Energy–Momentum Accounting in Photon Interactions: A Relativistic Interpretation of a Conceptual Gap in Standard Physics

Abstract

In high-energy photon interactions, such as Compton scattering and pair production, a measurable reduction in the energy and momentum of outgoing photons is observed. The principle of energy–momentum conservation requires that this loss be accounted for as a physical quantity that remains within the system. However, in many standard treatments, the fate of this energy is not explicitly formulated and is only stated in the general context of four-momentum conservation. In this paper, by directly relying on the invariant relation in special relativity, we address this gap by providing a clear, quantitative framework for the fate of lost photon energy and momentum. We show how the reduction in photon momentum corresponds directly to an increase in rest mass, forming the basis for a more detailed energy accounting in photon interactions.

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