Academic literature on the topic 'Nonequilibrium partition function'

A new approach to the statistical description of a self-gravitating system has been proposed. The approach employs a nonequilibrium statistical operator that involves into consideration inhomogeneous distributions of particles and temperature. The states with dominant contributions to the partition function are found in terms of the saddle-point method that yields all the thermodynamic relations for a self-gravitating system. The approach makes it possible to describe new peculiar features in the behavior of the gravitating system under various external conditions; it may be applied to describe the formation of stars and galaxies.

In this work we present a brief derivation of the periodic orbit expansion for simple dynamical systems, and then we apply it to the study of a classical statistical mechanical model, the Lorentz gas, both at equilibrium and in a nonequilibrium steady state. The results are compared with those obtained through standard molecular dynamics simulations, and they are found to be in good agreement. The form of the average using the periodic orbit expansion suggests the definition of a new dynamical partition function, which we test numerically. An analytic formula is obtained for the Lyapunov numbers of periodic orbits for the nonequilibrium Lorentz gas. Using this formula and other numerical techniques we study the nonequilibrium Lorentz gas as a dynamical system and obtain an estimate of the upper bound on the external field for which the system remains ergodic.

Showing that the location of the zeros of the partition function can be used to study phase transitions, Yang and Lee initiated an ambitious and very fruitful approach. We give an overview of the results obtained using this approach. After an elementary introduction to the Yang–Lee formalism, we summarize results concerning equilibrium phase transitions. We also describe recent attempts and breakthroughs in extending this theory to nonequilibrium phase transitions.

The lattice-gas method has been applied to solve the problem of kinetic theory of gas in the Gay–Lussac–Joule experiment. Numerical experiments for a two-dimensional gas were carried out to determine the number of molecules in one vessel (Nr), the ratio between the mean square values of the components of molecule velocity [Formula: see text], and the change in internal energy (ΔU) as a function of time during free expansion. These experiments were repeated for different sizes of an aperture in the partition between the two vessels. After puncturing the partition, the curve for the particle number in one vessel shows a damped oscillation for about half of the total number. The oscillations do not vanish after a sampling over different initial configurations. The system is in nonequilibrium due to the pressure equilibration, and here the flow is actually compressible. The equilibration time (in time steps) decreases with decreased size of aperture in the partition. For very small apertures (equal or less than [Formula: see text] lattice units), the number of molecules in one vessel changes with time in a smooth way until it reaches half of the total number; their curves obey the analytical solution for quasi-static processes. The calculations on [Formula: see text] and ΔU also support the results that the equilibration time decreases with decreased size of aperture in the partition.

A new approach based on the nonequilibrium statistical operator is presented that makes it possible to take into account the inhomogeneous particle distribution and provides obtaining all thermodynamic relations of self-gravitating systems. The equations corresponding to the extremum of the partition function completely reproduce the well-known equations of the general theory of relativity. Guided by the principle of Mach's "economing of thinking" quantitatively and qualitatively, is shown that the classical statistical description and the associated thermodynamic relations reproduce Einstein's gravitational equation. The article answers the question of how is it possible to substantiate the general relativistic equations in terms of the statistical methods for the description of the behavior of the system in the classical case.

The dissipation function is a key quantity in nonequilibrium statistical mechanics. It was originally derived for use in the Evans-Searles Fluctuation Theorem, which quantitatively describes thermal fluctuations in nonequilibrium systems. It is now the subject of a number of other exact results, including the Dissipation Theorem, describing the evolution of a system in time, and the Relaxation Theorem, proving the ubiquitous phenomena of relaxation to equilibrium. The aim of this work is to study the significance of the dissipation function, and examine a number of exact results for which it is the argument. First, we investigate a simple system relaxing towards equilibrium, and use this as a medium to investigate the role of the dissipation function in relaxation. The initial system has a non-uniform density distribution. We demonstrate some of the existing significant exact results in nonequilibrium statistical mechanics. By modifying the initial conditions of our system we are able to observe both monotonic and non-monotonic relaxation towards equilibrium. A direct result of the Evans-Searles Fluctuation Theorem is the Nonequilibrium Partition Identity (NPI), an ensemble average involving the dissipation function. While the derivation is straightforward, calculation of this quantity is anything but. The statistics of the average are difficult to work with because its value is extremely dependent on rare events. It is often observed to converge with high accuracy to a value less than expected. We investigate the mechanism for this asymmetric bias and provide alternatives to calculating the full ensemble average that display better statistics. While the NPI is derived exactly for transient systems it is expected that it will hold in steady state systems as well. We show that this is not true, regardless of the statistics of the calculation. A new exact result involving the dissipation function, the Instantaneous Fluctuation Theorem, is derived and demonstrated computationally. This new theorem has the same form as previous fluctuation theorems, but provides information about the instantaneous value of phase functions, rather than path integrals. We extend this work by deriving an approximate form of the theorem for steady state systems, and examine the validity of the assumptions used.