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Helvetica Physica Acta (Switzerland)

Deterministic evolutions and Schrödinger flows

Claude-Alain Faure, David Jeffrey Moore, C. Piron

Published Jul 14, 1995• 19 citations
Publisher

Abstract

Deterministic evolutions are defined without ad hoc hypotheses but in the context of the axiomatic approach due to Aerts and Piron. For systems described by a family of Hilbert spaces one can prove necessary conditions on the resulting flow using the tools of projective geometry. In particular we show that a deterministic evolution is always given by a family of partially defined unitary operators

DOI
10.5169/seals-116734

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Deterministic evolutions are defined without ad hoc hypotheses but in the context of the axiomatic approach due to Aerts and Piron. For systems described by a family of Hilbert spaces one can prove necessary conditions on the resulting flow using the tools of projective geometry. In particular we show that a deterministic evolution is always given by a family of partially defined unitary operators
Linear dynamics restricted to invariant submanifolds generally gives rise to nonlinear dynamics. Submanifolds in the quantum framework may emerge for several reasons: one could be interested in specific properties possessed by a given family of states, either as a consequence of experimental constraints or inside an approximation scheme. In this work we investigate such issues in connection with a one parameter group ϕt\phi_tϕt​ of transformations on a Hilbert space, H\mathcal{H}H, defining the unitary evolutions of a chosen quantum system. Two procedures will be presented: the first one consists in the restriction of the vector field associated with the Schr\"{o}dinger equation to a submanifold invariant under the flow ϕt\phi_tϕt​. The second one makes use of the Lagrangian formalism and can be extended also to non-invariant submanifolds, even if in such a case the resulting dynamics is only an approximation of the flow ϕt\phi_tϕt​. Such a result, therefore, should be conceived as a generalization of the variational method already employed for stationary problems.
This paper reconstructs the operational content of finite-dimensional quantum mechanics from deterministic Hamiltonian dynamics defined on a compact Kähler manifold Σ. An ontic microstate evolves under a Liouville-preserving flow generated by a Hamiltonian vector field, while observable statistics arise only after applying a smooth many-to-one projection π:Σ→CPⁿ⁻¹ to epistemic coordinates. The induced epistemic measure is taken to be absolutely continuous with respect to the Fubini–Study measure, which is selected as the canonical invariant density under the symmetry and regularity assumptions developed in companion work. For a class of Hamiltonians compatible with the projection structure, the projected dynamics on CPⁿ⁻¹ defines a holomorphic vector field. By the Ashtekar–Schilling correspondence, this reproduces the standard Schrödinger form of unitary evolution on the operational state space. Measurement interactions are modelled as Hamiltonian evolution on a joint system–apparatus surface, with outcomes represented by context-dependent regions on CPⁿ⁻¹ defined relative to the measurement setting. When evaluated with the Fubini–Study measure, the resulting region weights reproduce Born-form outcome statistics for the measurement constructions considered. Composite systems are represented by joint ontic structures that need not factorise, while the operational projection recovers the usual reduced-state formalism and subsystem dynamics. Overall, the analysis shows how unitary evolution, measurement statistics, mixed-state structure, and contextual dependence in finite-dimensional quantum mechanics can be realised within a deterministic geometric framework, without introducing fundamental stochastic processes or additional collapse postulates. Programme background: https://www.constraintsurfacedynamics.com — Glossary: https://glossary.constraintsurfacedynamics.com
The Unitary Dynamical Group in Schrödinger's Picture Up to now we have been concerned with the description of a physical system at a fixed instant of time. The time evolution of quantum systems constitutes a many-faced, wide issue which, however, is somewhat tangential to the main aim of this volume. We shall touch upon it only briefly, restricting ourselves to the more familiar aspects of the problem. We assume, to begin with, that the physical system has no superselection rules, so that states and physical quantities correspond one-to-one to density operators and self-adjoint operators, respectively, on the Hilbert space. To describe the dynamical evolution of the system one has to specify the way the representatives of states and physical quantities evolve in, which is fixed in time, since the physical system is supposed to preserve its identity. There is an old tenet according to which there are two kinds of time evolution: the discontinuous, nondeterministic evolution undergone by the quantum system when a measurement is made on it, and the continuous, deterministic evolution caused by the interaction with external forces or with other quantum systems. The first kind of evolution will be dealt with in Chapter 8, where we shall find it less alarming than is often claimed.
The quantum measurement problem asks how the smooth, unitary evolution of a superposition gives way to a single, definite outcome when a macroscopic detector interacts with a quantum system. This paper presents a geometric toy model of this interaction, with a deterministic factorization stage and a probabilistic selection step. The entangled state of a quantum system and a detector is embedded into semi-structured complex (ℍ) state space, where newly introduced projection operators, called trigger transformations, factorize the entangled state into a tensor product of two identical subsystems: one describing the quantum system and one describing what the detector registers. This mirrored factorization is shown to reduce exactly to the original entangled state on projection back to standard Hilbert space. Branch-indexed selector operators R, also introduced here, then return the state to Hilbert space and retain a single branch (or measurement outcome), with the standard quantum probabilities acting as structural weights of the branches. The model thereby confines the non-unitary step of measurement to a single, explicitly defined selection operation, offering an alternative to an ad-hoc wave function collapse. In its present form, the outcome rule and the Born weights are stated as an explicit postulate rather than derived, and a possible coupling to background vacuum fluctuations is identified as a route to future experimental tests.