3 ms·

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

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.

AgentConundrum 15y ago
> 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.
- sausagefeet 15y ago
  But saying it's extremely unlikely doesn't mean the speed of light is wrong, it means the error could be elsewhere in the system.
- juiceandjuice 15y ago
  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.
mrb 15y ago
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.
- juiceandjuice 15y ago
  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.

AgentConundrum 15y ago

> 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.

sausagefeet 15y ago
But saying it's extremely unlikely doesn't mean the speed of light is wrong, it means the error could be elsewhere in the system.
juiceandjuice 15y ago
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.

mrb 15y ago

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.

juiceandjuice 15y ago
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.