The Meaning Circuit Hypothesis (MCH) is a synthesis of ideas providing John Wheeler’s outline of ultimate physics, which he fine-tuned over several decades from the 1970s onward. It is a ‘working hypothesis’ in which ‘existence is a ‘meaning circuit”’ that portrays the world as a “system self-synthesized by quantum networking.” It was strongly advocated by him for roughly two decades and since then has had an increasingly strong impact on the approach of many investigators of quantum theory; in particular, elements (...) such as the quantum participator and ‘it from bit’ are now considered by others as candidate components of a foundation for quantum theory in which information is involved essentially. Therefore, it is worthy of review and critique. (shrink)

The ontology of Local Quantum Physics, Rudolf Haag’s framework for relativistic quantum theory, is reviewed and discussed. It is one of spatiotemporally localized events and unlocalized causal intermediaries, including the elementary particles, which come progressively into existence in accordance with a fundamental arrow of time. Haag’s conception of quantum theory is distinguished from others in which events are also central, especially those of Niels Bohr and John Wheeler, with which it has been compared.

The elementary particles of relativistic quantum field theory are not simple field quanta, as has long been assumed. Rather, they supplement quantum fields, on which they depend but to which they are not reducible, as shown here with particles defined instead as a unified collection of properties that appear in both physical symmetry group representations and field propagators. This notion of particle provides consistency between the practice of particle physics and its basis in quantum field theory.

The well-definedness of particles of any kind depends on the limits, approximations, or other conditions that may or may not be involved, for example, whether there are interactions and whether ostensibly related energy is localizable. In particular, their theoretical status differs between its non-relativistic and relativistic versions: One can properly define interacting elementary particles in single-system non-relativistic quantum mechanics, at least in the case of non-zero mass systems; by contrast, one is severely challenged to define even these properly in the (...) relativistic quantum field theories that now underlie the study of particle physics. Here, the impact of localizability on this status is reviewed in relation to the work of Paul Busch on positive operator valued measures that significantly probes the relevance of quantum unsharpness to it. (shrink)