The Wave Function: It or Bit?

Schroedinger's wave function shows many aspects of a state of incomplete knowledge or information ("bit"): (1) it is usually defined on a space of classical configurations, (2) its generic entanglement is, therefore, analogous to statistical correlations, and (3) it determines probabilities of measurement outcomes. Nonetheless, quantum superpositions (such as represented by a wave function) define individual physical states ("it"). This conceptual dilemma may have its origin in the conventional operational foundation of physical concepts, successful in classical physics, but inappropriate in quantum theory because of the existence of mutually exclusive operations (used for the definition of concepts). In contrast, a hypothetical realism, based on concepts that are justified only by their universal and consistent applicability, favors the wave function as a description of (thus nonlocal) physical reality. The (conceptually local) classical world then appears as an illusion, facilitated by the phenomenon of decoherence, which is consistently explained by the very entanglement that must dynamically arise in a universal wave function.Comment: Several comments added, in particular regarding the role of a "second" quantization and concerning some recently proposed cosmological models. -- 21 pages, Late

Remarks on the Compatibility of Opposite Arrows of Time

I argue that opposite arrows of time, while being logically possible, cannot realistically be assumed to exist during one and the same epoch of this universe.Comment: 10 pages, Latex (2 eps files

Comment on Carlo Rovelli's "An argument against the realistic interpretation of the wave function"

Rovelli's argument against the realistic interpretation of quantum states and in favor of an ontology of quantum events is refuted.Comment: 3 pages; v2: minor correction

Open Questions regarding the Arrow of Time

Conceptual problems regarding the arrow of time in classical physics, quantum physics, cosmology, and quantum gravity are discussed. Particular attention is paid to the dynamical role of the quantum indeterminism, and to various concepts of timelessness.Comment: 14 pages - To appear in The Arrow of Time, Mersini-Houghton, L., and Vaas, R., edts. (Springer). v2: minor changes and added comment

Time in Quantum Theory

The concept of time as used in various applications and interpretations of quantum theory is briefly reviewed.Comment: 6 pages pdf - entry for a forthcoming "Compendium of Quantum Physics". Version 2: minor revision

Roots and Fruits of Decoherence

The concept of decoherence is defined, and discussed in a historical context. This is illustrated by some of its essential consequences which may be relevant for the interpretation of quantum theory. Various aspects of the formalism are also reviewed for this purpose. Contents: 1. Definition of concepts. 2. Roots in nuclear physics. 3. The quantum-to-classical transition. 4. Quantum mechanics without observables. 5. Rules versus tools. 6. Nonlocality. 7. Information loss (paradox?). 8. Dynamics of entanglement. 9. Irreversibility. 10. Concluding remarks.Comment: 19 pages, 3 figures: Talk given at the Seminaire Poincare (Paris, November 2005)- version 2 is a slightly extended and updated version of the proceedings (identical to v1

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 introduction of axiomatic observables. The conventional (collapse) and the Everett interpretations of quantum theory may in principle lead to different dynamical consequences. Finally, a formal ensemble description not based on a concept of lacking information is discussed.Comment: Latex version of a paper published in 1973 (with corrections), 9 page

The role of the observer in the Everett interpretation

The role attributed to the observer in various interpretations of quantum mechanics as well as in classical statistical mechanics is discussed, with particular attention being paid to the Everett interpretation. In this context, the important difference between "quasi-classical" (robust against decoherence) and "macroscopically given" (rather than being part of a thermodynamic ensemble) is pointed out.Comment: v2-6: new abstract; further comments and references added; v3: published; v6: 14 page

Why Bohm's Quantum Theory?

This is a brief reply to Goldstein's article on ``Quantum Theory Without Observers'' in Physics Today. It is pointed out that Bohm's pilot wave theory is successful only because it keeps Schr\"odinger's (exact) wave mechanics unchanged, while the rest of it is observationally meaningless and solely based on classical prejudice.Comment: 5 pages, LaTeX. Several comments added. (To be published in Foundations of Physics Letters

The Problem of Conscious Observation in Quantum Mechanical Description

Epistemological consequences of quantum nonlocality (entanglement) are discussed under the assumption of a universally valid Schr\"odinger equation in the absence of hidden variables. This leads inevitably to a {\it many-minds interpretation}. The recent foundation of quasi-classical neural states in the brain (based on environmental decoherence) permits in principle a formal description of the whole chain of measurement interactions, including the {\it behavior} of conscious observers, without introducing any intermediate classical concepts (for macroscopic "pointer states") or "observables" (for microscopic particle positions and the like) --- thus consistently formalizing Einstein's {\it ganzer langer Weg} from the observed to the observer in quantum mechanical terms.Comment: Published version: new abstract, minor changes, some new references. 14 pages, Late