutrino apparatus, MINOS [The MINOS Experiment and NuMI Beamline] with a 734 km baseline and T2K with a 300 km baseline. The MINOS experimenters at Fermilab have discussed two directions for doing this. One direction is to go through old data trying to improve the timing information . This is worth doing but I am skeptical as to how much clarity this will provide. The other MINOS direction is to upgrade their timing system and take new data. But this will take of the order of a years or more. The experiment must be carried out, the data analyzed and the MINOS experimenters be sure they are right.
The T2K experiment [arXiv:1106.1238v2] in Japan can also look at old data and upgrade their timing and detection systems for their next runs. The latter direction will also take a year or more.
Thus we are in the midst of a dichotomy between (a) the short times- hours to days- required for us to think and calculate and then to report on the Internet and (b) the long times-years- required to carry out an experiment, analyze it, and be sure the result is right. The Internet gives us the illusion of speed in physics research.
It would be marvelous if the OPERA physicists in the end are correct. Their result would produce a revolution in our understanding of the relativistic world, equal to the revolution produced by quantum mechanics in the classical world. If the result is wrong then the superluminal muon neutrino concept will fade away. The fading process will be similar to what happened to the gravitational fifth force experiments and theories of the late 1980’s [E. Fischbach and C. Talmadge, Nature 356, 207 (1992)], the excitement, the puzzles, the mistakes to be remembered by old-timers such as myself. An odd sidelight is that a recent paper by Dvali and Vikman [arXiv:1109.5685v1] discusses superluminal neutrinos in connection with a gravitational fifth force.
November 20, 2011 Update
The OPERA experimenters have submitted their paper “Measurement of the neutrino velocity with the OPERA detector in the CNGS beam” to the on-line, referred Journal of High Energy Physics (JHEP). The conclusion remains about the same, relative difference of the muon neutrino velocity with respect to the speed of light (v-c)/c = (2.37 ± 0.32 (stat.) +0.34 (sys.) -0.24(sys) ×10-5. There is an important addition: the results of of a small data, low proton beam intensity run show that the beam time structure is understood. This new paper is arXiv:1109.4897v2.
This new paper and its submission to a referred journal appears to have changed few minds, most concerned physicists remain skeptical. But I am puzzled, the very experienced and competent OPERA experimenters have had time to consider the dozens of suggestions, mostly on blogs, as to why the results are wrong; suggestions ranging from GPS problems to timing problems to relativity problems. There are numerous comments that the next step is to wait for results from other experiments to be done, particularly from MINOS.
But I remain puzzled and patient and fascinated. I will continue to follow this fascinating physics on this post.