Suppose you watch two time-lapse video clips. In the first video clip you see progressively more and more specks of jelly in the peanut butter. In the second clip you see less and less specks of jelly in the peanut butter.
Which video clip shows forward direction of time? Which shows time in reverse?
Answer: the first video clip shows forward direction of time.
Initially the jelly and peanut butter were separate, i.e. “ordered”. Over time, as the children make peanut butter and jelly sandwiches more and more specks of jelly get into the peanut butter and there is a greater degree of mixing of the two, i.e. it becomes more “disordered”.
There is a general trend in our world toward increasing disorder, e.g. cars rust, buildings crumble, mountains erode, apples rot, and cream poured into coffee dissipates until it is evenly mixed.
In fact, scientists have created a “law” that states that the disorder of the whole universe is increasing. Over time, all order, structure, and pattern in the universe breaks down, decays and dissipates – the ultimate end point of the universe is a random, featureless, homogenized murkiness. It is this increasing disorder that gives time its arrow, e.g. it is the increasing disorder of the peanut butter which enabled you to know which video clip shows the forward direction of time.
Scientists call this disorder “entropy”, and the law which states that the universe becomes increasingly disordered is called The Second Law of Thermodynamics.
— Extracted from The Origin of Wealth by Eric D. Beinhocker