I'm hoping that someone has the patience to help a poor ignorant layman understand... and be warned that I am mathematically impotent. My struggle is in wrapping my mind around the concept of decoherence driven quantum collapse. As I understand it, the interaction between particles causes a quasi-measurement that "collapses" the quantum state of those particles (and all other entangled entities). If that is accurate, why does the photon strike in the double slit experiment not produce decoherence in the quantum state? Why the interference pattern?