Duality and ‘particle’ democracy

Among the significant philosophical issues raised by the central role of physical dualities in recent fundamental research, one has a specific ontological flavor, as it concerns ‘dual entities’, i.e. entities exchanged by a duality mapping between theories. In particular, the type of duality that is known as weak/strong duality, or S-duality according to current terminology, seems to imply new surprising features from an ontological point of view. Weak/strong duality has become a basic ingredient in field and string theories, especially since the 1990s (see Polchinski, this issue). In general terms, it is described as an equivalence map between two different theories of the same physics, such that the weak coupling regime of one theory is mapped to the strong coupling regime of the other theory. Hence the special interest in this form of duality, seen as a new tool for getting information on physical quantities in the case of large values of the coupling constant (where the usual perturbative methods fail) by exploiting the results obtained in the weak coupling regime of the dual description. This duality is usually accompanied by a novel, puzzling feature: the fact that under this kind of duality it often happens that what is viewed as ‘elementary’ in one description gets mapped to what is viewed as ‘composite’ in the dual description. To use the words of Ashoke Sen – one of the physicists who significantly contributed in the 1990s to extend dualities to the string setting – “the classification of particles into elementary and composite loses significance as it depends on which particular theory we use to describe the system” (Sen, 2001, p. 3). What does this mean? At first sight, this interchanging role of elementary and composite seems to have strong implications for reductionism