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].