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) of time.
Famous quotes containing the words arrow and/or time:
“O Russia! O my wife! Our long and narrow Road lies clear though distressed.
Our road with an old Tatar freedom’s arrow Has deeply pierced our breast.”
—Alexander Blok (1880–1921)
“If I ever see more clearly at one time than at another, the medium through which I see is clearer.”
—Henry David Thoreau (1817–1862)