By the lights of most realist interpretations of quantum theory, characteristically quantum phenomena appear surprising and highly counterintuitive. In this lecture, I will explain why I believe that most quantum mysteries are the result of a category mistake concerning the nature of the quantum state: that of interpreting the quantum state as a state of reality (an ontic state) rather than a state of incomplete knowledge (an epistemic state), and I will describe a research program that aims to provide a realist interpretation of quantum theory wherein quantum states are epistemic. The notion of realism that will be endorsed is a minimalist one – that correlations need to be explained causally. Two lines of argument that support the epistemic view of quantum states will be reviewed. (i) The bottom-up argument. Starting from a classical notion of reality, if one assumes a fundamental restriction on the amount of knowledge that any agent can acquire about reality, then one can recover the precise predictions of quantum theory for certain subsets of the operational procedures that the theory allows. (2) The top-down argument. By carefully distinguishing the causal influences that hold among systems (where manipulating one system allows one to control another) on the one hand from the Bayesian inferences one can make among systems (where learning about one system allows one to update one’s knowledge of another) on the other, it is possible to understand quantum theory in a way that is conservative about causation – in particular, not requiring any spooky actions at a distance – but radical about the formalism one should use to make inferences. Specifically, the correct theory of inference according to this approach is a generalization of classical probability theory wherein states of knowledge are represented not by probability distributions but by complex matrices.
This talk is part of a Lecture series on foundations of physics: scientific realism (260020 VO, 2.5 ECTS, ufind.univie.ac.at/en/course.html) organised by the students of natural science of the University of Vienna (see naturwissenschaftscafe.wordpress.com)
