“…In fact, by changing the boundary conditions of a given operator, one modifies the operator itself without changing its formal expression, not to mention the risk of losing the self-adjointness property (see appendix A). For example, in the Aharonov-Bohm effect, by choosing different boundary conditions, which preserve self-adjointness, one obtains different cross sections [4]; aside from other considerations, it is the experimental arrangement which selects the appropriate observable.…”