Irreversibility

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.

History
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:

It may, moreover, happen that instead of a descending transmission of heat accompanying, in the one and the same process, the ascending transmission, another permanent change may occur which has the peculiarity of not being reversible without either becoming replaced by a new permanent change of a similar kind, or producing a descending transmission of heat.

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.