As someone returning to a study of physics after many years, I would appreciate any help and comments on my unresolved questions after reading “popular science” type information regarding quantum effects. In considering (that old chestnut!) the “twin slit” experiment to demonstrate the quantum interference effects of photons, I guess many people try to formulate their own explanation of the observed results of this experiment. For what it’s worth, my model seems to have ended up having two sorts of ‘worlds’. The ‘real’ world in which we can observe time, movement, events and so forth – and the ‘quantum world’. In the twin slit experiment this ‘quantum world’ is first entered when a photon is emitted from the source (laser) and is exited when a photon appears on the viewing screen (camera or whatever). The intrinsic difference between these two worlds seems to be that the ‘real’ observed world consists of actualities whereas the ‘quantum world’ consists of possibilities (or probabilities). These possibilities are very happy to remain in the ‘quantum world’ (co-exist?) until they exit their world to become an actuality in the ‘real world’. Now to the core of my question! One of the atomic principles in maths and physics is that of symmetry, in a very simple and basic sense that it is the natural order of things until an outside influence acts to create an asymmetry. Now, viewing the quantum wave propagation of the photon from the source, it seems to emanate nice and symmetrically in all directions – passes equally through both slits and produces a perfectly symmetrical interference based probability distribution on the screen. The quantum maths is perfectly symmetrical. So, when it gets to “crunch time” and a photon decides to appear as an actuality on the screen, this symmetry is suddenly and dramatically lost. It has a probability to appear at certain points on the screen, but it can only appear at a specific point. What is this extra “outside influence” acting on the system at this point which manifests this asymmetry? I do realise the definition of a probabilty contains the answer, but I feel the source and mechanism that manifests the effect of this probability outcome is not clear. My second, and related question relates to the transition between (my!) ‘quantum world’ and ‘real world’. It seems that ‘movement’ of photons in spacetime is fine in the quantum world – happy to remain as multiple possibilities. So exactly which ‘events’ – (like absorption of a photon by an atom?) preclude the system from staying in the ‘quantum world’ and force it to become an actuality. This seems to me to relate to an event that could potentially be observed. Does anyone have a list of types of events that could ‘potentially’ be observed? If an event is ‘potentially’ observable, does it flip from the ‘quantum world’ to the ‘real world’ instantaneously or is there a transitional state? For those ‘real physicists’ please forgive my rather none scientific terminology and also (as I suspect) that these questions have been asked many times before.