Question

Entropy has sometimes been described as "time's arrow" because it is the property that determines the forward direction of time. Explain.

Short answer

The entropy of an isolated system either remains constant or increases due to all spontaneous processes, but never decreases. This continual increase gives a direction to time, equating to the perception of time's arrow always pointing from the past to the future. Therefore, entropy is often described as 'time's arrow'.

Step-by-step solution

Understanding of Entropy

Entropy is a concept in the field of thermodynamics that measures the number of specific ways in which a system may be arranged, also referred to as its randomness or disorder. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time, and is constant if and only if all processes are reversible. Isolated systems spontaneously evolve towards thermodynamic equilibrium, the state with maximum entropy.

Introducing the Concept of 'Time's Arrow'

The term 'time's arrow' is a concept that is used to refer to the one-directional or asymmetric nature of time. It's usually associated with the idea of time progressing from the past to the future, and it doesn't, in normal conditions, 'turn back'.

Link between Entropy and 'Time's Arrow'

Because the entropy of an isolated system either remains constant or increases in all spontaneous processes, and never decreases, entropy can be considered a measure of the passage of time. The continuing increase of entropy seems to give a direction to time, leading to our perception of time's arrow always pointing from past to future.