A nit, but this struck me as highly speculative. I, like the author, want to see the FGPA equipment analyzed further.
I kinda felt like I was led down the garden path a bit here. In the end, the entire essay (to me) boils down to "Stuff we don't know about could be stuff we should worry about" This is a true statement, but I'm not sure how useful it is to the reader.
Perhaps it would have been better to list all of the possible problems, then show why the FGPA was the biggest concern. That way I would be more informed about what the state of the discussion is. While it's really cool the author can outline the myriad ways an FPGA device reading can be misread, context here is more important than the ability to impress us with all the technical details (which are also important, no doubt, but only with context) Put another way, this was a bit of "nerd porn" -- lots of cool little technical details that need to be considered when dealing with FPGAs in places like this. For that, it's definitely great HN material.
Just a structural criticism of the essay, not the topic or author. The topic and writing quality were awesome.
I've worked with high resolution timing devices, data acquisition systems and NIST traceable timestamps, PMTs and APDs and FADCs. 10ns precision is a relative cakewalk, and can be done with about $600 in equipment. Often times some people neglect some latency introduced due to analog filtering, but it's nowhere near the order of 60ns. Furthermore, most of these systems are almost always calibrated daily with fiber optics, lasers, or other subatomic particles or particle accelerators. So often times the delay is actually relative to a particle (usually a photon) moving at or near the speed of light in a medium with a known refractive index. I'm sure it's not too hard to find the whitepaper for the equipment if you really wanted to look for it.
I'm not saying it's not possible, it's just extremely unlikely. The author is asking for FPGA analysis when he should be asking for calibration analysis.
I agree it is easy to design the system with 10 ns precision. Just like it is easy to calculate trajectories using different units, yet the $125 million dollar Mars Climate Orbiter space probe crashed due to a metric mixup that could have easily been avoided. Another way I look at it is that the OPERA experiment is either going to be concluded by brand new physics, or by a statement like "doh! I can't believe we screwed that simple thing". Out of all the systems they described, this one seems the more likely to be erroneous to me.
I understand that it's very possible the FPGA is the problem. However, it's a cakewalk to continuously monitor and calibrate FPGA latencies. In fact, it's probably done in conjunction with the calibration of APDs/PMTs. If they aren't doing this, then the whole experiment has much bigger problems. My point is that I personally feel it's incredibly wrong to single out the FPGA DAQ systems, as the discrepancy of 60ns is so incredibly large compared to what is easily attainable, and the delay is something that's always compensated for in calibration. I think that there's some other error in the system, and my gut feeling is that it's due either to geometry or calibration.
> I'm not saying it's not possible, it's just extremely unlikely.
Which is more unlikely though, that the FPGA has an issue where precision can be worse than 60ns, or that we're wrong about the speed of light / the ability of matter to travel beyond that speed? We're talking about hard to believe things here, so we shouldn't discard the possibility that this was the failure point simply because it's unbelievable.
I know you're not discarding the idea, and I welcome the context you bring to the discussion; I just wanted to point out that in this case there really isn't much that should be thrown out as impossible.
A systematic uncertainy is very possible, but a systematic uncertainty is a combination of things.
Assuming it's on the FPGA/DAQ side first and foremost is silly because there's multiple systems involved and they are geographically very far apart. Maybe someone forgot that they cut the cables shorter somewhere, or someone is using different cable with a different velocity factor than they are used to. Maybe their geometries are off by 18 meters.
The experimental setup and calibration, along with how it is performed, is much more important than any single component. Assuming it's the (seemingly) most complicated piece in the experiment is wrong. Everything needs to be checked and double checked at this point.
Still without the context, but those two FPGA mechanisms he suggests don't sound that compelling.
Why would they be using an FPGA that needs cached DRAM? It seems more likely that they're just using it for plain old logic. And you would assume that temperature-dependent changes affecting the crystal wouldn't be big enough and systemically consistent enough to account for the 60ns. But I guess we'll see.
The problem for the layman with this type of article is that the author can pick something that is unknown to us, dump a ton of speculation on the reader, and at the end you've got what, exactly? A big pile of technical speculation, that's all. Sort of a rehash of all the physics classes and texts that the author may have consumed. The ability to speculate on possible instrumentation problems does not correlate with the probability that the instrumentation in question is actually important.
Put another way, this article would be a great comment inside of a larger conversation about possible problems, but it's not so good as something that stands on its own. In the author's defense, perhaps it was written as part of a larger web conversation that he can then refer people to.
But it's not technical speculation - he pretty much says he knows nothing about these devices, then assumes they're not too dissimilar from a home PC. More just idle gossip.
I kinda felt like I was led down the garden path a bit here. In the end, the entire essay (to me) boils down to "Stuff we don't know about could be stuff we should worry about" This is a true statement, but I'm not sure how useful it is to the reader.
Perhaps it would have been better to list all of the possible problems, then show why the FGPA was the biggest concern. That way I would be more informed about what the state of the discussion is. While it's really cool the author can outline the myriad ways an FPGA device reading can be misread, context here is more important than the ability to impress us with all the technical details (which are also important, no doubt, but only with context) Put another way, this was a bit of "nerd porn" -- lots of cool little technical details that need to be considered when dealing with FPGAs in places like this. For that, it's definitely great HN material.
Just a structural criticism of the essay, not the topic or author. The topic and writing quality were awesome.