Reversibility

Reversibility is the property of a process or transformation that, in principle, can be undone to return the involved system and its surroundings to their initial state. In many contexts, reversibility denotes an idealized limit rather than a common occurrence, because real processes typically produce dissipative effects or require infinite time to reverse.

Thermodynamic reversibility refers to a quasi-static process that occurs with infinitesimal departures from equilibrium, so no entropy is generated and the surroundings are affected minimally. Such processes are idealizations; real processes generate entropy and are therefore irreversible. The distinction underpins the Carnot cycle and limits on efficiency.

Time-reversal symmetry in physics describes laws that remain unchanged when time is reversed. Classical mechanics and many fundamental equations possess this symmetry, but macroscopic phenomena exhibit an arrow of time due to statistical behavior of large ensembles and initial conditions.

Chemical reversibility describes reactions that can proceed in both directions, achieving dynamic equilibrium according to reaction rates. The forward and reverse processes occur simultaneously, and the position of equilibrium depends on temperature, pressure, and concentration.

In computation, reversible computing uses bijective operations so information is not erased; theoretically, it can reduce energy dissipation. In practice, physical limits (Landauer's principle) imply energy cost for irreversible erasure of information.

In mathematics and dynamical systems, a reversible system is one where there exists a symmetry that reverses the direction of time evolution, enabling a mapping that recovers past states from present states.

Reversibility as a concept guides design in physics, chemistry, and computation, but real-world constraints limit its realization and define the gap between ideal reversible models and actual processes.