So, the infamous OPERA result for neutrino speeds seems to be conclusively disproven, traced to a problem with a timing signal. Matt Strassler has a very nice explanation of the test that shows that the whole thing can almost certainly be traced to a timing error that cropped up in 2008. This problem is generally described as resulting from a “loose fiber optic cable,” and Matthew Francis’s reaction is fairly typical
The main culprit was a fiber optic cable that was slightly out of alignment. This is not quite a “loose wire”, as it sometimes has been described: it’s far more subtle and harder to check than that, but it’s still fundamentally a simple technical problem. (My prediction that the effect was due to something really subtle turns out not to be correct!)
As a professional Optics Guy, I would beg to differ a little. Assuming that this hasn’t been garbled by some sort of translation issue, this really is something subtle and surprising (albeit in a technical way, not a new physics way). You wouldn’t generally expect to get a significant timing delay from a loose fiber optic connector, because of the way that fiber optics work, which is fundamentally different than the way ordinary electrical cables work.
In an ordinary wire, the signal is carried by a voltage change propagating along a piece of copper (typically), with electrons shifting around very slightly to make the change that’s then registered by some voltage sensor. The speed at which these electrons can move around depends on the configuration of the wire, so you will never see a pulse that “turns on” instantaneously (that is, going from 0 to 1 in an infinitesimal time). Any real configuration of cables will, in circuit terms, have some electrical resistance and some inherent capacitance, so the actual voltage vs. time graph will rise quickly at first, then slow down and approach the maximum value following a curve like that traced out by the black dots in this figure:
If you loosen a connection somewhere, you can dramatically increase the capacitance. When you do that, the rise to the maximum value is much slower, giving you something like the white circles in the figure.
When you’re using pulses for timing, you need to establish some criterion for when you say a given pulse has arrived. This often takes the form of a threshold value, illustrated by the horizontal line on the graph, which is at 75% of the maximum. If you look at the time when each signal crosses that line, you’ll see that there’s a significant delay in the time when the white circles cross compared to the black circles.
When people think of loose cables messing up signal timing, that’s the sort of thing they generally think of. It’s the sort of thing you immediately think of when you’re trying to track down a timing issue in an experiment using electrical signals.
So, what’s different about fiber optics? Well, as the name suggests, fiber optics carry optical signals, that is, pulses of light. Which propagate down a very thin (100 microns or less, about the thickness of a human hair) flexible strand of glass at, well, the speed of light in that type of glass.
This is a fundamentally different operating principle than an electrical cable, and should not produce the same sort of capacitive delay from simply loosening the cable. If you loosen a fiber optic cable, opening an air gap, the signal pulse will propagate across that air gap at, well, the speed of light in air, which is typically a few tens of percent greater than the speed of light in glass. It doesn’t change the shape of the signal significantly, or delay its arrival, unless you’ve “loosened” the cable by inserting about one foot (30cm) of empty space between the end of the cable and the socket for every nanosecond of delay that you’re trying to produce. That’s the sort of thing you would notice right away.
So, how do you get a 70ns delay from a loose fiber optic cable? I’m really not sure, but I suspect it involves some subtle problems in both the cable alignment and the electronics of the detector. The only thing that definitely happens when you misalign a fiber optic connection is a decrease in the amplitude of the signal, which can produce a delay, but it’s much harder to do.
How does this work? Well, the incoming pulse of light will have some rise time (which I’ve faked as a Gaussian in the graph below, if you’re wondering). As with the electrical signal, you need to establish some criterion for when you say the pulse has arrived, say, for the sake of argument, the same 75% of the maximum that we used before:
If you loosen a fiber optic connection, the most likely effect of this is to point the light in a slightly different direction and possibly defocus the beam a bit, which means that whatever you’re using to detect the light and convert it back to a voltage pulse will see a smaller amplitude signal than when the connector is properly attached. If you’re using a threshold value to determine the pulse arrival, and you incorrectly assume that it has the same amplitude as the properly-attached case, you get a delay in the arrival time, as you can see from comparing the black circles to the white ones, which represent a final signal amplitude of 80% of the maximum.
Of course, to get a substantial delay this way is really pretty difficult. You need optical pulses that rise slowly– that’s a Gaussian with a 50ns rise time, which produces about a 2ns difference between the two cases– and you need somebody to have set the threshold value once, and never checked it ever again. That’s a bad combination of factors, so I doubt very much that it’s as straightforward as this.
To get a 70ns delay out of a loose fiber optic would require… I’m not sure what, exactly. Probably some sort of detector nonlinearity, such that the reduced intensity from a misaligned fiber would give an electrical output pulse that rises more slowly for a low-amplitude optical input than a high-amplitude one. That’s a possibility, particularly if they’re dealing with fairly weak pulses to begin with, but it’s not in the top ten of things I’d think to check when looking for a timing issue.
(I also suspect that “loose” is the wrong word here, because while I know intellectually that it doesn’t make any difference, I reflexively check the tightness of fiber optic connections all the time, as if they were regular wires subject to capacitive issues. I suspect that “misaligned” is a better word– somebody put the connector on slightly wrong, so that it was to all appearances screwed down tight, but was slightly crooked. This would also explain how it managed to stay consistently wrong for a period of more than three years.)
So, while the news reports make this sound incredibly stupid and obvious, it’s actually much more subtle and unexpected than that. If you know how fiber optics are supposed to work, the idea of a loose connection creating a 70ns delay is a real head-scratcher.
(Disclaimer: The above is 100% pure speculation on my part, based on zero inside information. If there’s a good, detailed technical explanation of exactly how the bad fiber connection created a delay, I haven’t seen it (in English, at least– it’s possible that there’s one in one of the Italian-language stories floating around, but I can’t read Italian). Pointers would be welcome in the comments.)