Thermodynamics. And once again, therefore, we find that the Second Law is associated with basic phenomena that we already saw early in this book.

But just how general is the Second Law? And does it really apply to all of the various kinds of systems that we see in nature?

Starting nearly a century ago it came to be widely believed that the Second Law is an almost universal principle. But in reality there is surprisingly little evidence for this.

Indeed, almost all of the detailed applications ever made of the full Second Law have been concerned with just one specific area: the behavior of gases. By now there is therefore good evidence that gases obey the Second Law—just as the idealized model earlier in this section suggests. But what about other kinds of systems?

The approach to equilibrium in a reversible cellular automaton with a variety of different initial conditions. Apart from exceptional cases where no randomization occurs, the behavior obtained with different initial conditions is eventually quite indistinguishable in its overall properties. Because the underlying rule is reversible, however, the details with different initial conditions are always at least slightly different—otherwise it would not be possible to go backwards in a unique way. The rule used here is 122R. Successive pairs of pictures have initial conditions that differ only in the color of a single cell at the center.