The famous Einstein-Podolsky-Rosen (EPR) “paradox” is a good example of how “before” and “after” arguments can be misleading. Wigner's friend is an even better case asking questions such as: when and how does the observer change or “reduce” the state vector? Perhaps the best and most insightful Lehrbeispiel of how to think about before and after issues comes from the quantum eraser. In this case Baysian logic helps clear up before and after confusion via detailed, but simple, calculations.
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