Quantum theory of measurement without wave packet collapse

Abstract

A schematization of the measurement process in quantum mechanics is presented leading to a unified treatment both of measurements performed by means of polarized counters and of measurements made with Stern-Gerlach-like set-ups. In this way it is shown that the so-called wave packet collapse is not an absolute postulate which should be added from outside to the laws of quantum mechanics, but rather a consequence —though not an exact one but valid to a very high degree of accuracy— of these laws. The limits of the deviations from exact collapse are expressed explicitly in terms of quantities related to the macroscopic character of the experimental device. The relationship between irreversibility and this (pseudo) collapse is discussed and shown to arise from their common origin represented by the large numbers implied in this macroscopic character. However, one can have collapse without irreversibility, although not viceversa. It is shown that all the so-called paradoxical features of the measurement problem stem from the confusion between the level of small quantum numbers and the level of very large ones. It is only at this latter level that the equivalence between the pure state vector of the total system «object+apparatus» and the statistical matrix representing the possible outcomes of their interaction ensures that the «observer» does not have any power of «creating» reality, but merely obtains from an objective, although probabilistic, representation of reality all the statistical information available.