2For a gas formed by 1022 molecules which has volume equal to 1 lt in room temperature and atmospheric pressure, the average velocity of its molecules is ≈ 100ms−1 . This means that the typical de Broglie wavelength of the molecules is λ ≈ 10−10m . If we estimate that the volume occupied by each molecule is of order λ3 , then the number of states that each molecule can be is ≈ 1027 . Therefore the system can be in ≈ (1027)1022 different states. If we assume that on the average the molecules collide 109 times per second, then we have ≈ 1031 changes of states per second. In order that the system visits all possible states, the time needed is 101023 times the age of the universe  [4].