The Measurement Problem, Decoherence and Education

Abstract

A study is made of the status of measurement, in particular the projection postulate, in quantum mechanics. Part of this will be a historical outlook: how was the concept of measurement viewed by the quantum architects? Then a more systematic investigation is made on what has been called the measurement problem which poses the question why superpositions of quantum states are never observed. Reasons are given why the projection postulate does not from a satisfactory solution, namely the non-unitary evolution of time and the lack of a precise de?nition of the word `measurement'. Since the 1980's active research has been undertaken in the ?eld of decoherence phenonema. Environment-induced decoherence has been proposed to solve the measurement problem by looking at open quantum systems instead of closed ones. It is inquired upon to what extent decoherence is indeed a solution. It has been shown that the fundamental problem survives, but a justi?cation is found for the appearance of classical properties in a universal quantum world. Although the discussion on the decoherence solution has arrived at an approximate equilibrium since the ?rst few years of this century, the terms `decoherence' and `collapse' are still used interchangeably in a vast amount of literature. This is pointed out to be an uncareful use of words. However, the decoherence program has inspired a new way of thinking about interpretations, giving preference to some modern interpretations - the non-collapse theories - over the early Copenhagen or subjective interpretations. It seems that the ideas behind decoherence theories have seeped through to the most modern textbooks. However, the textbooks currently in use mostly date back to orthodox times of the late 1980's. According to the recent textbooks, it seems that a shift in teaching interpretational quantum mechanics is to be expected, similar to the impact Bell's inequalities made on education in the early 1980's.