This paper re-examines the fundamental meaning of the speed of light within the framework of Time Field Theory (TFT). Departing from the conventional view that treats light as a physical entity moving at speed c, we demonstrate that light is ontologically a geodesic path satisfying the vanishing first variation of Fermat’s principle δ∫ds/γ=0. The constancy of the speed of light is not a description of “how fast light travels,” but the guarantee that the differential relation ds=cγdt is well-defined as a universal physical measurement language. The zero intrinsic time of photons (vt=0) is not a negative characterization of light, but the physical condition that enables the calculus of variations to function as a legitimate tool in physics. We establish a strict logical chain: metric self-consistency → flux conservation ∇·(γ∇γ)=0 → Fermat’s principle δ∫ds/γ=0 → vanishing first variation → zero intrinsic time of photons (vt=0) → locally determined γ → constant speed of light → differential relation ds=cγdt → calculus as a legitimate physical descriptive language. We further demonstrate that if any link in this chain is broken—if the speed of light were not constant, or if photons possessed intrinsic time—the logical foundation of the calculus of variations and calculus as physical tools would collapse. The speed of light is not a property of a moving entity; it is the mathematical signature of metric self-consistency in path space.
Huang (Thu,) studied this question.