
A 1639 allegory by F. Quarles, showing Father Time, holding an hourglass, who tells Death (see: death), holding an arrow, when to extinguish the light of life of a human, shown in front of the sun and a sundial; the inscription reading: “tempus erit” or the time will come; the overall depiction said to well-capture the conception of the arrow of time, in respect to thermodynamics, heat, and the second law. [3]

A 1639 allegory by F. Quarles, showing Father Time, holding an hourglass, who tells Death (see: death), holding an arrow, when to extinguish the light of life of a human, shown in front of the sun and a sundial; the inscription reading: “tempus erit” or the time will come; the overall depiction said to well-capture the conception of the arrow of time, in respect to thermodynamics, heat, and the second law. [3]

In thermodynamics, an arrow of time refers to the direction time flows in respect to irreversibility and entropy increase.

An alternative synonym for arrow of time is English astronomer Arthur Eddington's 1928 term "time's arrow", refers to the temporal effect or measure of irreversibility in nature. The phrase was popularized in the 1950s and into the 70s by those as Austrian-born English philosopher Karl Popper and Belgian thermodynamicist Ilya Prigogine. [1]

A newer synonym is "thermodynamic arrow", popularized in the 1980s by English theoretical physicist Stephen Hawking, who introduced this term in comparison to the psychological arrow. [2]

References
1. (a) Popper, K. (1956). “The Arrow of Time”, Nature, Vol. 177. Pg. 538.
(b) Prigogine, Ilya. (1984). Order Out of Chaos – Man’s New Dialogue with Nature, (section: "Boltzmann and the Arrow of Time", pgs 253-56, "Entropy and the Arrow of Time", pgs. 257-59). New York: Bantam Books.
2. Hawking, Stephen. (1996). The Illustrated - A Brief History of Time, (ch. 9: "The Arrow of Time", pgs. 182-95). New York: Bantam Books.
3. Mainzer, Klaus. (1996). Symmetries of Nature (§3.4. Symmetry and Thermodynamics, pgs. 317-18). Walter De Gruyter.

Further reading
● Hawking, Stephen W. (1985). “Arrow of Time in Cosmology”, Physical Review D, Nov. 15; in: Quantum Cosmology (pgs. 308-15) by Lizhi Fang and Remo Ruffini, World Scientific, 1987.
● Carroll, Sean. (2010). From Eternity to Here: the Quest for the Ultimate Theory of Time. Penguin Group USA.

External links
● Arrow of time – Wikipedia.
● Arrow of Time (comic) by Fenerit , Wed Nov 19, 2008.