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

by Seth Lloyd

In the middle of the nineteenth century, James Clerk Maxwell developed a detailed theory of heat in terms of the motion of atoms. He figured out how fast the atoms were moving as a function of temperature: the kinetic energy of an atom is proportional to its temperature. The hotter something is, the faster its atoms are jiggling around. This jiggling is also associated with entropy: the faster the atoms jiggle, the more information is required to describe their jiggling, and thus, the more entropy they possess. Temperature is a measure of the trade-off between information and energy: atoms at a high temperature require more energy to register a bit of information, and atoms at a low temperature require less energy to register a bit. Temperature is energy per bit. When energy in the form of heat flows from a hot thing to a cold thing, entropy increases: the same amount of energy registers less information when it’s hot than when it’s cold. The state of maximum entropy is obtained when everything is at the same temperature.