On the interpretation of quantum theory – from Copenhagen to the present day

A central feature in the Copenhagen interpretation is the use of classical concepts from the outset. Modern developments show, however, that the emergence of classical properties can be understood within the framework of quantum theory itself, through the process of decoherence. This fact becomes most crucial for the interpretability of quantum cosmology – the application of quantum theory to the Universe as a whole. I briefly review these developments and emphasize the importance of an unbiased attitude on the interpretational side for future progress in physics. Ich bin nicht damit zufrieden, wenn man eine Maschinerie hat, die zwar zu prophezeien gestattet, der wir aber keinen klaren Sinn zu geben vermögen. Albert Einstein in a letter to Max Born (3.12.1953) 1 Copenhagen interpretations and alternatives A physical theory contains both a mathematical formalism and an interpretational scheme. Although their relation may be subtle already for classical physics, it becomes highly non-trivial in quantum theory. In fact, although the mathematical framework had been basically fixed by 1932, the debate about its meaning is going on. As we shall see, this is due to the possibility of having conflicting concepts of reality, without contradicting the formalism. This is why all physicists agree on the practical application of the formalism to concrete problems such as the calculation of transition probabilities. It is often asserted that the orthodox view is given by the Copenhagen interpretation of quantum theory. This is the standpoint taken by most textbooks. What is this interpretation? It is generally assumed that it originated from intense discussions between Bohr, Heisenberg, and others in Copenhagen in the years 1925-27. However, there has never been complete agreement about the actual meaning, or even definition, of this interpretation even among its main contributors. In fact, the Copenhagen interpretation has remained until today an amalgamation of different views. As has been convincingly argued in [1], it is the incompatibility between Bohr’s and Heisenberg’s views that sometimes gives the impression of inconsistencies in the Copenhagen interpretation. Historically, Heisenberg wanted to base quantum theory solely on observable quantities such as the intensity of spectral lines, getting rid of all intuitive (anschauliche) concepts such as particle trajectories in space-time [2]. This attitude changed drastically with his paper [3] in which he introduced the uncertainty relations – there he put forward the point of view that it is the theory which decides what can be observed. His move from positivism to operationalism can be clearly understood as a reaction on the advent of Schrödinger’s wave mechanics [1] which, in particular due to its intuitiveness, became soon very popular among physicists. In fact, the word anschaulich (intuitive) is contained in the title of Heisenberg’s paper [3]. Bohr, on the other hand, gave the first summary of his interpretation in his famous lecture delivered in Como in September 1927, cf. [4]. There he introduced the notion of complementarity between particles and waves –