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?