Editing T-symmetry (section)

===The second law of thermodynamics===

[[Image:teeter-totter.png|frame|A toy called the [[teeter-totter]] illustrates, in cross-section, the two aspects of time reversal invariance. When set into motion atop a pedestal (rocking side to side, as in the image), the figure oscillates for a very long time. The toy is engineered to minimize friction and illustrate the reversibility of [[Newton's laws of motion]]. However, the mechanically stable state of the toy is when the figure falls down from the pedestal into one of arbitrarily many positions. This is an illustration of the law of increase of [[entropy]] through [[Boltzmann]]'s identification of the logarithm of the number of states with the entropy.]]

Daily experience shows that T-symmetry does not hold for the behavior of bulk materials. Of these macroscopic laws, most notable is the [[second law of thermodynamics]]. Many other phenomena, such as the relative motion of bodies with friction, or viscous motion of fluids, reduce to this, because the underlying mechanism is the dissipation of usable energy (for example, kinetic energy) into heat.

The question of whether this time-asymmetric dissipation is really inevitable has been considered by many physicists, often in the context of [[Maxwell's demon]]. The name comes from a [[thought experiment]] described by [[James Clerk Maxwell]] in which a microscopic demon guards a gate between two halves of a room. It only lets slow molecules into one half, only fast ones into the other. By eventually making one side of the room cooler than before and the other hotter, it seems to reduce the [[entropy]] of the room, and reverse the arrow of time. Many analyses have been made of this; all show that when the entropy of room and demon are taken together, this total entropy does increase. Modern analyses of this problem have taken into account [[Claude E. Shannon]]'s relation between [[information entropy|entropy and information]]. Many interesting results in modern computing are closely related to this problem—[[reversible computing]], [[quantum computing]] and [[physical limits to computing]], are examples. These seemingly metaphysical questions are today, in these ways, slowly being converted into hypotheses of the physical sciences.

The current consensus hinges upon the Boltzmann–Shannon identification of the logarithm of [[phase space]] volume with the negative of [[information theory|Shannon information]], and hence to [[entropy]]. In this notion, a fixed initial state of a macroscopic system corresponds to relatively low entropy because the coordinates of the molecules of the body are constrained. As the system evolves in the presence of [[dissipation]], the molecular coordinates can move into larger volumes of phase space, becoming more uncertain, and thus leading to increase in entropy.