Many of Georgescu-Roegen’s insights can be expressed in terms of his “entropy hourglass analogy.”

First, the hourglass is an isolated system: no sand enters, and no sand exits.

Second, within the glass there is neither creation nor destruction of sand, the amount of sand in the glass is constant. This of course is the analog of the first law of thermodynamics—conservation of matter–energy.

Third, there is a continuing running down of sand in the top chamber, and an accumulation of sand in the bottom chamber. Sand in the bottom chamber, since it has used up its potential to fall and thereby do work is high-entropy or unavailable matter/energy. Sand in the top chamber still has potential to fall, thus it is low-entropy or available matter/energy. This is the second law of thermodynamics: entropy increases in an isolated system. The hourglass analogy is particularly apt since entropy is time’s arrow in the physical world.

One more thing—unlike a real hourglass, this one cannot be turned upside down!

This Informational Box is an excerpt from An Introduction to Ecological Economics by Robert Costanza, John H Cumberland, Herman Daly, Robert Goodland, Richard B Norgaard. ISBN: 1884015727