Suppose I have a system in my mind which consists of two elementary particles interacting with each other via fundamental interactions. We know that fundamental interactions have $T$-symmetry and there is no arrow of time.
Now, Suppose I add another particle to the system, then another, then another. At what instant, The time-reversal symmetry will break down?
As we know that macroscopic bodies do have an arrow of time. Is there any sharp boundary or what's the parameter that defines, How much sharp the arrow of time is? As I asked this question lately, but not seems to get any useful answer, Everyone just explaining the second law for macroscopic objects.
