Fluctuation theorem

A family of relations in nonequilibrium statistical mechanics connecting the probabilities of positive and negative fluctuations in entropy production or related quantities.

Entropy production: The thermodynamic quantity measuring irreversible change in a system and its surroundings. Fluctuation relations compare the likelihood of trajectories producing entropy with their time reversals.
Evans–Searles fluctuation theorem: A finite-time fluctuation relation for dissipation in systems evolving from specified initial conditions. It is an early formulation that treats transient fluctuations before steady state.
Nonequilibrium statistical mechanics: The study of statistical laws governing systems that exchange matter or energy or sustain macroscopic currents. The fluctuation theorem is a relation within this broader framework.
Single-molecule force spectroscopy: Experimental methods that measure forces and motions in individual molecules. Work relations can infer free-energy differences from repeated nonequilibrium pulling measurements.
Time reversal: The transformation that reverses the direction of time in a physical description. The theorem pairs forward trajectories with reversed trajectories to derive probability ratios.
Denis Evans: An Australian physicist known for work on nonequilibrium statistical mechanics and molecular dynamics. His simulations helped reveal fluctuation relations in driven systems.
Boltzmann's H-theorem: A kinetic-theory result showing that the Boltzmann H quantity decreases under molecular-chaos assumptions. Its apparent one-way behavior highlights the reversibility puzzle fluctuation relations address.
Optical tweezers: Instruments that use focused laser beams to trap and manipulate microscopic particles. They enable controlled work protocols used to test fluctuation relations in small systems.
Microscopic reversibility: The principle that microscopic dynamics respect time-reversal symmetry under appropriate conditions. It links the probabilities of a trajectory and its time-reversed counterpart.
E. G. D. Cohen: A Dutch-American physicist whose work spans statistical mechanics and kinetic theory. He co-authored the early numerical work that motivated the theorem's development.

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