We prove by contradiction that any causal system-physical, biological or informational-must exhibit a strictly positive minimal response delay τ 0 > 0. Assuming τ 0 = 0 leads to three independent absurdities: (1) violation of the second law via a Maxwell's demon engine; (2) superluminal signaling breaching relativistic causality; and (3) reversible quantum decoherence contradicting the irreversibility of measurement.These jointly collapse modern physics, so τ 0 > 0 is a logical necessity, on par with the finiteness of the speed of light or nonzero Planck's constant.
We prove by contradiction that any causal system-physical, biological or informational-must exhibit a strictly positive minimal response delay τ 0 > 0. Assuming τ 0 = 0 leads to three independent absurdities: (1) violation of the second law via a Maxwell's demon engine; (2) superluminal signaling breaching relativistic causality; and (3) reversible quantum decoherence contradicting the irreversibility of measurement.These jointly collapse modern physics, so τ 0 > 0 is a logical necessity, on par with the finiteness of the speed of light or nonzero Planck's constant.
Causality is fundamental to science, but it appears in several different forms. One is relativistic causality, which is tied to a spacetime structure and forbids signaling outside the future. A second is an operational notion of causation that considers the flow of information between physical systems and interventions on them. In [V. Vilasini and R. Colbeck, General framework for cyclic and fine-tuned causal models and their compatibility with space-time, Phys. Rev. A 106, 032204 (2022).PLRAAN2469-992610.1103/PhysRevA.106.032204], we propose a framework for characterizing when a causal model can coexist with relativistic principles such as no superluminal signaling, while allowing for cyclic and nonclassical causal influences and the possibility of causation without signaling. In a theory without superluminal causation, both superluminal signaling and causal loops are not possible in Minkowski spacetime. Here we demonstrate that if we only forbid superluminal signaling, superluminal causation remains possible and show the mathematical possibility of causal loops that can be embedded in a Minkowski spacetime without leading to superluminal signaling. The existence of such loops in the given spacetime could in principle be operationally verified using interventions. This establishes that the physical principle of no superluminal signaling is not by itself sufficient to rule out causal loops between Minkowski spacetime events. Interestingly, the conditions required to rule out causal loops in a spacetime depend on the dimension. Whether such loops are possible in three spatial dimensions remains an important open question.
A central goal in the foundations of physics is to understand the structure of physical theories, such as quantum theory, from physical principles. This is often explored by considering various information-theoretic principles. Here, we initiate a similar approach considering relativistic causality principles. No superluminal causation (NSC) and no superluminal signaling (NSS) are distinct relativistic principles, requiring respectively that causal influence/the ability of agents to signal are within the future lightcone. After formalizing their distinction, we investigate how well theories constrained by NSC and NSS perform in a task that involves generating nonclassical correlations. We find a spacetime configuration in which this task cannot be achieved in any theory (classical, quantum, or postquantum) satisfying NSC. However, we show that theories violating NSC but satisfying NSS can perfectly achieve the task. We give a protocol that would, in a world allowing superluminal causation, enable its operational certification without violating NSS, in general spacetimes. In the case of (1 + 1)-dimensional Minkowski spacetime, the task remains achievable in a configuration where measurement outcomes occur arbitrarily earlier in time than the settings, allowing a form of certifiable retrocausality without violating NSS. We illustrate our results by linking two different types of nonclassical postquantum resources: PR boxes and jamming. Our work offers insights into the role of different relativistic causality principles in fundamental physics and paves the way for characterizing the information-theoretic structure of theories obeying such principles.
This paper advances the core thesis that the speed of light is the limit of physical quantities, not the limit of spacetime quantities. The "superluminal" correlations of quantum entanglement and the "superluminal" recession of distant galaxies in cosmic expansion are not violations of relativity, but rather because these two phenomena belong to the category of spacetime quantities and fall outside the jurisdiction of the speed of light. Through the methodology of "hierarchical reclassification," this paper separates physical quantities (matter, energy, signals, causal chains) from spacetime quantities (spacetime structure, expansion, connectivity, topological relations), pointing out that the definition of velocity (m/s) presupposes "matter moving through space"; when the object of discussion is spacetime itself, the concept of "velocity" no longer applies. This paper further clarifies that there is no fundamental contradiction between relativity and quantum mechanics requiring "unification"—they describe effective laws at different levels (the L4 physical level and the L1–L2 quantum level). The "superluminal paradox" that has long troubled physics is, in essence, a result of category misplacement.
The quantum measurement problem—why observation appears to collapse superposition states—has resisted resolution despite decades of progress in decoherence theory. While environmental decoherence explains the emergence of classicality, it lacks a quantitative criterion distinguishing "measured" from "merely decohered" systems. This work proposes such a criterion: measurement occurs when environmental coupling strength, quantified by the dimensionless parameter W = (I * tau) / tau0, exceeds a universal threshold W0. Here I is path distinguishability (Englert’s D), tau is information retention time, and tau0 is system coherence time. The framework predicts continuous visibility decay V(W) = 1 / [1 + exp(kappa * (W - W0))] rather than binary collapse, with threshold W0 ≈ 1–3 across platforms after normalization. Testable predictions include: (1) I × tau trade-off (different combinations of distinguishability and retention yielding identical visibility at matched W); (2) quantum eraser timing dependence (recovery window ~ tau0); and (3) finite collapse propagation speed, formalized as the Causal Limit Postulate (v = c in vacuum). The model predicts a measurable delay (Delta t ≈ L / c) and introduces Causal Viscosity, where n_eff ~ Gamma^(1/2), to describe propagation drag in decohering media. A cavity-QED toy model with 87Rb atoms validates the framework with realistic parameters, independently confirmed by dispersive regression.
We argue that quantum randomness is a logical necessity arising from the nature of intrinsically relational properties—properties lacking definite states outside specific contexts of causal interaction. This motivates Relational Actualism (RAQM), an ontological framework classifying physical properties as invariant, classical, or intrinsically relational. RAQMgrounds wave function actualization in physical interaction rather than observer-relative facts. The actualization criterion is physically grounded and mathematically precise: an interaction constitutes an actualization event when it produces an irreversible increase in the entanglement entropy of the system with its asymptotic environment, generating a robust, redundant causal record. In weakly coupled QFT, a sufficient condition is the exchange of a real, on-shell excitation (pμ pμ = m2c2). This criterion is basis-independent, nonperturbative, and requires no new fundamental constants. To reconcile a globally consistent wave function with special relativity, the Tomonaga-Schwinger equation establishes a foliation-independent global update mechanism. We derive a consistency condition demonstrating that RAQM’s actualization map preserves identical observable statistics at spacelike-separated points regardless of hypersurface choice—a derived result, not a postulate—strictly forbidding superluminal signaling via algebraic microcausality. The no-signaling result applies rigorously to the non-selective (outcome-averaged) actualization map; conditioned branches are treated explicitly. Three specific, discriminating empirical predictions are stated: a finite actualization propagation velocity bounded by the Lieb-Robinson speed, an environment-dependent transition duration scaling as ℏ/ΔEenv, and a sharp reversibility threshold at the actualization criterion. RAQM is distinguished from Relational Quantum Mechanics (which relativizes facts), dynamical collapse models (which ground collapse in stochastic gravitational thresholds rather than causal interaction), decoherence-only models (which leave the outcome problem open), and Everettian Many-Worlds (which denies definite events). Classical behavior emerges dynamically when continuous actualization events eliminate all unconstrained degrees of freedom. The companion papers RAGC and RACI extend these results to gravity, cosmology, and biological complexity; RAHC [Sandeman, 2026e] extends the framework further to the emergence of agentic intelligence, formalizing the recursive coarse-graining hierarchy that generates successive levels of causal closure from the microscopic DAG; key theorems in those papers have been formally verified in Lean 4/Mathlib. A second Lean 4 file (RA_AQFT_Proofs_v10.lean) formalises the finite-dimensional AQFT proof program and proves the frame-independence and Rindler stationarity theorems; see the note on formal verification in the companion paper RAGC [Sandeman, 2026b].