30 research outputs found
Is mereology empirical? Composition for fermions
How best to think about quantum systems under permutation invariance is a
question that has received a great deal of attention in the literature. But
very little attention has been paid to taking seriously the proposal that
permutation invariance reflects a representational redundancy in the formalism.
Under such a proposal, it is far from obvious how a constituent quantum system
is represented. Consequently, it is also far from obvious how quantum systems
compose to form assemblies, i.e. what is the formal structure of their
relations of parthood, overlap and fusion.
In this paper, I explore one proposal for the case of fermions and their
assemblies. According to this proposal, fermionic assemblies which are not
entangled -- in some heterodox, but natural sense of 'entangled' -- provide a
prima facie counterexample to classical mereology. This result is puzzling;
but, I argue, no more intolerable than any other available interpretative
option.Comment: 24 pages, 1 figur
Qualitative individuation in permutation-invariant quantum mechanics
In this article I expound an understanding of the quantum mechanics of
so-called "indistinguishable" systems in which permutation invariance is taken
as a symmetry of a special kind, namely the result of representational
redundancy. This understanding has heterodox consequences for the understanding
of the states of constituent systems in an assembly and for the notion of
entanglement. It corrects widespread misconceptions about the inter-theoretic
relations between quantum mechanics and both classical particle mechanics and
quantum field theory. The most striking of the heterodox consequences are: (i)
that fermionic states ought not always to be considered entangled; (ii) it is
possible for two fermions or two bosons to be discerned using purely monadic
quantities; and that (iii) fermions (but not bosons) may always be so
discerned.Comment: 58 pages, 5 figure
Symmetries and Paraparticles as a Motivation for Structuralism
This paper develops an analogy proposed by Stachel between general relativity (GR) and quantum mechanics (QM) as regards permutation invariance. Our main idea is to overcome Pooley's criticism of the analogy by appeal to paraparticles. In GR the equations are (the solution space is) invariant under diffeomorphisms permuting spacetime points. Similarly, in QM the equations are invariant under particle permutations. Stachel argued that this feature - a theory's "not caring which point, or particle, is which" - supported a structuralist ontology. Pooley criticizes this analogy: in QM the (anti-)symmetrization of fermions and bosons implies that each individual state (solution) is fixed by each permutation, while in GR a diffeomorphism yields in general a distinct, albeit isomorphic, solution. We define various versions of structuralism, and go on to formulate Stachel's and Pooley's positions, admittedly in our own terms. We then reply to Pooley. Though he is right about fermions and bosons, QM equally allows more general types of symmetry, in which states (vectors, rays or density operators) are not fixed by all permutations (called 'paraparticle states'). Thus Stachel's analogy is revived
Symmetries and Paraparticles as a Motivation for Structuralism
This paper develops an analogy proposed by Stachel between general relativity
(GR) and quantum mechanics (QM) as regards permutation invariance. Our main
idea is to overcome Pooley's criticism of the analogy by appeal to
paraparticles.
In GR the equations are (the solution space is) invariant under
diffeomorphisms permuting spacetime points. Similarly, in QM the equations are
invariant under particle permutations. Stachel argued that this feature--a
theory's `not caring which point, or particle, is which'--supported a
structuralist ontology.
Pooley criticizes this analogy: in QM the (anti-)symmetrization of fermions
and bosons implies that each individual state (solution) is fixed by each
permutation, while in GR a diffeomorphism yields in general a distinct, albeit
isomorphic, solution.
We define various versions of structuralism, and go on to formulate Stachel's
and Pooley's positions, admittedly in our own terms. We then reply to Pooley.
Though he is right about fermions and bosons, QM equally allows more general
types of symmetry, in which states (vectors, rays or density operators) are not
fixed by all permutations (called `paraparticle states'). Thus Stachel's
analogy is revived.Comment: 45 pages, Latex, 3 Figures; forthcoming in British Journal for the
Philosophy of Scienc
On Kinds of Indiscernibility in Logic and Metaphysics
Using the Hilbert-Bernays account as a spring-board, we first define four
ways in which two objects can be discerned from one another, using the
non-logical vocabulary of the language concerned. (These definitions are based
on definitions made by Quine and Saunders.) Because of our use of the
Hilbert-Bernays account, these definitions are in terms of the syntax of the
language. But we also relate our definitions to the idea of permutations on the
domain of quantification, and their being symmetries. These relations turn out
to be subtle---some natural conjectures about them are false. We will see in
particular that the idea of symmetry meshes with a species of indiscernibility
that we will call `absolute indiscernibility'. We then report all the logical
implications between our four kinds of discernibility. We use these four kinds
as a resource for stating four metaphysical theses about identity. Three of
these theses articulate two traditional philosophical themes: viz. the
principle of the identity of indiscernibles (which will come in two versions),
and haecceitism. The fourth is recent. Its most notable feature is that it
makes diversity (i.e. non-identity) weaker than what we will call individuality
(being an individual): two objects can be distinct but not individuals. For
this reason, it has been advocated both for quantum particles and for spacetime
points. Finally, we locate this fourth metaphysical thesis in a broader
position, which we call structuralism. We conclude with a discussion of the
semantics suitable for a structuralist, with particular reference to physical
theories as well as elementary model theory.Comment: 55 pages, 21 figures. Forthcoming, after an Appendectomy, in the
British Journal for the Philosophy of Scienc
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Issues of identity and individuality in quantum mechanics
This dissertation is ordered into three Parts. Part I is an investigation into identity, indiscernibility and individuality in logic and metaphysics. In Chapter 2, I investigate identity and discernibility in classical first-order logic. My aim will will be to define four different ways in which objects can be discerned from one another, and to relate these definitions: (i) to the idea of symmetry; and (ii) to the idea of individuality.
In Chapter 3, the four kinds of discernibility are put to use in defining four rival metaphysical theses about indiscernibility and individuality.
Part II sets up a philosophical framework for the work of Part III. In Chapter 4, I give an account of the rational reconstruction of concepts, inspired chiefly by Carnap and Haslanger. I also offer an account of the interpretation of physical theories. In Chapter 5, I turn to the specific problem of finding candidate concepts of particle. I present five desiderata that any putative explication ought to satisfy, in order that the proposed concept is a concept of particle at all.
Part III surveys three rival proposals for the concept of particle in quantum mechanics. In Chapter 6, I define factorism and distinguish it from haecceitism. I then propose an amendment to recent work by Saunders, Muller and Seevinck, which seeks to show that factorist particles are all at least weakly discernible. I then present reasons for rejecting factorism. In Chapter 7, I investigate and build on recent heterodox proposals by Ghirardi, Marinatto and Weber about the most natural concept of entanglement, and by Zanardi about the idea of a natural decomposition of an assembly. In Chapter 8, I appraise the first of my two heterodox proposals for the concept of particle, varietism. I define varietism, and then compare its performance against the desiderata laid out in Chapter 5. I argue that, despite its many merits, varietism suffers a fatal ambiguity problem. In Chapter 9, I present the second heterodox proposal: emergentism. I argue that emergentism provides the best concept of particle, but that it is does so imperfectly; so there may be no concept of particle to be had in quantum mechanics. If emergentism is true, then particles are (higher-order) properties of the assembly, itself treated as the basic bearer of properties.This work was supported by the Arts and Humanities Research Council [grant number 2007/134560]
Structural basis of the leukocyte integrin Mac-1 I-domain interactions with the platelet glycoprotein Ib
Cell-surface receptor interactions between leukocyte integrin macrophage-1 antigen (Mac-1, also known as CR3, aMb2, CD11b/CD18) and platelet glycoprotein Iba (GPIba) are critical to vascular in?ammation. To de?ne the key residues at the binding interface, we used nuclear magnetic resonance (NMR) to assign the spectra of the mouse Mac-1 I-domain and mapped the residues contacting the mouse GPIba N-terminal domain (GPIbaN) to the locality of the integrin metal ion-dependant adhesion site (MIDAS) surface. We next determined the crystal structures of the mouse GPIbaN and Mac-1 I-domain to 2 ?A and 2.5 ?A resolution, respectively. The mouse Mac-1 I-domain crystal structure reveals an active conformation that is stabilized by a crystal contact from the a7-helix with a glutamatesidechaincompletingtheoctahedralcoordinationsphereoftheMIDASMg21 ion. The amino acid sequence of the a7-helix and disposition of the glutamic acid matches the C-terminal capping region a-helix of GPIba effectively acting as a ligand mimetic. Using these crystal structures in combination with NMR measurements and docking analysis, we developed a model whereby an acidic residue from the GPIba leucine-rich repeat (LRR) capping a-helix coordinates directly to the Mac-1 MIDAS Mg21 ion. The Mac-1:GPIbaN complex involves additional interactions consolidated by an elongated pocket ?anking the GPIbaN LRR capping a-helix. The GPIbaN a-helix has an HxxxE motif, which is equivalent by homology to RxxxD from the human GPIbaN. Subsequent mutagenesis of residues at this interface, coupled with surface plasmon resonance studies, con?rmed the importance of GPIbaN residues H218, E222, and the Mac-1 MIDAS residue T209 to formation of the complex