Copenhagen: Heisenberg and von Neumann

Quantum measurement has relatively recently become a topic of practical concern, as well as one of prodigious growth. The need to illuminate the principles and problems of measurement in presentations that can be easily grasped within a lecture period have led to some excellent developments. A survey of modern texts reveals a diversity of treatments of the Copenhagen interpretation. Tracing these variations to the primary literature reveals differences of opinion and treatment among the individuals who comprised the Copenhagen school. Since these presentations are already in use in classrooms, they can be adapted to present a much richer picture of the development of the problem of quantum measurement within a context that fits in teachable lecture blocks. This paper reviews key discussions from Heisenberg and von Neumann. The results suggest there were indeed differences in the sensibilities concerning quantum measurement between the participants at Copenhagen. Further, the tensions not just between those participants, but between the questions they raise, reflect a true difficulty in the nature of the foundations of quantum mechanics that can be clearly identified in Feynman’s treatments (which also came in lecture-sized chunks). These issues provide the foundation for almost all of the new treatments of quantum measurement that have emerged in recent years, and which still deeply affect the controversial character of the topic.