^{2}For a gas formed by molecules which has volume equal to 1 lt in room
temperature and atmospheric pressure, the average velocity of its molecules is .
This means that the typical de Broglie wavelength of the molecules is . If we
estimate that the volume occupied by each molecule is of order , then the number of states
that each molecule can be is . Therefore the system can be in
diﬀerent states. If we assume that on the average the molecules collide times per
second, then we have changes of states per second. In order that the system
visits all possible states, the time needed is times the age of the universe
[4].