OPERA review serves up a feast for physics geeks
Superluminal neutrinos and silly science writers
Let’s get the “big news” out of the way first: there’s a lot of excitement due to one paper published on Arxiv.org, which asks whether the CERN OPERA experiment – the one that seemed to detect superluminal neutrinos – took into account the “satellite reference frame” in its calculations.
So the short version of the paper is this: did the calculations conducted on the OPERA data treat the clocks on the GPS satellites as being stationary with respect to the detector (more precisely for The Register’s crowd-sourced scientific sub-editors, I mean “in the same reference frame as the detector”)? If so, van Elburg argues, it would be a source of error, because the satellites aren’t stationary; and the error he predicts a worst-case correction of 64 nanoseconds.
The media excitement is comprehensible for several reasons: the paper, by Roland van Elburg of the University of Groningen, yields a calculation quite close to the 60 nanoseconds that the neutrinos seemed to travel too fast; it would therefore restore relativity (and give journalists an opportunity for some “you was wrong!” schadenfreude).
It also suits journalists’ sense of narrative: Einstein’s theory used to prove that Einstein was right.
The most sensible discussion of this hypothesis I've seen is on the Bad Astronomy blog, here.
What the sudden leap for “Einstein still right” headlines fails to convey is the ongoing scrutiny being applied to the OPERA experiment. It hasn’t been called into question by one paper; what’s happening is an intense scrutiny that’s mostly passing unnoticed.
By my count, there have been 47 papers published at Arxiv.org discussing the OPERA results one way or another. So why just focus on one? Instead, let’s scan a few of the ideas, discussions and quirks of this “gold standard peer review” (as one reader described it recently).
Causality safe, for now
One of the first things that got everybody excited when the OPERA results first hit the wires was that the possibly-superluminal neutrinos traveled “backwards in time”. By my admittedly-poor understanding of relativity, this was mostly a journalistic construct, since in a human time-frame, the neutrinos still arrived after they left.
Nonetheless, time travel grabs the attention like few other topics, apart perhaps from climate conspiracies and crop circles. So please don’t direct any hate mail to me or to the physicists for telling you it ain’t so, at least not yet.
According to a German-Hungarian group of physicists, even if the neutrinos could travel faster than light, they can’t send information backwards in time. Their paper reminds us all that the neutrino beam would need, somehow, to be modulated. For us to be satisfied that a bit of information has been transferred, the trailing edge of the modulated pulse, not the leading edge, has to be observed fast enough to satisfy the “faster than light” requirement.
It may be, they admit, that a setup could be devised to defeat this, but at the moment, causality is safe from messages-from-the-future.