Measurement perturbs the system, always. Most of the time (radar speed check on a car, e.g.), the perturbation is so small as to be insignificant. When one bou

Measurement perturbs the system, always. Most of the time (radar speed check on a car, e.g.), the perturbation is so small as to be insignificant. When one bounces a photon off an electron to measure its position, though, one moves it. Likewise, when one uses a magnetic field to detect an electron, one deflects it. Add to that that position is poorly defined for such quantized objects in the first place, and things only get fuzzier.