Talk:Introduction to entropy: Difference between revisions

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::::::::Non-equilibrium is characterized by some sequence of measurements drifting a significant 'distance' through phase space. The drift may involve repeated distinct visits of the whole-system instantaneous microstate to some region of phase space, but it must be evident that they are repeated distinct and separate visits, not just little excursions in a permanent and persistent hovering pattern. In general, for a non-equilibrium trajectory through the phase space of whole-system instantaneous microstates, over some long observation time interval <math>(t_{\mathrm{initial}},t_{\mathrm{final}})</math>, the trajectory will drift from some region <math>R_{\mathrm {initial}} \subset R_0</math> to some other region <math>R_{\mathrm {final}} \subset R_0</math>, with negligible overlap <math>R_{\mathrm {initial}} \cap R_{\mathrm{final}}</math>. Thermodynamic entropy does not apply here. Other so-called 'entropies' may be defined ''ad lib'', but they refer to some kind of 'time rate of entropy production'.[[User:Chjoaygame|Chjoaygame]] ([[User talk:Chjoaygame|talk]]) 20:09, 19 December 2020 (UTC)
 
:::::::::I think of it this way: It is an *assumption* that every trajectory will visit any neighborhood in phase space with a probability proportional to the "volume" of that neighborhood. This is just another way of saying that each microstate is equally probable. Phase space may be divided up into a large number of macrostates, each with their own information entropy. For systems with a large number of particles, the microstates corresponding to the equilibrium macrostatesmacrostate hugely outnumber the volume of the nonequilibrium microstates combined. It follows that, starting from a nonequilibrium microstate, the trajectory will wander into the equilibrium macrostate region and practically never leave. Observationally, that is the signature of an equilibrium state - the macrostate is unchanging. Since the information entropy of a macrostate (and, by Boltzmann's equation, the thermodynamic entropy) is proportional to the log of the phase space volume occupied by that macrostate, the information entropy of the equilibrium macrostate is the largest. A trajectory from a non-equilibrium microstate does not "drift" in any particular direction any more than a trajectory from an equilibrium microstate does. A sort of random walk from any point in phase space will almost certainly walk you into an equilibrium microstate, and almost certainly not walk you into a non-equilibrium microstate, no matter what kind of state you started from. In phase space, trajectories do not "hover" around equilibrium microstates. The macrostate variables do "hover" around their means, however. [[User:PAR|PAR]] ([[User talk:PAR|talk]]) 21:17, 19 December 2020 (UTC)
 
== Outstanding questions ==