 The Second Law of thermodynamics has long been connected to the seeming asymmetry of the arrow of time, that time seems to flow, so to speak, in only one direction for all systems no matter how different in kind they are and no matter how spatiotemporally separated. Indeed, one of the fundamental problems is that different types of system seem prima facie to give rise to independent arrows of time, e.g., thermodynamical, electromagnetic, cosmological, and so on, with no a priori reason why they should all point in the same direction. (See Zeh 2014 for a thorough recent review; see also the Encyclopaedia entry  Thermodynamic Asymmetry in Time.)  The Generalized Second Law and the corresponding idea of general gravitational entropy  (section 5.5)  introduces a new possible arrow of time, the gravitational. 