We published a news story recently suggesting that Albert Einstein, the Mighty Hip Einie, got one thing wrong, or at least not quite totally right: the universal upper limit constituted by the speed of light. Precise-timing GPS receivers in a Geneva lab helped indicate that subatomic neutrinos can travel at a velocity just a smidge faster than the speed of light. Someone at a burning idea factory in the Netherlands riposted that the scientists erred in their conclusion because they failed to take into account the relative movement of the GPS clocks in space and thus miscalculated the neutrinos’ time of flight. We hereby refute that assertion with our heavy-lifting Innovation columnist, Richard B. Langley.
The original news story, derived from a breathless announcement out of the European Organization for Nuclear Research (CERN) in Geneva, Switzerland, reported that Septentrio’s precise-timing GPS receivers PolaRx2eTR synchronize the time bases at CERN and Gran Sasso Laboratory in Italy, 730 kilometers away, for the OPERA experiment. Researchers at the two labs synchronized to an accuracy of a couple of nanoseconds and then measured transit speeds of 15,000 neutrino events in a neutrino beam between the two sites.
Light moves at 299,792,458 meters per second. Let’s see, doing the math, that’s 299,792.5 kilometers per second, divide into the distance from Geneva to Gran Sasso, carry the one, cross the fingers, spit downwind, gives 0.002435017553808 seconds. Two-and-a-half thousandths of a second. 24,350 nanoseconds. If the neutrinos got to sunny Abruzzo any sooner, well then, they were the new universe record-holders.
It turns out, the little buggers made the trip 60 nanoseconds faster than that. Killing it. Just killing it. And poking a hole in the Mighty Hip Einie’s Special Theory of Relativity.
“This result comes as a complete surprise,” said Oscillation Project with Emulsion-tRacking Apparatus (OPERA) spokesperson Antonio Ereditato.
Then Ronald van Elburg of the University of Groningen in the Netherlands climbed into the ring. The OPERA project researchers did not take into account the relative movement of GPS clocks in space and thus miscalculated the distance, he said. “From the perspective of the clock, the detector is moving towards the source and consequently the distance travelled by the particles as observed from the clock is shorter.”
Thus, according to van Elburg, the travel time measured by the GPS was shorter than the travel time measured in the reference frame on the ground. Accounting for the changing distances between the GPS clocks and the neutrino detectors would lengthen the observed time of flight by 32 nanoseconds on each end of the experiment, making for a total time delay of 64 nanoseconds — close to the interval that OPERA observed using the difference between the speed of neutrinos and that of light. Case dismissed. Einstein restored.
Unable to parse this myself, I asked Richard Langley of the University of New Brunswick whether it seemed reasonable.
“No, I don’t think so,” Langley replied. “Special Relativity is already taken into account whenever GPS is used, whether for timing or positioning, which amounts to the same thing, since 1 nanosecond of timing error equals about 30 centimeters of distance error (simply using the speed of light). Of course, anyone can use GPS incorrectly or infer something incorrectly. There is an error (likely) somewhere but I don’t think it is in the “standard way” that clocks are synchronized using GPS. The error is either a timing error (unrelated to Special Relativity but perhaps related to the electronics and associated delays) or a neutrino-path-length measurement error. The OPERA folks have put online their internal reports on the calibration of the GPS time link between the neutrino emitter and detector:
and on how the distance travelled by the neutrinos was determined:
I haven’t had time to read these reports yet but it appears, at face value, that the work was quite thorough.
“By the way,” Richard said, “there have been a number of relevant articles in GPS World over the years. And we do apologize that some of these are no longer available digitally, due to a trashing of this site by its former owner, Questex Media Group.
- GPS time transfer: Using precise point positioning for clock comparisons
- Time and frequency dissemination: Advances in GPS transfer techniques
- New IGS clock products: A global time transfer assessment
- Time and frequency transfer: High-precision using GPS phase measurements
- Relativity and GPS
- He must have kept on thinking about it, in the way that scientists do, because the next day he wrote me again:“Here is a good overview of some of the suggestions that have been proposed to track down the problem with the OPERA results: http://www.theregister.co.uk/2011/10/18/reviewing_the_opera_papers/Some pretty intelligent discussion here, too: http://blogs.discovermagazine.com/badastronomy/2011/10/15/followup-ftl-neutrinos-explained-not-so-fast-folks/,” Langley concluded.
If you haven’t slaked your thirst yet, there is a Quantum diaries blog post: “Elementary, my dear neutrino . . . “ and quite a few photos photos from the OPERA collaboration:
Richard adds an interesting fact: “How many physicists does it take to time a neutrino: at least 174. That’s the number of authors on the OPERA paper! The names of the authors and their affiliations takes up the first two pages of the paper!”