[r9etb]

Sort of hard to believe that they could measure so accurately at 3 feet.

I think the primary error source would be due to timing the arrival of the pulses.

With modern frequency standards, you can get timing to fractions of a nanosecond, which corresponds to distances of a lot less than 3 feet. For a properly calibrated system, the "error regions" around each detector would not overlap, and you should be able to detect the phenomenon in question, if it occurs.

Or ... you could measure the speed of light.

[TommyDale]

Current test equipment can locate a fault within inches over a distance of 25 miles, but measuring the actual speed isn’t feasible. Like you said, the precise “arrival time” cannot be measured accurately and would be definitely be difficult to prove that a lightwave arrived before it actually left. That entire idea is preposterous.