Abstract: A formalism based on a four-vector fundamental equation is used to derive the equations governing mechanics and thermodynamics of a process, the absorption of laser photons by a blackbody in an adiabatic process, and the body entropy variations observed from two reference frames in the standard configuration. The classical (Clausius) definition of entropy variations, dS = dQrv/T (Qrv stands for heat exchanged in a reversible process), is generalised as dS¯ = || dÊmpN ||/T, with || dÈmpN || the norm of the four-vector associated to the set of thermal phonons exchanged in frame ^K as heat between the blackbody and the thermal reservoir. This relativistic formalism addresses the so-called Ott-Planck imbroglio, arising from the transformation of thermodynamic equations, such as the first law of thermodynamics, and related variables, including temperature, heat and entropy, within the framework of Einstein's special theory of relativity.

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