Thursday, September 22, 2011

Superluminal neutrinos - a case study in how good science is done

As many people have now heard, the OPERA collaboration is reporting a very surprising observation.  The OPERA experiment is part of CERN, and is an experiment meant to study neutrino flavor oscillations.  The idea is, the proton beam at CERN creates a beam of neutrinos.  Since neutrinos hardly interact with normal matter, they move in a straight line right through the earth, and pass through the experimental station in Gran Sasso, Italy, where some small fraction of them are then detected.  There are (according to the Standard Model) three flavors of neutrinos, the electron neutrino, muon neutrino, and tau neutrino.  It has been determined experimentally that those flavors are not exact "mass eigenstates".  That means that if you start off with a tau neutrino of particular energy, for example, and let it propagate for a while, it will change into a muon neutrino with some probability that oscillates in time.  Anyway, OPERA wanted to study this phenomenon, and in doing so, they measured the time it takes neutrinos to go from their production point at CERN to the detector in Gran Sasso, using precisely synchronized special clocks.  They also used differential GPS to measure the distance between the production point and the detector to within 20 cm.  Dividing the distance by the time, they found much to their surprise that the neutrinos appear to traverse the distance about 60 ns faster than would be expected if they traveled at the speed of light in vacuum.

So, what could be going on here?  There are a few possibilities.  First, they could have the distance measurement wrong.  This seems unlikely, given the use of differential GPS and the sensitivity (they could clearly see the change in the distance due to a 2009 earthquake, as shown in Fig. 7 of the paper).  Second, they could have a problem in their synchronization of the clocks.  That seems more likely to me, given that the procedure is comparatively complicated.  Third, there is some other weird systematic at work that they haven't found.  Fourth, neutrinos are actually tachyons.  That would be all kinds of awesome, but given how challenging it would be to reconcile that with special relativity and causality, I'm not holding my breath.

Why is this an example of good science?  The collaboration spent three years looking hard at their data, analyzing it many different ways, checking and cross-checking.  They are keenly aware that a claim of FTL neutrinos would be the very definition of "extraordinary" in the scientific sense, and would therefore require extraordinary evidence.  Unable to find the (highly likely) flaw in their analysis and data, they are showing everything publicly, and asking for more investigation.  I want to point out, this is the diametric opposite of what happens in what I will term bad science (ahem.  Italian ecat guys, I'm looking at you.).   This is how real experimental science works - they're asking for independent reproduction or complementary investigation.  I hope science journalists emphasize this aspect of the story, rather than massively sensationalizing it or portraying the scientists as fools if and when a flaw is found.

10 comments:

Don Monroe said...

Since everyone agrees that this is likely to be an artifact, why should it be all over the news? The stories I've seen, like the one in the New York Times, mostly report the unlikely nature of the result well, but why report it at all?

Your point is good, that after doing all the checking they can, the authors are putting it out to be shot down. But I can't see what the public gets out of watching this particular sausage get made.

Luis Gregório Dias said...

I partially agree with the "good science" part: there are many motivations behind such an "extraordinary" claim and "doing good science" is only one of them.

It's interesting, though, that a FTL neutrino claim had already been reported 4 years ago (with a smaller statistical significance) so there might be something into it...

David McGloin said...

I might be wrong here, but the implication from the stuff I read on this yesterday was that the press release was out before the arxiv paper. Good science is not via press release. Far better, I think, would have been to get the results verified (or more likely refuted) by other experiments and then publish. There seem to be increasing reports like this in the press from CERN and other high energy labs, which ultimately turn out to be wrong. It's great to see physics on the front pages, but this just gives the wrong impression to the public, who vaguely remember the headline, but won't see the follow up in a couple of years saying it was a instrumentation error. They'll remember, "Einstein was wrong." This is akin to a tabloid newspaper 'exclusive shocker' headline.

Doug Natelson said...

I do share your misgivings about the level of attention this is getting, given that it's almost certainly a flawed result.

David, as far as I can tell, there was no press release. There was a blog post rumor. The preprint went up on the arxiv last night, and there is a talk at CERN today. Now, it's completely reasonable to argue about whether the media should pay attention to arxiv preprints that are not peer-reviewed.

DanM said...

Putting aside the issue of sensationalization, Eric Berger's blog post on this topic has a comment linking to an analysis of the statistics from the OPERA result, suggesting that they screwed up the statistics in a really rather naive way, and reducing their 'sensational' result to a mere curiousity. I only glanced at it briefly, but it's hard to imagine they were that careless. Whaddayathink?

Doug Natelson said...

Dan - I agree, it's hard to imagine that these guys don't know how to do their statistics properly, particularly since they've been staring at some of this data for three years. The most likely scenario is still some kind of timing problem, for my money. I did think it was interesting that they brought in two independent sets of people to check their clock synchronization, though.

ingilizce tercüme said...

İ do agree with this part at all!
İngilizce Tercüme

Nalliah said...

All of my investigations seem to point to the conclusion that they are small particles, each carrying so small a charge that we are justified in calling them neutrons. They move with great velocity, exceeding that of light - Nikola Tesla 1932

Experimental tests of Bell inequality have shown that microscopic causality must be violated, so there must be faster than light travel. According to Albert Einstein's theory of relativity, nothing with nonzero rest mass can go faster than light. But zero rest mass particles can go faster than the light. Neutrinos have a small nonzero rest mass. Faster than light interactions are a necessity and they provide the non local structure of the universe. We should understand the relation between local and nonlocal events like the dynamics of universal structure. In any physical theory, it is assumed that there is some kind of nonlocal structure violates causality. If neutrinos are traveling faster than light, then neutrinos must be on the otherside of the light barrier going backwards in time, where the future can interact with the past.

- Nalliah Thayabharan

Zach said...

The bartender says, "Hey, we don't serve neutrinos in here!"

A superluminal neutrino walks into a bar.

dis said...

I suspect there will be soemthing wrong with the timing too. But if neutrinos can move faster than light, yes there will be problems with lorentz causality, but no problems with galilean causality. whether once could make a consistent theory with lorentz physics as a low energy phenomena rather than a fundamental law might be hard though