The Arrow of Time

He [Sean Carroll] explains how, at a subatomic or quantum level, it is far from obvious why the arrow of time should point the way it does, for the laws of physics stipulate that particle interactions are reversible. In the quantum world, time might as well run backward as forward. Imagine a film clip of two billiard balls moving at a steady pace, colliding and bouncing off each other. How can you be sure that the film was not shown to you in reverse—that what you actually saw was time running backward? So Mr. Carroll persists: "Why then, in the observable universe, does time appear to run in one direction only? Why, for instance, when an egg is broken and scrambled into an omelette, if the quantum processes that allowed this to happen are reversible, why does the omelette never reassemble itself into an egg?"

- Alexander Waugh in the WSJ


It is exactly for the reason that billiard balls would not be moving at a steady pace, rather would be slowing down at each instant through resistance, that the arrow of time points the way it does. And this resistance, or reaction activation requirements in the language of interactions reversible or otherwise, would accumulate at each infinitesimal moment leading to a formidable requirement at the meta level manifested in unidirectional time, just as the billiard ball noticeably slows down. This may be more evident in the cracking of an egg as each quantum crack, falsely proposed to be reversible, has a very low activation requirement to be overcome in one direction, yet a much larger one in the opposite direction due to the reduction in entropy. Hence, the overall reaction is irreversible.