We formalize the concept of subtime -- a reversible mode of information interchange within entangled systems -- and show how classical time emerges as an asymptotic limit through decoherence. Building on the photon clock model, in which a single photon confined between two ideal mirrors creates an alternating causality regime, we develop a process-theoretic formalization using the Oreshkov--Costa--Brukner framework extended with an explicit time-reversal duality condition. We introduce Perfect Information Feedback (PIF) as the information-theoretic realization of this reversibility, demonstrating that mutual information is conserved in any closed causal loop and that entropy quantifies the degree of unreflected causality. We define the Reversible Causal Principle (RCP): every causal relation possesses a conjugate dual, and entropy, energy dissipation, and the classical arrow of time appear only when these alternating components decohere or fail to reflect perfectly. The framework unifies Wheeler--Feynman absorber theory, Bennett's reversible computation, Shannon's communication theory, and the process matrix formalism under a single symmetry principle, and identifies experimentally accessible signatures in reversible digital links and quantum switch experiments. The arrow of time, in this picture, records the universe's imperfect causal echo.

翻译：我们形式化亚时间概念——纠缠系统内部的一种可逆信息交换模式——并阐明经典时间如何通过退相干作为渐近极限而涌现。基于光子钟模型（其中单个光子被约束于两个理想反射镜之间，形成交替因果机制），我们利用经显式时间反演对偶条件扩展的Oreshkov-Costa-Brukner框架，建立了过程理论形式化体系。我们引入完美信息反馈作为这种可逆性的信息理论实现，证明互信息在任何封闭因果循环中守恒，且熵量化了未反射因果性的程度。我们提出可逆因果原理：每个因果关系都具有共轭对偶，而熵、能量耗散和经典时间箭头仅在这些交替分量退相干或未能完美反射时出现。该框架将惠勒-费曼吸收体理论、贝内特可逆计算、香农通信理论及过程矩阵形式体系统一于单一对称原理之下，并在可逆数字链路与量子开关实验中识别出实验可观测特征。在此图景中，时间箭头记录着宇宙不完美的因果回响。