In thermodynamics, time's arrow refers a conceptualized forward directional one-way property of time. [1]

In 1928, English astronomer Arthur Eddington, in his The Nature of the Physical World, devoted an eight-page section to the topic of Time’s Arrow or the relationship between entropy and time. In short, he stated “without any mystic appeal to consciousness it is possible to find a direction of time on the four-dimensional map by a study of organization”. He continues “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.” On this logic, he states “I shall use the phrase time’s arrow to express this one-way property of time which has no analogue to space.” [1]

Then, in a few pages of discussion on changes of energy and changes in the organization of energy during various natural process, such as dropping a stone, operating a steam engine, opening a stopcock connecting one empty and one filled vessel of gases, etc., and each situations relation to the changes in the positions of the atoms and molecules involved, Eddington states that dissections of these sorts “raises organization from a vague descriptive epithet to one of measureable quantities of exact science”. Moreover, he states, “a common measure can … be applied to all forms of organization”, and that “any loss of organization is equitably measured by the chance against its recovery by an accidental coincidence”. Then in a form of over-simplified extrapolation, he states “entropy (which he defines as: the practical measure of the random element which can increase in the universe by can never decrease) is the same as measuring by the chance explained in the last paragraph”; where he adds that “only the unmanageably large numbers are transformed (by a simple formula) into a more convenient scale of reckoning.” This latter statement is likely in reference to Austrian physicist Ludwig Boltzmann’s logarithmic formula of entropy. I

Eddington states, in conclusion:

“So far as physics is concerned, time’s arrow is a property of entropy alone.”

In 1965 philosophical discussions, a followup rehash of this model is: [2]

“Though we first lay hold of time’s arrow in consciousness, our sense of time-direction is probably seated in an entropy-clock in the brain.”

See also
Arrow of time

1. Eddington, Arthur. (1928). The Nature of the Physical World, (section: "Time's Arrow", pgs. 68-75). Michigan: The University of Michigan Press.
2. Staff Writer. (1965). Proceedings of the American Catholic Philosophical Association (pg. 105). National Office of the American Catholic Philosophical Association, Vol. 39.

Further reading
● Blum, Harold F. (1951). Time's Arrow and Evolution. Princeton: Princeton University Press.
● Morris, Richard. (1986). Time's Arrow: Scientific Attitudes Toward Time. Touchstone.
● Mackey, Michael C. (1992). Time's Arrow: the Origin of Thermodynamic Behavior.
● Price, Huw. (1997). Time’s Arrow and Archimedes’ Point. Oxford University Press.

External links
Time’s arrow and Boltzmann’s entropy (Joel Lebowitz) – Scholarpedia.

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