More About Superpositions

The superposition principle is an automatic consequence of QP's Hilbert space structure: If a quantum can be in any one of several different states, then it can be in all of them simultaneously. This would be weird if quanta were anything like Newtonian particles, but it is commonplace for waves. We should regard this principle as a direct expression of the wave nature of photons, electrons, and other quanta. Superpositions crop up everywhere in QP. This chapter discusses several striking examples, including the “encounter-delayed-choice experiment” that breaks new ground by experimentally inserting a beam splitter while the photon is interfering with itself at the crossing point of an interferometer. We study several experiments in which increasingly massive objects are superposed. One purpose of such experiments is to test proposed alterations of standard QP based on mechanisms designed to force superpositions of very massive objects to collapse spontaneously (i.e. without being detected). Such theories have been proposed as solutions to the quantum detection problem.