Analysis of the Experimental Meaning of Coherent Superposition and the Nonexistence of Superselection Rules

The experimental meaning of coherent superposition of state vectors is analyzed. It is shown that the involvement of the measuring instrument is of basic importance and that it is erroneous to study superposition without it. From this analysis it is found that there are no such things as superselection rules. The meaning of the sign change of spinors under $2\pi$ rotation is considered, and it is found to have experimental consequences. The nonmeasurability of the relative parity of states with different $z$ components of angular momentum is shown. An appendix discusses the application of some of the considerations of the paper to decaying states and elementary particles.