Causality and the Criterion of Physical Realizability in Quantum Field Theory

Luka Gluvić

Modern physics provides empirically complete and highly successful descriptions of local dynamics. General relativity specifies the causal evolution of spacetime geometry, and quantum mechanics specifies the unitary evolution of states. Despite this success, both frameworks leave implicit a foundational question: when a dynamically admissible configuration can be regarded as physically realized as part of a coherent world. This work introduces a minimal, non-dynamical realization criterion that formally closes this gap. Physical realization is defined as a global, temporally organized admissibility condition constrained by relativistic causality. The criterion does not modify any local equations of motion, introduce new forces, fields, particles, or degrees of freedom, nor alter the mathematical structure of existing theories. Instead, it identifies realization as an organizational condition required for dynamical solutions to become physically instantiated. A minimal causal bound T = L / c is shown to follow directly from relativistic causal structure, implying a finite organizational time for global coherence across a spatial extent L. The associated inverse scale Ω = c / L is clarified as a structural constant rather than a dynamical frequency: it has no oscillatory interpretation, carries no energy, and does not represent a physical process. Its role is purely limiting—it marks the minimal global temporal orientation required for realization to persist. Within this framework, classical gravitational optics, delayed global geometric organization, and quantum outcome realization arise as direct consequences of the same realization criterion. The contribution of this work is exclusively structural: it makes explicit a condition already implicit in successful physical theories and provides a formal closure without extending or revising their dynamics. This paper concludes a broader research program on temporal realization, gravitational organization, and global coherence by isolating realization as a necessary condition for physics to describe a world at all.

N. N. Bogolyubov

V.Ya. Fainberg

Jan Mandrysch, R. Simmons, Miguel Navascués

As noted by Sorkin, regarding quantum instruments whose Kraus operators are localizable within some spacetime region as operations accessible therein leads to superluminal communication. This so-called Sorkin paradox can be resolved by further constraining the set of allowed local operations in quantum field theory (QFT). In this spirit, Fewster and Verch proposed a framework for local QFT operations that generalizes non-relativistic quantum measurement theory and does not lead to Sorkin-like paradoxes. Shortly afterwards, Jubb (and later Oeckl) identified the minimal conditions that QFT instruments must satisfy to be compatible with Einstein's causality. In this work, we study both approaches in the quantum field theory of the free scalar field. First, we prove that a very wide class of causal instruments is FV-realizable: namely, those whose measurement channels are random displacements of the field operators. As we show, this class allows implementing arbitrary instruments in a heralded, probabilistic way, as well as non-demolition measurements deterministically. Second, we construct examples of causal channels that do not admit an approximate FV realization. Some of such causal, not FV-realizable channels violate basic physical principles, so they should not be part of any measurement theory for QFT. Third, we investigate the difficulty of characterizing the set of QFT channels that can be generated through the composition of several FV schemes. In this regard, we find a countable family of simple QFT channels for which no Turing machine can discriminate between channels within or far away from the implementable set.

Miklós Rédei, Stephen J. Summers