Entropy, Gravity, and an Apparent Violation of the Second Law

Abstract

An interesting question to explore in physics classes is whether gravity violates the second law of thermodynamics. Standard physics textbooks provide little to no discussion of the relationship between entropy and gravity, and the same is often true of specialized texts. The aim of this work is to address this question by analyzing the behavior of an ideal gas in two simple scenarios: one in which gravity is negligible and another in which its effects are significant. We show that although systems influenced by gravity may exhibit counterintuitive behavior, such as local ordering through structure formation, the second law of thermodynamics remains valid when the entire system is considered, including all emitted energy and radiation. Given the educational focus of this work and the complexity of the entropy-gravity relationship, we omit detailed calculations that are not strictly necessary and instead focus on the simplest physical scenarios. In this context, we analyze four representative examples through simple calculations: the Sun, the limit of extreme contraction in black holes, the protostellar contraction sequence, and core collapse with neutrino cooling.