The Physics of Turning Off a Light: Quantum Electrodynamics, Photonic Decay, and Thermalization Dynamics in Enclosed Macro-Environments

When an incandescent or solid-state light source in a closed macroscopic environment is de-energized, the illuminated space transitions into darkness almost instantaneously to human perception. This paper presents a rigorous theoretical and quantitative analysis of the physical dynamics governing the cessation of room illumination. Grounded in quantum electrodynamics (QED), condensed matter physics, and classical thermodynamics, we demonstrate that photons possessing zero rest mass ($m_0=0$) and propagating strictly at the phase velocity $c \approx 3 \times 10^8$ m/s cannot slow down, accumulate, or gradually fade. Within 100 to 150 nanoseconds, virtually all visible-spectrum photons are absorbed by electronic transitions in surface materials and converted into thermal lattice vibrations (phonons), strictly preserving the Law of Conservation of Energy across quantum and macroscopic scales.