We will try to understand exactly where is the trend of the universe to evolve into chaos, taking as example a pool of a rather unusual kind: a pool on which the balls ride without being hindered. Once set in motion, they can stop more ...

We will now place, as drawn below, a plate in the middle of the pool, which nevertheless leaves space for the balls to move from side to side. It leaves six balls move in this new environment, and pay attention to the side of the plate where they are. We see that in fact it's quite rare to find them all by chance on the same side of the plate. And in any case, it never lasts very long. Most of the time they are 3 on one side and 3 on the other. We can therefore say that, on average, over time, the balls do not all alone on the same side of the plate, but they are distributed.

Now, you said it has launched twenty balls on the billiard table, and we took two photos, one at the initial time, and the other a little later. It shows you two pictures, one on which you see as many balls on each side of the plate, the other where you see them all the same side. And you are asked which represents the initial time, which is taken later. How to respond?

Well it's simple: you have much, much less likely to go wrong if you answer that the balls were first launched all the same side, and they are then distributed. This is most likely, by far. Of course, the converse is not strictly impossible, but it is so rare that you have more chances to have it all wrong in assuming that the balls spontaneously gathered. And with 20 balls, it takes a long time when they were thrown at random, before you see them all the same side.

So finally, we can consider that appeared almost irreversible phenomenon: the balls have a tendency to divide, to increase their disorder, if left to themselves. It is simply that it is much more likely. You know say what photo was taken the first with very little chance of making mistakes! The pinballs therefore obey the second law of thermodynamics, since the disorder increases with time!

In fact, the billiard balls are very much like gas molecules. And a gas, such as billiard balls, tends to occupy all the space that is available, thus maximizing its disorder. If you look at gas in a box, it will never spontaneously shaking in a corner. Even if nothing seems to prohibit it. But remember, if with 20 billiard balls, it seems you already long wait they are on one side of the box, then with a gas, it will seem even longer: even a cubic centimeter of gas contains many billions of billions of molecules. At least. So for them to put all on one side of the box, and the second principle is violated, you should wait at least several billion years, without even being sure it could happen while waiting long enough. It is extremely unlikely.

Consider now the example of two cubes, a hot and cold, touch each other. On average, the hot particles, the more agitated, the particles bump against cold, and communicate their agitation, by losing. So the thermal agitation tends to s'homogénéiser. Ie the cold cube and the cube will be hot warm both. But this too is a purely statistical trend, because the behavior of particles of these cubes, can not say that the reverse is impossible. It is possible, but highly unlikely!

If we consider a drop of dye in water, we imagine that the colorant particles are subjected to shocks from the water particles. These random shocks tend to disperse the dye. But again, by extraordinary coincidence, as no one has ever seen, and never probably ever seen, it is possible that the dye comes together rather than disperse. Nothing really prohibited in the basic laws that govern the particles. But it's so unlikely that we can consider that it is impossible. Moreover, before being explained, the second principle is an empirical principle, ie from the experience - what we see around us, in fact.

It is said that the second law of thermodynamics is a statistical origin.