Random Fluctuations of Diathermal and Adiabatic Pistons

A comparison between the standard adiabatic piston dynamics and that of a perfectly conducting (diathermal) piston helps to clarify their different behaviors and, in particular, the anomalously large random displacement of the adiabatic piston as compared to the diathermal one. It is shown to be associated with a situation where the presence of a single massive "particle" (the piston), acting as an internal constraint in a many-particle system, plays a somewhat unexpected relevant role. A significant physical insight accounting for the above difference is gained by means of a simple analysis of the phase space available to our system.