The double-slit experiment: A paradox-free kinematic description

Two physical experiments have captured the paradoxical nature of quantum mechanics in an elementary yet essentially complete way: the double-slit experiment and the EPR experiment. Few experiments in the history of science have generated so many ferocious debates, prompted so many controversial interpretations and at the end left us with such a deep feeling of discomfort with the current state of affairs. Recall the mysterious notions of the “wave-particle duality” and of “quantum non-locality” originating in the experiments. The double-slit and the EPR experiments are essentially similar. In fact, in both of them one deals with superpositions of the classically meaningful states and those superpositions are the source of all controversies in the theory. Accordingly, there is essentially only one mystery in quantum mechanics: the existence of superpositions of classically meaningful states. So, to understand quantum mechanics is to understand superpositions of states. In particular, the electron in the double-slit experiment is in a superposition of states that describe electron passing through one of the slits. There are three logical possibilities for such an electron: