I have read in several places (such as this StackExchange reply) that wavefunction collapse is 'non-physical'. The explanations I have read seem clear and understandable to me and have helped me to understand EPR and the implications of Bell's Theorem. I am trying to reconcile this in my mind with how an interference pattern from a double slit setup is destroyed when a particle detector is activated at one of the slits. Clearly, whatever is happening when the detector is switched on is physical, as we can see its effect. But up to now, my understanding was that the impact of the detector was to collapse the wave function of the particles to an eigenket of the position operator. However, if wavefunction collapse is non-physical, we should not be able to observe its effects. Can someone please point out where my thinking has gone wrong? Is the particle detector doing something other than collapsing a wavefunction? Am I misunderstanding what nonphysical means? Is it something else? Thank you.