 It is most surprising, therefore, to learn that the Second Law is a deep, rigorous theorem that follows only from the fundamental mathematics of relativistic spacetimes (Hawking 1971), and does not depend in any essential way on the particulars of relativistic dynamics as encapsulated in the Einstein field equation (Curiel 2017). This is in strict opposition to the Second Law in classical thermodynamics, which stands as a more or less phenomenological principle derived by empirical generalization, perhaps justified in some sense by a “reduction” to statistical mechanics, with the temporal asymmetry of entropy non-decrease argued to hold based on the likelihood of initial states in conjunction with the forms of dynamical evolution “physically permissible” for matter fields. (See the entry  Philosophy of Statistical Mechanics.)  