Underdetermination, realism and objectivity in quantum mechanics

Abstract

Underdetermination of theories by empirical data is a central theme in debates surrounding scientific realism. Underdetermination undermines epistemological optimism: if empirical evidence cannot decide between theories, skepticism about the progress of science seems justified. Philosophical defenses have been developed against this skeptical threat. Typical themes in these defenses are that significant scientific examples of empirical equivalence (as opposed to imaginary armchair cases) are virtually non-existent, as it is already difficult enough in scientific practice to develop one single satisfactory theory; that in the rare instances where empirical equivalence can be maintained to occur it is defeasible and only temporary; and that there usually will be substantial differences in empirical support, even if theories are empirically equivalent. Examples are usually constructed cases within classical physics that have not played an important role in actual history. In this article we draw attention to the present-day situation in quantum mechanics, which we think is very relevant to the issue. There exist several realist interpretations of quantum mechanics, each of which depicts a quite distinctive physical world, and each of which has its own circle of devotees in the scientific community. Most of these interpretations are empirically equivalent in a quite strong sense: they predict the same results for all experiments that can be expected to be feasible. The usual arguments against the significance of theoretical underdetermination seem to lose a great deal of their effectiveness here. One may wonder whether non-uniqueness of theories is not part and parcel of the practice of modern science after all, and much less threatening than often thought.