A Historical and Philosophical Account ofthe Modal Interpretations of Quantum Mechanics

Nahuel Sznajderhaus

University of Leeds, UK

The first modal interpretation (MI) of quantum mechanics (QM) was put forward by van Fraassen ([6]). By rejecting the well known projection postulate, he formally considered the measurement in QM as a passage from the possible to the actual. This move brought about an original approach to the measurement problem of QM. His MI, called the “Copenhagen variant", was claimed to be a continuation of Bohr's interpretation, and it was presented as embedded in his stance on the philosophy of science, namely: constructive empiricism (CE). Whether van Fraassen's MI can be appropriately accommodated within CE is a matter of current debate. According to CE, empirical adequacy is what matters the most for a theory, and it remains agnostic about whether the theoretical statements are true so long as they “save the phenomena". Soon after van Fraassen's publication, though, a number of variants - - disregarding the tenets of CE - were inspired. Here we will concentrate on Dieks' variant ([2], [3], [4]), which, at least in his early articles, presented a realist version of the MI. This version considers quantum systems to possess properties objectively, in a man-independent world. Dieks also argues that his approach is a continuation of Bohr's view. But contrary to van Fraassen, according to Dieks reading, Bohrs interpretation of QM can be considered as a realistic stance. In the first part of our work, we intend to shed some light on the apparent dilemma of whether van Fraassen or Dieks are close to Bohr's thought. All MIs try to go beyond instrumentalism, which denies that “interpretation" is important (cf. [5]), while accepting the standard formalism of QM. However, we will favour the idea that this instrumentalist attitude can be partially seen as a consequence of considering Bohr's work in a neo-Kantian light (cf. [1]), in which the wave function plays the role of a computing algorithm, and the measurement problem simply dissolves. In the second part, we will contrast both attempts by van Fraassen and Dieks. A tension between them is shown by the fact that Dieks goes beyond empirical adequacy, building on an ontological view about quantum systems. Moreover, we will investigate the extent to which an instrumentalist approach fulfills the tenets of CE, and therefore how far van Fraassen's original MI is justified. We will claim that one reason which van Fraassen could give is that he is not an instrumentalist - who denies all metaphysics - but favours CE, an epistemic kind of anti-realism - which accepts the useful fictions provided by the interpretations, without believing in their truth. Therefore, by denying the orthodox projection postulate, a more suitable _ction needs to be told. We will also contribute to the current debate about whether the MI can be appropriately accommodated within CE.