Quantum Probabilities & Objectivity: New Pragmatist Insights

Quantum theory provides us with an apparently infallible recipe to compute the probability that a certain result is obtained if the suitable experiment is performed. When several measurements are considered, however, quantum probabilities exhibit a quite peculiar feature: they are incompatible with any straightforward explanation of the expected results in terms of correlations between the putative properties of the measured objects. While the most popular strategies to deal with this problem consist in working out less straightforward variants of the same realist pattern of explanation, in the present project we scrutinize the alternative approach which takes the operational character of quantum probabilities, that is, the reference to mere measurement results, as fundamental. In order to avoid the problematic references to the observer that this methodological choice is often taken to imply, we propose to rely on non-standard theories of language and knowledge, which conceive of meaning and objectivity in normative rather than representational terms. In this framework, some recent attempts to understand quantum mechanics as a generalized theory of probability can be analyzed from a neo-Kantian perspective with the purpose of bringing out the link between mathematical structure and (conceptual) presuppositions of empirical knowledge. The analysis can be extended to formal schemes that generalize quantum theory: we will undertake this task by focusing in particular upon the possible operational interpretations of such schemes and experimental tests thereof. The project involves two partners: Michel Bitbol (a philosopher of science based at the Husserl Archives of the Ecole Normale Supérieure of Paris) and Caslav Brukner (a theoretical physicist based at the Institute for Quantum Optics and Quantum Information of the Österreichische Akademie der Wissenschaften of Vienna). It will bring out previously unexplored transdisciplinary links and combine various methodological approaches, including mathematical techniques from quantum information theory, philosophical reflection inspired by the Kantian and Wittgensteinian traditions, and pragmatist theories of meaning in contemporary analytic philosophy.

Although the effectiveness of the "operational" interpretations of quantum mechanics is widely acknowledged by physicists, the logical challenges posed by quantum probabilities (in particular, the measurement problem) are not really dissolved by the mere endorsement of an instrumentalist stance. The goal of our project was twofold: to provide a critical assessment of, and a logical basis for, the pragmatic approaches to the quantum paradoxes; and to use the insight thus gained - particularly with regard to the notion of information - to devise an original theoretical framework in which to undertake the reconstruction and generalization of quantum mechanics. A guiding hypothesis of our project was that, within such a framework, the normative structure of quantum mechanics would emerge (through what the philosophers call "a transcendental argument") from the analysis of the conceptual presuppositions of empirical inquiry qua rational practice based on the processing of information. In order to carry out this programme, we undertook to develop a semantic framework inspired by the inferentialist theory of conceptual content and discursive practice - a framework in which the normative structure of probabilistic languages, on the one hand, and the constitutive process of objectivity, on the other hand, could be understood independently of any representational and foundationalist concern. These ideas were elaborated, in particular, through a thorough analysis of the Wigner's-friend paradox, which borrowed some fundamental conceptual tools from the most recent developments of the pragmatist tradition in the philosophy of language and the philosophy of mind. While our project explicitly addressed issues relating to the foundations of quantum mechanics as well as possible theoretical developments of the theory, the critical analysis of the conditions under which the proposed approach can be taken to dissolve the interpretive puzzles of quantum mechanics provides an original contribution to the current philosophical debate on inferentialist (vs "representational") semantics, especially in so far as epistemological implications (relating, in particular, to the status of logic and probability laws) are at stake.