Constraint rescaling in refined algebraic quantisation: momentum constraint

TL;DR

This paper examines how rescaling a momentum constraint affects refined algebraic quantisation, revealing cases where quantisation is equivalent, fails, or ambiguous, and discusses implications for systems with multiple constraints.

Contribution

It introduces a detailed analysis of the impact of constraint rescaling on quantisation, including new insights into self-adjoint extensions and superselection structures.

Findings

Quantisation can be equivalent to identity scaling under certain conditions.

Quantisation may fail due to nonexistence of self-adjoint extensions.

Quantum ambiguities influence the superselection structure of the physical space.

Abstract

We investigate refined algebraic quantisation within a family of classically equivalent constrained Hamiltonian systems that are related to each other by rescaling a momentum-type constraint. The quantum constraint is implemented by a rigging map that is motivated by group averaging but has a resolution finer than what can be peeled off from the formally divergent contributions to the averaging integral. Three cases emerge, depending on the asymptotics of the rescaling function: (i) quantisation is equivalent to that with identity scaling; (ii) quantisation fails, owing to nonexistence of self-adjoint extensions of the constraint operator; (iii) a quantisation ambiguity arises from the self-adjoint extension of the constraint operator, and the resolution of this purely quantum mechanical ambiguity determines the superselection structure of the physical Hilbert space. Prospects of generalising the analysis to systems with several constraints are discussed.