Entropy as an arrow of time

Use of the second law of thermodynamics to distinguish past from future

Entropy is one of the few quantities in the physical sciences that requires a particular direction for time, sometimes called an arrow of time. As one goes "forward" in time, the second law of thermodynamics says, the entropy of an isolated system can increase, but not decrease. Thus, entropy measurement is a way of distinguishing the past from the future.

From Wikipedia, under CC BY-SA. More on occurri.