The Essence of Radiation Pressure (Final Version)

Mainstream physical theories generally attribute all forms of radiation pressure to momentum transfer via photon collision, yet this oversimplified interpretation cannot reasonably explain why materials bear varying light-induced forces under identical natural illumination. Based on the energy coupling rule between light and matter, this paper establishes a complete theoretical framework by classifying scenarios into ultrahigh vacuum and normal atmospheric environments, differentiating momentum contribution and thermal expansion effect. In ultrahigh vacuum, radiation pressure originates entirely from photon momentum exchange, with the pressure on reflective surfaces roughly twice that on fully absorbing surfaces. Under standard atmospheric pressure on Earth, observable macroscopic apparent radiation pressure mainly stems from thermal expansion inside substances after selective light absorption boosts internal particle thermal motion, while the pressure contributed by photon momentum is numerically negligible at the macroscopic scale. Black materials absorb wide-range visible light efficiently and experience greater temperature rise and thermal expansion, hence larger mechanical force; white and specular substances reflect most incident light with little energy intake, mild temperature change and smaller thermally induced pressure. By distinguishing environmental conditions and separating momentum and thermal expansion effects, this theory fixes defects of conventional doctrines and consistently accounts for diverse experimental observations of radiation pressure.