Quantum Haecceity

Abstract

There is an extensive philosophical literature on the interrelated issues of identity, individuality and distinguishability in quantum systems. A key consideration is whether quantum systems are subject to a strong form of individuality termed ‘haecceity’ (from the Latin for ‘this-ness’). I argue that the traditional, strong form of haecceity does not apply at the quantum level, but that in order to properly account for the need for symmetrization in quantum systems, a weaker kind of haecceity must be involved, which I call quantum haecceity . In the process, I also question some generally accepted tenets of the current debate, such as the idea that symmetrization of states for identical quanta must be postulated and reflects permutation invariance. Instead, I note that a perturbative Hamiltonian is required for exchange effects, which suggests that the need for symmetrization arises from specific physical conditions. This article is part of the theme issue ‘Identity, individuality and indistinguishability in physics and mathematics’.

Identity, individuality and indistinguishability in physics and mathematics

“…In the wake of the works by Ghirardi and co-workers [ 96 – 98 ], Bigaj also analyses the relation between the notion of entanglement and the non-factorizability that merely arises from the (anti)symmetrization of a product state. In Kastner [ 99 ], a notion of quantum haecceity is proposed in the framework of Kastner’s transactional interpretation of quantum theory. This author maintains that the exchange of labels associated with a symmetrization of the wave function should be understood as an exchange of a quantum form of ‘transcendental individuality’ or ‘primitive thisness’ that Kastner calls quantum haecceity .…”

“… 10 For other discussions regarding the applicability of the notion of ‘primitive thisness’ to quantum systems see [ 100 – 102 ] and [ 79 , pp. 16–35], as well as Kastner [ 99 ]. …”