Synopsis: Creating—or destroying—entropy in a small system

Statistical physicists have long known that brief reductions in entropy are possible. Now, entropy changes in two nonequilibrium systems have been calculated exactly.
Synopsis figure
Illustration: Alan Stonebraker

The second law of thermodynamics forbids a decrease in entropy of an isolated system. However, in statistical mechanics this strict prohibition is softened to a probabilistic statement, which allows transient decreases in entropy to occur with a small probability. The likelihood of such fluctuations is vanishingly small in macroscopic bodies, but in smaller systems, such as a stretched DNA molecule, they can actually be observed.

This possibility has motivated recent work on fluctuation theorems, which compare the probability of a system reducing its entropy (over short times) in out-of-equilibrium processes to those in which the entropy increases. In a paper appearing in Physical Review E, Arnab Saha of the S. N. Bose National Center For Basic Sciences in Kolkata, India, and Sourabh Lahiri and Arun Jayannavar, of the Institute of Physics, in Bhubaneswar, India, build on this work and study the total entropy produced in a simple system that is driven out of equilibrium. They model a Brownian particle in a harmonic trap and consider the change in entropy when a time-dependent external force is applied to the particle, or the trap itself is moved in an arbitrary manner.

In nonequilibrium steady states, the detailed fluctuation theorem relates the probabilities of observing entropy changes of equal magnitude but opposite sign. Saha et al. show, somewhat surprisingly, that the Brownian particles also obey this theorem even in the nonstationary, transient regime, provided the system is prepared in equilibrium.

These and other advances given in this paper should stimulate the analysis of thermal fluctuations in small systems, and their application in determining free energy differences. – Ron Dickman


Features

More Features »

Announcements

More Announcements »

Subject Areas

Statistical Physics

Previous Synopsis

Atomic and Molecular Physics

Making monopoles

Read More »

Next Synopsis

Nonlinear Dynamics

Swinging and springing

Read More »

Related Articles

Synopsis: Straying from the Norm in Pedestrian Movements
Complex Systems

Synopsis: Straying from the Norm in Pedestrian Movements

Experiments tracking people as they walk down a corridor reveal universal behaviors that, if incorporated into models, could ensure safe flow in large crowds. Read More »

Synopsis: Number of Cycles Matters for a Quantum Engine
Statistical Physics

Synopsis: Number of Cycles Matters for a Quantum Engine

Theoretical calculations show that the performance of a quantum heat engine over several cycles can’t be judged by analyzing just a single cycle. Read More »

Focus: Grid Outages from Failures of Power Line Clusters
Complex Systems

Focus: Grid Outages from Failures of Power Line Clusters

Specific clusters within a network tend to fail consistently as part of large-scale network failures, such as those in electrical grids or airline transportation systems. Read More »

More Articles