 When explaining time’s arrow many philosophers and physicists have focused their attention upon the unimpeachable fact that real systems are open systems that are subjected to interactions of various sorts. Thermodynamic systems cannot be truly isolated. To take the most obvious example, we can not shield a system from the influence of gravity. At best, we can move systems to locations feeling less and less gravitational force, but we can never completely decouple a system from the gravitational field. Not only do we ignore the weak gravitational force when doing classical thermodynamics, but we also ignore less exotic matters, such as the walls in the standard gas in a box scenario. We can do this because the time it takes for a gas to reach equilibrium with itself is vastly shorter than the time it takes the gas plus walls system to reach equilibrium. For this reason we typically discount the effects of the box walls on the gas.