REDUCTION OF THE WAVE-FUNCTION

The present status of the notion of reduction (R) of the wave function is discussed with a special emphasis placed on the elucidation of the operational meaning of this notion. The main part of the paper is pre sented in the form of a dialogue between supporters of two extreme vie wpoints on the R in the interpretation of several characteristic exper iments. According to the first point of view, R is a real physical phe nomenon. According to the second approach, R is a purely speculative p rocess or a procedure for the processing of the experimental data. Our conclusion is that the choice between these two viewpoints is a matte r of taste because such a choice cannot be based on an experiment. For the supporters of the operational approach, the notion of R as areal process, as well as the assignment of a wave function to an individual particle, has no physical meaning. We also clarify the operational me aning of some other notions of quantum mechanics, such as preparation, filtration, simple and correlation measurements, back action, and dis tinguishable and indistinguishable alternatives. The mathematical aspe ct of the problem is presented in Appendices, where the relevant infor mation from the quantum theory of measurements is summarized and a mea surement postulate is proposed that is equivalent in its form to the p rojection postulate but does not involve the notion of R. We also cons ider a simple model that describes correlation experiments of the Time -of-Flight type with a freely moving electron.