Quantum Theory - What the Collapse Theory Says

Collapse theory says that when the photon is finally detected at detecta one, all the possibilities have collapsed into one. But if any one were to observe the photon mid flight, for example, if they knew it was travelling along the d path and not the u path, then the interference effect would be destroyed. In other words, there is a special kind of physics of observation in quantum theory. Observation does not cause the collapse of the wave function. There is nothing special about observations. On this view, the so called measurement problem, as it's sometimes called, is solved by saying there is no special physics of measurement.