This paper presents a threshold-based interpretation of the cosmic ray energy spectrum, with particular focus on the origin of the knee. The observed steepening of the spectrum at energies of a few petaelectronvolts (PeV) is typically attributed to a combination of source limitations and propagation effects, often requiring multiple independent mechanisms to reproduce the full structure. In this work, the knee is interpreted as a propagation-driven threshold phenomenon arising from the interaction between particle structure and the medium through which cosmic rays travel. A dimensionless stability parameter is introduced, defined by the balance between effective tension, structural scale, and coherence capacity. The threshold condition marks a transition in which coherent propagation is reduced, leading to a suppression of observable flux. The resulting formulation yields species-dependent threshold energies, producing a layered structure in the cosmic ray spectrum. A semi-calibrated multi-species model demonstrates that this approach naturally reproduces a proton-dominated knee in the few-PeV range, followed by higher-energy transitions for helium and heavier nuclei. The all-particle spectrum is therefore understood as the superposition of multiple structural thresholds rather than a single universal cutoff. The same threshold framework can be extended beyond particle-scale systems by replacing energy-dependent loading with time-dependent accumulation. This suggests a general stability condition applicable across scales, linking particle propagation limits with the behavior of larger bound systems. Cosmic Forge Research Series This work is part of the Cosmic Forge research series, which develops a structural approach to physics based on tension, coherence, and scale. Foundational papers: Single Force Theory (SFT) Cosmic Seed Theory (CST) Related papers: Hydrogen Genesis Rethinking the Particle: Proton Knots and Electron Sheaths Cosmic Seed and Galactic Bang Dynamics Redshift in CST CMB Reinterpreted Rapid Transitions in Active Galactic Nuclei
Robert Fairbanks (Tue,) studied this question.