Georgescu-Roegen coined the fourth law of thermodynamics according to which matter continuously and irrevocably degrades from an available to an unavailable state in a closed system including the Earth and predicted that we would soon arrive at the state of maximum entropy, material death rather than heat death. He also insisted that recycling could not avoid this material death, because the ideal of complete recycling is based on the illusion of the thermodynamically impossible perpetual motion. But what he called the fourth law of thermodynamics is false. A closed system such as the Earth can reduce material entropy in exchange for increasing thermal entropy, keeping the former low by dumping the latter in outer space. That is to say, a sustainable economy based on recycling is possible, at least theoretically.

Clausius, one of the main founders of the modern thermodynamics, is the originator of the term “entropy,” but before this coinage he used an expression “equivalence value of transformations” instead. The term “entropy,” which is now used as a physical term, was originally economically conceived as equivalent value of exchange. Thermodynamics itself was a field of physics that was economically motivated to improve the thermal efficiency of heat engines. So, it was half physics and half economics. Apart from Clausius’s interest, the term has now become an important keyword for solving the environment and resource problems.

A heat engine is a device that repeatedly converts thermal energy into kinetic energy. It needs the difference of temperature and a working substance with a high rate of expansion to do useful work for us. It can be done in two ways: utilizing the change of volume of the working substance that hot and cold reservoirs make alternately expand or compress, and utilizing convection that the change of density resulting from the change of volume causes. A heat engine is not confined to an artificial one. The Earth also does useful work for living systems as a heat engine. The heat engine whose hot reservoir is solar radiation heat maintains the circulation of air and water, while the heat engine whose hot reservoir is geothermal heat maintains the circulation of mantle and mineral nutrition.

The shortage of natural resources and the destruction of the natural environment are two colossal problems that are now threatening our existence. These seemingly two problems are just one problem ― the problem of entropy. On this page, I will show that the essence of the two problems is entropy and that the key to resolving them consists in how to decrease entropy. So, first, let me explain what entropy is.

In modern Europe, engineers tried to invent perpetual motion machines, only to fail. Perpetual motion machines are imaginary machines that eternally work without being supplied with any fuel once they are set up. Invention of perpetual motion machines is modern alchemy.

There are two fundamental laws in thermodynamics. The first law of thermodynamics is the law of the conservation of energy. The energy of the universe is constant. The second law is the principle of maximum entropy. The entropy in an isolated system does not decrease.