Gravitational entropy in black hole transformations

There are several reasons to support the idea that entropy might be associated to gravity itself. In the absence of a quantum theory of gravity, classical estimators for the gravitational entropy have been proposed. Any viable description of the gravitational entropy should reproduce the Hawking–Bekenstein entropy at the event horizon of black holes. Furthermore, in any black hole transformation, these estimators must satisfy the second law of black hole thermodynamics. In this work, we analyze whether two entropy estimators, one based on the Weyl tensor and the other on the Bel–Robinson tensor, satisfy the second law in the transformation process from a Schwarzschild to a Reissner–Nordström black hole by the absorption of a charge test particle. We also address the inverse process. We show that depending whether the process is reversible or not, both estimators fulfill the second law.