Arrow of time
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Arthur Stanley Eddington
For other uses, see Time's arrow (disambiguation).
This article is an overview of the subject. For a more technical discussion and for information related to current research, see Entropy (arrow of time).
The arrow of time, or time's arrow, is a concept developed in 1927 by the British astronomer Arthur Eddington involving the "one-way direction" or "asymmetry" of time. This direction, which can be determined, according to Eddington, by studying the organization of atoms, molecules and bodies, might be drawn upon a four-dimensional relativistic map of the world ("a solid block of paper").
Physical processes at the microscopic level are believed to be either entirely or mostly time-symmetric: if the direction of time were to reverse, the theoretical statements that describe them would remain true. Yet at the macroscopic level it often appears that this is not the case: there is an obvious direction (or flow) oftime.
3.1 The thermodynamic arrow of time
3.2 The cosmological arrow of time
3.3 The radiative arrow of time
3.4 The causal arrow of time
3.5 The particle physics (weak) arrow of time
3.6 The quantum arrow of time
3.7 The psychological/perceptual arrow of time
4 See also
6 Further reading
7 External links
In the 1928 book The Nature of the Physical World, which helped to popularize the concept, Eddington stated:
Let us draw an arrow arbitrarily. If as we follow the arrow we find more and more of the random element in the state of the world, then the arrow is pointing towards the future; if the random element decreases the arrow points towards the past. That is the only distinction known to physics. This follows at once if our fundamental contention is admitted that the introduction of randomness is the only thing which cannot be undone. I shall use the phrase ‘time’s...