Discerning “Indistinguishable” Quantum Systems

Abstract

In a series of recent papers, Simon Saunders, Fred Muller and Michael Seevinck have collectively argued, against the folklore, that some non-trivial version of Leibniz's principle of the identity of indiscernibles is upheld in quantum mechanics. They argue that all particles-fermions, paraparticles, anyons, even bosons-may be weakly discerned by some physical relation. Here I show that their arguments make illegitimate appeal to nonsymmetric, i.e. permutation-non-invariant, quantities, and that therefore their conclusions do not go through. However, I show that alternative, symmetric quantities may be found to do the required work. I conclude that the Saunders-Muller-Seevinck heterodoxy can be saved after all. *

Is Mereology Empirical? Composition for Fermions.

“…Another interesting consequence is that in any non-GMWentangled joint state, any two individual fermions are discernible by monadic predicates (which Muller & Saunders (2008) call absolutely discernible). This is contrary to the orthodoxy in the quantum literature, in which bosons and fermions are taken to be either merely weakly discernible or utterly indiscernible (French & Redhead 1988;Butterfield 1993;Huggett 2003;French & Krause 2006;Muller & Saunders 2008;Muller & Seevinck 2009;Caulton 2013). But this orthodoxy relies on adopting an alternative interpretation of permutation invariance, in which we can still give physical meaning to the labels of the factor Hilbert spaces in the joint Hilbert space.…”

“…Indeed, this interpretative gloss is offered by many authors (e.g. French & Redhead 1988;Butterfield 1993;Huggett 1999Huggett , 2003French & Krause 2006;Muller & Saunders 2008;Muller & Seevnick 2009;Caulton 2013). However, it may be argued that the physical emptiness of the factor Hilbert space labels offers the best explanation of the empirical fact that permutation invariance seems always to hold true.…”

On discernibility in symmetric languages: the case of quantum particles

“…10 The possibility of the weak discernment of quantum particles has been extensively studied e.g. in Saunders (2003Saunders ( , 2006a, Muller and Saunders (2008), Muller and Seevinck (2009), Muller (2011, Caulton (2013), Huggett and Norton (2014), Bigaj (2015b).…”

Structuralism in the philosophy of physics

“…As a consequence of permutation invariance, two such similar elementary quantum systems cannot be (absolutely) discerned (and individuated) by some intrinsic physical property expressed by a physically relevant formula with one free variable that applies to one system but not to the other. However, they have been shown to be weakly discernible by an irreflexive symmetric physical relation holding between them and expressed by a physically relevant formula with two free variables that is satisfied by the two systems in either order, but not by either system taken twice (Saunders ; Muller & Saunders ; Muller & Seevinck ; Caulton ; Huggett & Norton ). Similar elementary quantum systems therefore satisfy a weak version of the principle of the identity of the indiscernibles (PII), allowing for their identity and individuality to be relationally and structurally grounded.…”