The measurement postulate (postulate 4) relates the quantum theoretic predictions with experimental observations. Those predictions are found to be in conformity with experimental observations. That postulate, however, has been the subject of debate right since its conception. The issues debated are: (i) whether the measurement postulate is consistent with the postulate specifying the time evolution (postulate 5) and (ii) whether it denies the objective reality per the criterion of the Einstein, Podolsky and Rosen (EPR). The said issues are highlighted in the form of paradoxes by the thought experiment (Gedanken experiment in German) of Schrödinger, and that of EPR. See [40] for collection of articles and papers on the problem of measurement. See in particular [42] and [43].

The Measurement Problem

We recall first the measurement postulate. Let a system S (hereafter called the object) be subject to the measurement of an observable . Let the eigenvalues of be non-degenerate with as the corresponding orthonormal eigenvectors. Let the object be in the state jsi before the measurement, where

According to the measurement postulate, the measurement of would make the state of the object collapse to one of the eigenstates of and the outcome of the measurement would be the eigenvalue corresponding to the eigenstate to which the state of the object has collapsed. Hence, if the outcome of the measurement is the eigenvalue kk then we know that the state of the object after the measurement is, written symbolically as.