H. D. Zeh

The emergence of classical properties through interaction with the environment

The dependence of macroscopic systems upon their environment is studied under the assumption that quantum theory is universally valid. In particular scattering of photons and molecules turns out to be essential even in intergalactic space in restricting the observable properties by locally destroying the corresponding phase relations. The remaining coherence determines the ‘classical’ properties of the macroscopic systems. In this way local classical properties have their origin in the nonlocal character of quantum states.The effect of the interaction depends essentially on whether it permanently ‘measures’ discrete or continuous quantities. For discrete variables (here exemplified by two-state systems) the classical properties are given by the measurement basis. The continuous case, studied for translational degrees of freedom, leads to a competition between destruction of coherence by the interaction and dispersion of the wave packet by the internal dynamics. A non-phenomenological Boltzmann-type master equation is derived for the density matrix of the center of mass. Its solutions show that the much-discussed dispersion hardly ever shows up even for small dust particles or large molecules. Instead the coherence length decreases towards the thermal de Broglie wave length of the object, whereas the incoherent spread increases. The Ehrenfest theorems are shown nevertheless to remain valid for recoil-free interactions. Some consequences of these investigations for the quantum theory of measurement are pointed out.

On the interpretation of measurement in quantum theory

It is demonstrated that neither the arguments leading to inconsistencies in the description of quantum-mechanical measurement nor those “explaining” the process of measurement by means of thermodynamical statistics are valid. Instead, it is argued that the probability interpretation is compatible with an objective interpretation of the wave function.

TOWARD A QUANTUM THEORY OF OBSERVATION

The program of a physical concept of information is outlined in the framework of quantum theory. A proposal is made for how to avoid the intuitive introduction of observables. The conventional and the Everett interpretations in principle may lead to different dynamical consequences. An ensemble description occurs without the introduction of an abstract concept of information.

Dynamics of quantum correlations

Abstract The density matrix describing the state of a sybsystem of a physical system whose time dependence is assumed to follow a Schrodinger equation does not itself obey a von Neumann equation. The behavior of the eigenvalues and eigenfunctions of this density matrix is studied. An expression for the rate at which initially separating systems are de-separated is derived in perturbation theory. Indications are given that coherent photon states are more stable in the presence of charged particles than photon number eigenstates. The possible dynamical origin of super selection rules is discussed. A simple model is solved analytically.

THE PROBLEM OF CONSCIOUS OBSERVATION IN QUANTUM MECHANICAL DESCRIPTION

Epistemological consequences of quantum nonlocality (entanglement) are discussed under the assumption of a universally valid Schrödinger equation and the absence of hidden variables. This leads inevitably to a many-minds interpretation. The recent foundation of quasi-classical neuronal states in the brain (based on environmental decoherence) permits in principle a formal description of the whole chain of measurement interactions, including the behavior of a conscious observer, without introducing any intermediate classical concepts (for macroscopic “pointer states”) or “observables” (for microscopic particle positions and the like)—thus consistently formalizing Einsteins ganzer langer Weg from the observed to the observer in quantum mechanical terms.

Time in quantum gravity

Abstract The intrinsic time concept of quantum gravity allows one to derive thermodynamical and quantum mechanical time arrows correlated with cosmic expansion only. Tube-like standing waves subject to a “final” condition may resemble unparametrised orbits of the universe, with “quantum Poincare cycles” coinciding with its durations. A recent criticism by Qadir is answered.

Symmetries, superselection rules, and decoherence

Abstract We discuss the applicability of the programme of decoherence to cases where it was suggested that the presence of symmetries would lead to exact superselection rules. We discuss, in particular, superpositions of states with different charges, as well as with different masses, and suggest how the corresponding interference term, although they exist in principle, become inaccessible through decoherence.

Why Bohm's Quantum Theory?

This is a brief reply to S. Goldsteins article “Quantum theory without observers” in Physics Today. It is pointed out that Bohms pilot wave theory is successful only because it keeps Schrödingers (exact) wave mechanics unchanged, while the rest of it is observationally meaningless and solely based on classical prejudice.

What is Achieved by Decoherence

A short critical review of the concept of decoherence, its consequences, and its possible implications for the interpretation of quantum mechanics is given.

Quantum cosmology as an initial value problem

Abstract The Wheeler-DeWitt equation for a closed Friedmann universe allows one to impose an (intrinsic) symmetric initial condition (SIC!) at the “big brunch” (superposition of big bang and big crunch). All degrees of freedom are essentially absent close to this singularity. They enter existence through states of low excitation.