Arrow of time refers to the capability of distinguishing previous events from subsequent events by observing significant differences between the two events. The concept throughout history has represented a philosophical issue. However, it became a scientific issue with the advent of the theory of relativity and again with the discovery of phenomena at the subatomic level.

The second law of thermodynamics, or the law of entropy, states that the universe and all of its energy systems will increase in disorder as time moves forward. In physics, disorder means the breakdown of energy into useless heat, from which no work can be done.

The concept of entropy, the increase in disorder over time, can be expected to provide a direction of time. A movement into the future can be recognized by observing deterioration and decay of things in the past. The laws of entropy may...