I've been trying to think about what is really meant by "detection" in double-slit style experiments. I've had different explanations, most amounting to the effect of "if the path information could even in theory be known" (in regards to having detectors and not looking at the results, etc.). So, I was thinking of a classic interferometer experiment with a beam splitter and photons. What happesn if you have some 100% (or near enough) mirrors in the path of one or both of them? Or maybe they use mirrors already to cause the photon to turn corners in the machines? My question is, isn't it in theory possible to then detect which path a photon took because bouncing off a mirror will impart momentum to the mirror which could in theory be detected?