Programming the Universe: A Quantum Computer Scientist Takes on the Cosmos

by Seth Lloyd

we could look at matter at the atomic scale, we would see atoms dancing and jiggling every which way at random. The energy that drives this random atomic dance is called heat, and the information that determines the steps of this dance is called entropy. More simply, entropy is the information required to specify the random motions of atoms and molecules—motions too small for us to see. Entropy is the information contained in a physical system that is invisible to us. Entropy is a measure of the degree of molecular disorder existing in a system: it determines how much of the system’s thermal energy is unavailable for conversion into mechanical work—how much of its energy is useful. The second law of thermodynamics states that the entropy of the universe as a whole does not decrease; in other words, the amount of unusable energy is increasing. Manifestations of the second law are all around us. Hot steam can run a turbine and do useful work. As steam cools, its jiggling molecules transfer some of their disorder into increased disorder in the surrounding air, heating it up. As the molecules of steam jiggle slower and slower, the air molecules jiggle faster and faster, until steam and air are at the same temperature. When the difference in temperatures is minimized, the entropy of the system is maximized. But room temperature steam will do no work.