Generalized gauge independence and the physical limitations on the von Neumann measurement postulate

Journal Article · Mon Dec 01 00:00:00 EST 1986 · Found. Phys.; (United States)

DOI:https://doi.org/10.1007/BF00732120· OSTI ID:6921737

Feuchtwang, T E; Cutler, P H; Kazes, E

An analysis is presented of the significance and consequent limitations on the applicability of the von Neumann measurement postulate in quantum mechanics. Directly observable quantities, such as the expectation value of the velocity operator, are distinguished from mathematical constructs, such as the expectation value of the canonical momentum, which are not directly observable. A simple criterion to distinguish between the two types of operators is derived. The nonobservability of the electromagnetic four-potentials is shown to imply the nonmeasurability of the canonical momentum. The concept of a mechanical gauge is introduced and discussed. Classically the Lagrangian is nonunique within a total time derivative. This may be interpreted as the freedom of choosing a ''mechanical'' (M) gauge function. In quantum mechanics it is often implicitly assumed that the M-gauge vanishes. However, the requirement that directly observable quantities be independent of the arbitrary mechanical gauge is shown to lead to results analogous to those derived from the requirement of electromagnetic gauge independence of observables. The significance of the above to the observability of transition amplitudes between field-free energy eigenstates in the presence (and absence) of electromagnetic fields is discussed. E- and M-gauge independent transition amplitudes between field-free energy eigenstates in the absence of electromagnetic fields are defined. It is shown that, in general, such measurable amplitudes cannot be defined in the presence of externally applied time-dependent fields. Transition amplitudes in the presence of time-independent fields are discussed. The path dependence of previous derivations of E-gauge independent Hamiltonians and/or transition amplitudes in the presence of electromagnetic fields are related to the inherent M-gauge dependence of these quantities in the presence of such fields.

• Research Organization: Pennsylvania State Univ., University Park, PA
• OSTI ID: 6921737
• Journal Information: Found. Phys.; (United States), Vol. 16:12
• Country of Publication: United States
• Language: English

Related Subjects

71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
QUANTUM MECHANICS
GAUGE INVARIANCE
MEASURE THEORY
EIGENSTATES
ELECTROMAGNETIC FIELDS
HAMILTONIANS
HERMITIAN OPERATORS
LAGRANGIAN FUNCTION
PARTICLE MODELS
PROJECTION OPERATORS
SCHROEDINGER EQUATION
TRANSITION AMPLITUDES
AMPLITUDES
DIFFERENTIAL EQUATIONS
EQUATIONS
FUNCTIONS
INVARIANCE PRINCIPLES
MATHEMATICAL MODELS
MATHEMATICAL OPERATORS
MATHEMATICS
MECHANICS
PARTIAL DIFFERENTIAL EQUATIONS
QUANTUM OPERATORS
WAVE EQUATIONS
657002* - Theoretical & Mathematical Physics- Classical & Quantum Mechanics