Thermodynamic time

Although it is easy to imagine apparently reversible processes in time (the swing of a pendulum, the motion of the planets, the trajectory of a marble bouncing between the sides of a billiard table), microscopic analysis shows that even these processes are in fact irreversible thermodynamic processes that define a unique way in which time flows, that is, always in the same direction.

For it is well known that of the energy supplied to a system to induce any transformation in it, an ineradicable fraction must still go out irretrievably in the form of disordered energy, such as heat.

The decreasing amplitude of the pendulum oscillations due to friction (heat generators), the heating of the sphere as a result of collisions, and the dissipation of kinetic energy in the form of gravitational radiation by the orbiting planet provide as many keys to defining the direction in which time flows. In general, all real, non-interacting systems tend to reach states of increasing internal disorder as they undergo thermodynamic transformations.

Strictly speaking, the laws of thermodynamics dictate that real transformations of isolated systems take place in the sense of increasing their entropy. Irreversibility and increasing entropy are synonyms which, in the same way that they denote the properties which events obey in their becoming, also encapsulate the essential property which gives meaning to time, since the latter would lose all meaning if there were no production of events or if there were no means of determining the universal direction of flow.

Thermodynamic arrow of time

The arrow of time is the “one-way direction” or “asymmetry” of time. The thermodynamic arrow of time is provided by the second law of thermodynamics, which states that in an isolated system, entropy tends to increase with time. Entropy can be thought of as a measure of microscopic disorder, so the second law implies that time is asymmetric with respect to the amount of order in an isolated system: as a system progresses through time, it becomes more statistically disordered. This asymmetry can be used empirically to distinguish between the future and the past, although measuring entropy does not accurately measure time. In an open system, entropy can also decrease over time.

The British physicist Sir Alfred Brian Pippard wrote There is therefore no justification for the view, often glibly repeated, that the second law of thermodynamics is only statistically true, in the sense that microscopic violations occur repeatedly, but never violations of any serious magnitude. On the contrary, no evidence has ever been presented that the second law “breaks down” under any circumstances1. However, there are a number of paradoxes regarding violations of the second law of thermodynamics, one of which is due to Poincaré’s recurrence theorem.

Harold Blum’s 1951 book Time’s Arrow and Evolution2 discusses “the relationship between the arrow of time (the second law of thermodynamics) and organic evolution. This influential text explores “irreversibility and direction in evolution and order, negentropy and evolution”3. Blum argues that evolution followed specific patterns predetermined by the inorganic nature of the Earth and its thermodynamic processes4.


  1. A. B. Pippard, Elements of Chemical Thermodynamics for Advanced Students of Physics (1966), p. 100. ↩︎
  2. Blum, Harold F. (1951). Time’s Arrow and Evolution (First ed.). Princeton University Press. ISBN 978-0-691-02354-0. ↩︎
  3. Morowitz, Harold J. (September 1969). “Book review: Time’s arrow and evolution: Third Edition”. Icarus. 11 (2): 278–279. Bibcode:1969Icar…11..278M. doi:10.1016/0019-1035(69)90059-1. PMC 2599115. ↩︎
  4. McN., W. P. (November 1951). “Book reviews: Time’s Arrow and Evolution”. Yale Journal of Biology and Medicine. 24 (2): 164. PMC 2599115. ↩︎
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