Dictionary Definition

irreversibility n : the quality of being irreversible (once done it cannot be changed) [ant: reversibility]

User Contributed Dictionary




  1. The quality of being irreversible; the lack of an ability to be reversed.
    • 1806, Samuel Barnard, The Essence, Spirituality, and Glorious Issue, of the Religion of Christ Jesus, to All God's Chosen, W. Nicholson, page 279,
      Now if the irreversibility of God's gifts to his people, be considered, what joy for those who feel within a wicked heart.
    • 1911, Power, Hill Pub. Co., page 171,
      But because of irreversibilities in the engine, all of this theoretical conversion is not realized and some internal energy remains in the steam exhausting from the engine.
    • 2007, Mark Currie, About Time: Narrative, Fiction and the Philosophy of Time, Edinburgh University Press, ISBN 0748624244, page 103,
      Paul Horwich, for example, in Asymmetries in Time, includes the laws of thermodynamics in a list of ten temporally asymmetric phenomena which operate in our conceptual system. Seen in this way, where the irreversibility of time is inseparable from rationality itself, the argument against backwards time tends to be expressed in terms of consequences, or the fear that time reversal is paramount to irrationality.

Extensive Definition

In science, a process that is not reversible is called irreversible. This concept arises most frequently in thermodynamics, as applied to processes. Irreversibility is also used in economics to refer to investment or expenditures that involve large sunk costs.
From a thermodynamics perspective, all natural processes are irreversible. The phenomenon of irreversibility results from the fact that if a thermodynamic system of interacting molecules is brought from one thermodynamic state to another, the configuration or arrangement of the atoms and molecules in the system will change as a result. A certain amount of "transformation energy" will be used as the molecules of the "working body" do work on each other when they change from one state to another. During this transformation, there will be a certain amount of heat energy loss or dissipation due to intermolecular friction and collisions; energy that will not be recoverable if the process is reversed.

Absolute versus Statistical reversibility

Thermodynamics defines the statistical behaviour of large numbers of entities, whose exact behavior is given by more specific laws. Since the fundamental laws of physics are all time-reversible, it can be argued that the irreversibility of thermodynamics must be statistical in nature, that is, that it must be merely highly unlikely, but not impossible, that a system will lower in entropy.


The German physicist Rudolf Clausius, in the 1850s, was the first to mathematically quantify the phenomenon of irreversibility in nature through his introduction of the concept of entropy. In his 1854 memoir “On a Modified Form of the Second Fundamental Theorem in the Mechanical Theory of Heat” Clausius states:

Complex Systems

The difference between reversible and irreversible events has particular explanatory value in complex systems (such as living organisms, or ecosystems). According to the biologists Humberto Maturana and Francisco Varela, living organisms are characterized by autopoiesis, which enables their continued existence. More primitive forms of self-organizing systems have been described by the physicist and chemist Ilya Prigogine. In the context of complex systems, events which lead to the end of certain self-organising processes, like death, extinction of a species or the collapse of a meteorological system can be considered as irreversible. Even if a clone with the same organizational principle (e.g. identical DNA-structure) could be developed, this would not mean that the former distinct system comes back into being. Events to which the self-organizing capacities of organisms, species or other complex systems can adapt, like minor injuries or changes in the physical environment are reversible. However, adaptation depends on import of negentropy into the organism, thereby increasing irreversible processes in its environment. Ecological principles, like those of sustainability and the precautionary principle can be defined with reference to the concept of reversibility.


irreversibility in Belarusian: Незваротны працэс
irreversibility in German: Irreversibilität
irreversibility in Spanish: Irreversibilidad
irreversibility in French: Réversibilité et irréversibilité en thermodynamique
irreversibility in Hungarian: Irreverzibilitás
irreversibility in Turkish: Tersinmezlik
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