It’s not a good week for me to be writing about anything remotely controversial, but if I want to keep my physics blogging license, I need to say something about the latest fast neutrino news. This has followed the usual trajectory of such stories, with the bonus farcical element of people who blasted the media for buying into the initial release seizing triumphantly on an initial rumor in the press that was garbled into incomprehensibility. With a little more time, it’s become more clear how their result has become less clear, and the best place to look for a description of this is Matt Strassler’s blog, where he has not just one, not just two, but three excellent posts on the news, laying out what’s really going on.
Having written at the time that “if something is wrong with their experiment, it’s something pretty subtle, because they’ve checked all the obvious problem areas carefully,” though, I probably need to say something about whether this counts as “subtle.” Because “look for loose wires” might seem too obvious to count as subtle, at least if you’re not familiar with experimental physics.
If you are familiar with experimental physics, though, this definitely would count as “subtle,” because real experiments aren’t like block diagrams, with single wires running in and out of a single detector. In an experiment on this scale, you’re probably talking about hundreds of individual cables connecting different boxes, and checking all of them is a non-trivial matter. What’s more, there’s nothing about their measurement that suggests a bad connection as the first thing you would look for– a loose cable, particularly a fiber-optic cable, shouldn’t produce tens of nanoseconds of signal delay unless by “loose” you mean “on the other side of the room from where it’s supposed to be.”
What’s more, as Strassler notes, there are actually two timing issues, moving the signal in opposite directions. Fixing one would tend to reduce the apparent speed, while fixing the other would increase it, making matters worse. So the whole situation is, as Strassler says in the third post linked above, completely confused. At this point, it’s not clear what if anything they can say about the apparent speed, other than that it’s worth testing again in a new data run, with more neutrinos and more detectors, coming later this spring.