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Introduction

The relationship between entropy and living things has been widely discussed since the last century. In 1871, Maxwell unnerved thermodynamicists by suggesting a way that a living being could break the Second Law of Thermodynamics [Maxwell, 1904,Ehrenberg, 1967,Rothstein, 1951]. One of the many implications of the Second Law [Jaynes, 1988] is that when a quantity of gas is separated into two compartments, both initially at the same pressure and temperature, it is not possible to raise the temperature in one of the compartments and lower the other without performing work. Maxwell proposed that a tiny intelligent being could open and close a hole between two such compartments to allow only fast molecules from the first compartment to pass into the second compartment and slow molecules to move in the other direction. The first compartment would become cool while the other compartment would become hot. Assuming that this demon does not need to do any work to open and close the hole, one could use the heat difference to run an engine. This perpetual motion machine would violate the Second Law of Thermodynamics.

The problem of Maxwell's demon was partially resolved by Szilard in 1929 [Szilard, 1929,Szilard, 1964,Feld & Szilard, 1972] and more completely by Brillouin in 1951 [Brillouin, 1951a,Brillouin, 1951b]. They recognized that the demon would have to obtain one bit of information about the approaching molecules. To distinguish the molecules from the background of thermal radiation, the demon could use a flashlight. Brillouin showed that more energy would be lost by operating the flashlight than could be gained by the demon's tricks. Thus the information that the demon gains must be paid for by a loss of some energy, and the Second Law is not broken.

Brillouin and Szilard's arguments are not convincing because the problem has been posed for an imaginary beast. It is not obvious, for example, that controlled opening and closing of a door can be done without energy dissipation. The difficulty of guaranteeing that a photon from the flashlight reaches the eye of the demon and the problem of what happens to the photon's energy in the eye of the demon have also been ignored. To bring this problem into the concrete world of molecular biology [Watson et al., 1987], we can focus on the mechanisms of molecules that can be investigated in the laboratory. The question of what Maxwell's demon can do becomes a question of how rhodopsin in the eye and actomyosin in muscle operate. Indeed, it becomes the question of how all molecular machines operate.