Thermodynamics and Relativity, A Condensed Explanation of their Close Connection

Although its practical efficiency is unquestionable, it is well known that thermodynamics presents conceptual difficulties from the theoretical point of view. It is shown that the problem comes from an imperfect compatibility between the first law and the second law. The solution suggested consists of connecting thermodynamics with relativity (and, by extension, with gravitation) through the Einstein mass-energy relation.