Thermodynamical Versus Optical Complementarity

We establish correspondence between macroscopic thermodynamical quantities and complementarity in wave interference. The well known visibility and predictability in a double slit--like experiment are shown to be connected to magnetic susceptibility and magnetization of a general interacting spin chain. This gives us the ability to analyze the tradeoff between thermodynamical quantities in the same information--theoretic way that is used in analyzing the wave--particle duality. We thus obtain new physical insights into usually complicated thermodynamical models, such as viewing a phase transition simply as a change from an effective single slit diffraction to a double slit interference.