Philosophical Issues in Quantum Theory (Stanford Encyclopedia of Philosophy)

Despite its status as a core part of contemporary physics, there is no consensus among physicists or philosophers of physics on the question of what, if anything, the empirical success of quantum theory is telling us about the physical world. This gives rise to the collection of philosophical issues known as "the interpretation of quantum mechanics". One should not be misled by this terminology into thinking that what we have is an uninterpreted mathematical formalism with no connection to the physical world. Rather, there is a common core of interpretation that consists of recipes for calculating probabilities of outcomes of experiments performed on systems subjected to certain state preparation procedures. What are often referred to as different "interpretations" of quantum mechanics differ on what, if anything, is added to the common core. Arguably, two of the major approaches, hidden-variables theories and collapse theories, involve formulation of physical theories distinct from standard quantum mechanics; this renders the terminology of "interpretation" even more inappropriate. Much of the philosophical literature connected with quantum theory centers on the problem of whether we should construe the theory, or a suitable extension or revision of it, in realist terms, and, if so, how this should be done. Various approaches to the "Measurement Problem" propose differing answers to these questions. There are, however, other questions of philosophical interest. These include the bearing of quantum nonlocality on our understanding of spacetime structure and causality, the question of the ontological character of quantum states, the implications of quantum mechanics for information theory, and the task of situating quantum theory with respect to other theories, both actual and hypothetical.