Arrow of time

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The arrow of time is how the passage of time is always associated with an overall increase in disorder, or entropy. Hence time is not symmetric, like space, but is directional. The term was coined in 1927 by astronomer Sir Arthur Eddington.[1][2]

Time-symmetric systems

Physical systems with a few degrees of freedom usually are time-symmetric; for example, analysing the breakup of a binary asteroid with the capture of one of them as a planet's satellite yields equations where the time can be reversed: the analysis can be made for the event where a rogue asteroid captures a planet's satellite.

When the freedom degrees increase, it becomes improbable that the time can be reversed: a rogue star that passes close to a solar system and disrupts the nice orbits of the planets might be seen as the inverse process where a rogue star "fixes" the disrupted orbits, but this scenario is very improbable.

Fundamental physics

It's usually believed (and backed by experiments) that most "forces" in nature (gravitation, electromagnetism, strong nuclear force) are time-symmetric, at least in the microscropic level. The possible exception is the weak nuclear force, that is not time-symmetric but has CPT symmetry (CPT stands for "charge", "parity" and "time" - if we reverse time, the only way to have a realistic process is by reversing charge and parity).


  1. Edgar Anrews (2010). Who made God? Searching for a theory of everything.. Carlisle, PA, USA: EP Books, 112. ISBN 978-0-85234-707-2. 
  2. Arthur Eddington (1927, 2012). The Nature of the Physical World: Gifford Lectures. Cambridge University Press, 80, 295. ISBN 9781107663855.