Introduction

It is well known that all natural physical processes are irreversible. Three brief examples will demonstrate this.

1. (i) It has never been observed that heat flowing from a warmer to a colder system will suddenly change its direction and flow from the colder to the warmer system. Nevertheless, the first law of thermodynamics does not prohibit this reversal of direction.

2. (ii) Consider a system consisting of two chambers. One of these is filled with a gas, the second chamber is completely evacuated. If the separating wall is pierced, a mass flow will take place until the pressure in both chambers is the same. It has never been observed that the original situation was restored by a return flow.

3. (iii) A stone is dropped into a water container resulting in an increase of the internal energy of the water container and, therefore, of its temperature. It never happens that the water container cools off spontaneously using the change of the internal energy to expel the stone.

In case of irreversible processes, the original state can be restored only by means of interactions with other systems which then suffer a remaining change. For example, to restore the original system in the second example, energy in form of work is required to evacuate the second chamber.

Irreversible processes taking place in isolated systems run in one direction only and thus provide the possibility of discerning between the past, the present and the future.