This study reinterprets temperature not merely as a thermodynamic state variable, but as a fundamental physical parameter that reflects the degree of electron activation and resonant states. On this basis, it seeks to electromagnetically define the temperature range within which biological reactions are possible, herein termed the life reaction window. Absolute zero (−273 °C) is regarded as a reference state in which electron motion and electromagnetic resonance are effectively halted, while an increase in temperature represents the degree of deviation from this state, corresponding to the activation of electromagnetic reactivity. From this perspective, the thermal stability of Earth’s environment is not simply a climatic condition, but a fundamental prerequisite for the persistence of life, enabling sustained electron transport, chemical reactions, and metabolic processes. This study further extends the interpretation of surface and oceanic temperature variations on Earth as a physical problem associated with the displacement or potential collapse of the life reaction window, and proposes a new electromagnetic framework applicable to studies of the origin of life, Earth’s environmental changes, and the assessment of habitability in extraterrestrial environments.