chapter  Five
48 Pages

Reversible Computation and the Thermodynamics of Computing

ByRichard P. Feynman

For an irreversible process, the equality is replaced by an inequality, ensuring that the entropy of an isolated system can only remain constant or increase — this is the Second Law of Thermodynamics. In fact, it was contemplation of Maxwell's demon that partly led workers such as Charles Bennett and Rolf Landauer to their conclusions about reversible computing and clarified the link between information and entropy. The physical state of the molecule before and after the compression appears to be the same. In the device, the movement of billiard balls on a plane is used to simulate the movement of electronic signals through logic gates. The easiest way to get the dot from one trough to the other would be to bring the model up rapidly to bias the troughs in the desired way. The denominator is the average rate of transition — it is a measure of the degree to which computer is oscillating back and forth.