Is it correct to say that a measurement occurs whenever a wavefunction interacts with a macroscopic object? If this is the case why does the wavefunction make a distinction between a photon detector and say a half silvered mirror used in quantum experiments. Both are macroscopic objects but in one case the wavefunction collapses and in the other case it doesn't. I don't understand what is so different physically about a photon detector and a mirror. Both are fundamentally ensembles of particles. Why the difference in the way a wavefunction reacts to them?