Introduction: Scope, Method, and Interpretive Commitments
Modern fundamental physics rests on a small number of highly successful formal frameworks—general relativity, quantum mechanics, and thermodynamics—each of which has been validated across an extraordinary range of experiments and observations. Despite this success, persistent conceptual tensions remain at their interfaces, particularly concerning time, measurement, irreversibility, inertia, and cosmology. These tensions are frequently interpreted as evidence that new microscopic entities, additional degrees of freedom, or modified dynamical laws are required. Less frequently examined is the possibility that the difficulty lies not in what is missing from existing theories, but in how their physical meaning is being interpreted.
A recurring problem in foundational and thermodynamic discourse is that conceptual vocabulary is often learned and propagated independently of the physical constraints it was intended to represent, allowing formally consistent descriptions to persist even when their dynamical and irreversible content has become ambiguous.
A recurring feature of contemporary theoretical language is the tacit assumption that spacetime, vacuum, or background structure can be treated as fundamentally inert or purely formal. Geometry is often taken as primitive, with dynamics layered upon it; time is treated as a universally flowing parameter; and the vacuum is described either as a mathematical null or as an abstract energy reservoir. These assumptions are rarely stated explicitly, yet they strongly shape which explanatory strategies are considered admissible and which questions are regarded as physically meaningful.
Reactive Substrate Theory (RST) adopts a deliberately different methodological stance. Rather than introducing new particles, fields, dimensions, or modifying established equations, RST asks whether the shared successes and shared failure modes of existing theories point to a common physical constraint that has not been made explicit. The framework is built around a single hardware-level commitment: physical interactions occur through a continuous substrate with finite, nonlinear, and dissipative response capacity. This commitment is minimal in scope yet restrictive in consequence. It is not intended as a revival of historical mechanical media, nor as a proposal of specific microphysical constituents, but as the least structure required to support finite rates, resistance to reconfiguration, and irreversibility as genuine physical features rather than calculational artifacts.
The core move of RST is interpretive rather than formal. General relativity, quantum mechanics, and thermodynamics are retained unchanged as effective, software-level descriptions. Divergences, singularities, and infinities are treated not as physically real objects but as indicators that an effective description has been extended beyond the domain in which bounded response can be neglected. In this sense, RST functions as a constraint-first framework: it restricts admissible interpretations based on finite response, rather than proposing new dynamical laws to repair inconsistencies after the fact.
This interpretive stance motivates a systematic reinterpretation of several foundational notions. In v1.1, time is treated as an operational rate defined by the accumulation of accessible substrate-coupled transitions. In v1.2, measurement is analyzed as irreversible substrate coupling that exhausts coherence bandwidth while preserving the Born rule. In v1.3, energy, temperature, entropy, and thermodynamic irreversibility are unified as expressions of finite transition accessibility governed by dissipative response. In v1.4, inertia is reinterpreted as substrate impedance—the resistance associated with retuning coherent configurations under acceleration— preserving the equivalence principle without introducing inertial primitives. In v1.5, interactions, including gravity, are treated as response-field phenomenology, with spacetime geometry understood as an emergent bookkeeping description of organized substrate stress.
Across these developments, the unifying claim remains narrow but stringent: any physically meaningful interpretation must respect finite response, finite coherence capacity, and irreversible dissipation. Interpretive frameworks that implicitly rely on unbounded response, environment-independent rates, or globally reversible evolution may retain mathematical utility while silently violating the conditions required for time, causation, and measurement to exist at all.
Reactive Substrate Theory does not present a parameter-free predictive model or an alternative cosmological theory. Instead, it defines a bounded response manifold within which existing formalisms operate as effective descriptions. Empirical confrontation occurs not through new equations, but through the identification of regime boundaries—contexts in which established theories begin to exhibit systematic tensions as response limits become dynamically relevant.
Reader roadmap and scope clarification. This work does not attempt to modify general relativity, replace quantum mechanics, derive cosmological parameters, or propose new particles or fields. It does not offer numerical predictions or alternative data fits. Its purpose is disciplinary rather than reconstructive: to impose explicit physical constraints on interpretation by requiring finite, nonlinear, dissipative substrate response, and to demonstrate how multiple foundational problems—time, measurement, irreversibility, inertia, and interaction—collapse into a single coherent explanatory structure once those constraints are made explicit.
Reactive Substrate Theory should therefore be read as an effort to restore alignment between conceptual language and physical constraint. Its guiding principle is that structure precedes description: before new formalisms are introduced, the interpretive vocabulary used to read existing ones must be shown not to violate the conditions under which physical dynamics, causation, and observation are possible.
