Time arrow

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The time arrow represents the idea of ​​a clear and directed connection between the past and the future . Different meanings are associated with this idea in the sciences, but also in everyday life. The term " time's arrow " was first coined by Arthur Stanley Eddington in 1927 ( Gifford Lectures ).

Time arrows


The psychological arrow of time describes our subjective distinction between past and future events. We can remember the past, but not the future. The western perspective of the time arrow looks at the future in front (i.e. in the direction of view).

Pictorial representation of a time arrow after Arthur Eddington


Conversely, however, the local adverb “in front” is often used for the past, e.g. B. “before”, “before”, before (English), avant (French), while the word for locally “after” is used for the future, e.g. B. after (English), après (French).

Also in Andean cultures (e.g. Quechua , Aymara ) the future is viewed as lying behind (i.e. opposite to the direction of vision, because the future is unknown), which is expressed accordingly in the languages ​​of the Andes ( Quechua , Aymara ), in which the local Adverb for "in front" (Quechua: "ñawpa") in the temporal sense means "earlier, past", while the adverb for "behind" (Quechua: "qhipa") in the temporal sense means "in the future".


According to this, causes always precede their effects. The causal arrow of time is a postulate that reflects everyday experience. However, it is not clear whether causality is mandatory or whether it is only generated through perception .


The thermodynamic time arrow is based on the 2nd law of thermodynamics : The future is the time direction in which the entropy increases. An interesting point is that this time arrow does not exist in thermodynamic equilibrium: for a state of equilibrium there is no thermodynamically defined past and future; the state of equilibrium is timeless, so to speak.


The universe started with the Big Bang and may have expanded since then. It is not known for sure whether it will extend into eternity. According to the currently prevailing calculations and theories, it looks like this. Thus, one can read the past time from the size of the universe: The future is the direction of the larger universe.

But even if the universe contracts again, the old, collapsing cosmos will look different than the early, expanding cosmos: It contains burned-out stars , some of which have collapsed into black holes , and heavy elements that have arisen in supernova explosions. Thus one can also read off one's age and thus the time direction from the composition of the universe.

The T Injury and CP Injury

While the difference between the past and the future is omnipresent at the macroscopic level, it has long been true of the well-known microscopic laws of matter that they were invariant in time : if a process can run forwards, then it can just as well run backwards, provided that the prerequisites are met . For example, the fact that an excited atom can fall into the ground state with emission of a photon means that the reverse process, the excitation of an atom in the ground state by an absorbed photon, is also possible via the same mechanism.

In 1964, measurements on certain elementary particles , the kaons , revealed a violation of the CP invariance for the first time. This previously assumed invariance means that the (spatially) mirror-inverted process in antimatter also exists for every process in matter and can take place in the same way.

The violation of the CP invariance is interesting at this point because of the CPT theorem , which states that for every process with matter, the mirrored and time-reversed process with antimatter takes place in the same form. This theorem is a fundamental part of any quantum field theory , so it is expected to hold exactly. But if the CPT theorem holds, a violation of the CP invariance also means a violation of the time-reversal invariance.

This does not necessarily mean that the fundamental laws of physics know a difference between the past and the future, but only that the exactly time-reversed analogue of a process also requires a space reflection and an exchange of matter and antimatter. In 2012, a direct violation of the T symmetry was observed for the first time.


  • Dieter Zeh : The physical basis of the direction of time , first 1984 ( Die Physik der Zeitrichtung , Springer), 5th edition, Springer Verlag, 2010, ISBN 3-540-42081-9 .
  • PCW Davies : The physics of time asymmetry , University of California Press, 1976 (and his popular science book About Time , Penguin 1995)
  • David Layzer : The Arrow of Time , Scientific American, December 1975
  • Claus Kiefer : Cosmological Foundations of Irreversibility , Physikalische Blätter 1993, p. 1027
  • Peter Coveney, Roger Highfield: The Arrow of Time , Verlag WH Allen, 1990 (popular science)
  • Roger Penrose : Singularities and time asymmetry , in: Hawking, Israel (editor) General Relativity - An Einstein Centenary Survey , Cambridge 1979 (as well as his broader audience books The emperors new mind , The road to reality )
  • Ilya Prigogine , Isabelle Stengers : The Paradox of Time. Piper Verlag, 1993, ISBN 3-492-03196-X .
  • Stephen Hawking : The Illustrated Brief History of Time. 3. Edition. Reinbek bei Hamburg, Rowohlt Taschenbuch, 2010, ISBN 978-3-499-61968-7 (original title: The Illustrated A Brief History of Time, 1996), pp. 182-195 (popular science).
  • Hans Reichenbach : The Direction of Time. Dover Publications 2000 (first edition 1956), ISBN 0-486-40926-0 .
  • Laura Mersini-Houghton, Rüdiger Vaas : The Arrows of Time. Springer Verlag 2012, ISBN 978-3-642-23258-9 .

Web links

Individual evidence

  1. ^ Arthur Eddington : The Nature of the Physical World (1928) , in: The Gifford Lectures 1927, University of Edinburgh, Cambridge University Press 1928, pp. 68ff.
  2. Peter Coveney, Roger Highfield: Anti-Chaos. The arrow of time in the self-organization of life. Rowohlt Verlag 1992, p. 19.
  3. from A. Eddington : Space Time and Gravitation. Cambridge University Press 1920
  4. BaBar makes first direct measurement of time-reversal violation. November 21, 2012, accessed December 18, 2017 .