Arrow of time

The arrow of time, or time's arrow is the concept positing the 'one-way direction' or 'asymmetry' of time. It wasdeveloped in 1927 by the British astronomer Arthur Eddington, and is an unsolved general physics question. This direction, according to Eddington, could be determined by studying the organization of atoms, molecules, and bodies, and might be drawn upon a four-dimensional relativistic map of the world ('a solid block of paper').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 arrow' to express this one-way property of time which has no analogue in space.The Moving Finger writes; and, having writ,  Moves on: nor all thy Piety nor WitShall lure it back to cancel half a Line,  Nor all thy Tears wash out a Word of it.— Omar Khayyám (translation by Edward Fitzgerald). The arrow of time, or time's arrow is the concept positing the 'one-way direction' or 'asymmetry' of time. It wasdeveloped in 1927 by the British astronomer Arthur Eddington, and is an unsolved general physics question. This direction, according to Eddington, could be determined by studying the organization of atoms, molecules, and bodies, and 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. The symmetry of time (T-symmetry) can be understood simply as the following: if time were perfectly symmetrical, a video of real events would seem realistic whether played forwards or backwards. Gravity, for example, is a time-reversible force. A ball that is tossed up, slows to a stop, and falls is a case where recordings would look equally realistic forwards and backwards. The system is T-symmetrical. However, the process of the ball bouncing and eventually coming to a stop is not time-reversible. While going forward, kinetic energy is dissipated and entropy is increased. Entropy may be one of the few processes that is not time-reversible. According to the statistical notion of increasing entropy, the 'arrow' of time is identified with a decrease of free energy. In the 1928 book The Nature of the Physical World, which helped to popularize the concept, Eddington stated: