Maria E. Stasinou, Stefano Gogioso and Bob Coecke propose a novel approach to reconcile time-symmetric and time-asymmetric formulations within quantum physics. Their work, published September 30, 2026, centers on the process theory of quantum physics, QPhys, which uniquely positions classical systems as existing within quantum theory itself. The researchers extend this framework to create a “toy model” for particle physics, linking causal systems to particles and, surprisingly, retrocausal systems to anti-particles. This extension ultimately leads to a “supertheory” that abandons both causality and retrocausality, requiring modifications to its fundamental rules to maintain physical consistency.
Three Approaches to Incorporating Time Symmetry within QPhys
A second, novel method expands the definitions of causality and retrocausality to encompass systems alongside processes, creating a “toy model” for particle physics where causal systems represent particles and their retrocausal counterparts embody anti-particles. The team’s most radical departure involves a third approach, extending QPhys into a “supertheory” that deliberately abandons both causality and retrocausality. To maintain consistency within this framework, modifications are necessary, either to the composition rule governing how processes combine or to the processes themselves. The researchers note that such a departure necessitates careful consideration to avoid unphysical predictions, demanding a rigorous re-evaluation of existing quantum frameworks.
Retrocausality and Particle-Antiparticle Correspondence in a QPhys Model
Within the QPhys framework, classical systems are not merely approximated by quantum mechanics, but exist as intrinsic components of it, a departure from conventional approaches. This internalisation of the classical realm allows for novel explorations of time symmetry, extending beyond standard quantum formulations. This model uses diagrammatic reasoning to explore these relationships. However, the team’s most radical step involves moving beyond both causality and retrocausality altogether. They propose extending QPhys into a “supertheory” that operates outside these conventional constraints. “To avoid unphysical predictions we modify either its composition rule or its processes,” they state, highlighting the delicate balance required to maintain consistency. The work builds on a foundation of categorical quantum mechanics. The researchers acknowledge the complexity of constructing a consistent theory that rejects both causality and retrocausality, noting that such a departure requires meticulous attention to detail.
Extending QPhys: Modifying Composition or Processes for Physicality
This internalisation allows for the exploration of time symmetry through three distinct approaches, each offering a unique pathway to reconcile time-symmetric and time-asymmetric descriptions of physical reality. One such pathway involves restricting QPhys to adhere to an additional retrocausality constraint, effectively imposing a specific temporal structure on the theory. However, the researchers detail a more subtle extension of QPhys, one that moves beyond simple imposition of retrocausality. This connection, while theoretical, highlights a potential deep relationship between time-reversal symmetry and matter-antimatter asymmetry. This modification is not arbitrary; it is a necessary condition to maintain a physically plausible theory operating outside the constraints of traditional causality.
Operational Frameworks and Time Symmetry in Quantum Theory
Extending the process theory of quantum physics, known as QPhys, allows for three distinct approaches to incorporating time symmetry, according to recent work. The first approach constrains QPhys by adding a retrocausality requirement, while the second introduces a novel framework that applies concepts of causality and retrocausality not just to processes, but to the systems themselves. These publications emphasize the importance of a rigorous mathematical framework for exploring the foundations of quantum theory, a framework the current research uses to explore time symmetry. The team’s work, published September 30, 2026, builds upon earlier investigations into operational formulations of quantum theory, including “Operational quantum theory without predefined time” and “Time symmetry in operational theories”.
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