Arrow of time
The arrow of time, also called time's arrow, is the concept positing the "one-way direction" or "asymmetry" of time. It was developed in 1927 by the British astrophysicist 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.
The arrow of time paradox was originally recognized in the 1800s for gases as a discrepancy between microscopic and macroscopic description of thermodynamics / statistical physics. At the microscopic level physical processes 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 of time.
Overview
The symmetry of time 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 his book The Big Picture, physicist Sean M. Carroll compares the asymmetry of time to the asymmetry of space: While physical laws are in general isotropic, near Earth there is an obvious distinction between "up" and "down", due to proximity to this huge body, which breaks the symmetry of space. Similarly, physical laws are in general symmetric to the flipping of time direction, but near the Big Bang, there is an obvious distinction between "forward" and "backward" in time, due to relative proximity to this special event, which breaks the symmetry of time. Under this view, all the arrows of time are a result of our relative proximity in time to the Big Bang and the special circumstances that existed then.
Conception by Eddington
In the 1928 book The Nature of the Physical World, which helped to popularize the concept, Eddington stated: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.
Eddington then gives three points to note about this arrow:
- It is vividly recognized by consciousness.
- It is equally insisted on by our reasoning faculty, which tells us that a reversal of the arrow would render the external world nonsensical.
- It makes no appearance in physical science except in the study of organization of a number of individuals.
Arrows
Psychological/perceptual arrow of time
A related mental arrow arises because one has the sense that one's perception is a continuous movement from the known past to the unknown future. This phenomenon has two aspects: memory and volition. The two aspects are a consequence of the causal arrow of time: past events are the cause of our present memories, as more and more correlations are formed between the outer world and our brain ; and our present volitions and actions are causes of future events. This is because the increase of entropy is thought to be related to increase of both correlations between a system and its surroundings and of the overall complexity, under an appropriate definition; thus all increase together with time.Past and future are also psychologically associated with additional notions. English, along with other languages, tends to associate the past with "behind" and the future with "ahead", with expressions such as "to look forward to welcoming you", "to look back to the good old times", or "to be years ahead". However, this association of "behind ⇔ past" and "ahead ⇔ future" is culturally determined. For example, the Aymara language associates "ahead ⇔ past" and "behind ⇔ future" both in terms of terminology and gestures, corresponding to the past being observed and the future being unobserved. Similarly, the Chinese term for "the day after tomorrow" 後天 literally means "after day", whereas "the day before yesterday" 前天 is literally "preceding day", and Chinese speakers spontaneously gesture in front for the past and behind for the future, although there are conflicting findings on whether they perceive the ego to be in front of or behind the past. There are no languages that place the past and future on a left–right axis, although at least English speakers associate the past with the left and the future with the right, which seems to have its origin in the left-to-right writing system.
The words "yesterday" and "tomorrow" both translate to the same word in Hindi: कल, meaning " day remote from today." The ambiguity is resolved by verb tense. परसों is used for both "day before yesterday" and "day after tomorrow", or "two days from today".
The other side of the psychological passage of time is in the realm of volition and action. We plan and often execute actions intended to affect the course of events in the future. From the Rubaiyat:
The Moving Finger writes; and, having writ,
Moves on: nor all thy Piety nor Wit.
Shall lure it back to cancel half a Line,
Nor all thy Tears wash out a Word of it.
— Omar Khayyam.
In June 2022, researchers reported in Physical Review Letters finding that salamanders were demonstrating counter-intuitive responses to the arrow of time in how their eyes perceived different stimuli.
Thermodynamic arrow of time
The arrow of time is the "one-way direction" or "asymmetry" of time. The thermodynamic arrow of time is provided by the second law of thermodynamics, which says that in an isolated system, entropy tends to increase with time. Entropy can be thought of as a measure of microscopic disorder; thus the second law implies that time is asymmetrical with respect to the amount of order in an isolated system: as a system advances through time, it becomes more statistically disordered. This asymmetry can be used empirically to distinguish between future and past, though measuring entropy does not accurately measure time. Also, in an open system, entropy can decrease with time. An interesting thought experiment would be to ask: "if entropy was increased in an open system, would the arrow of time flip in polarity and point towards the past."British physicist Sir Alfred Brian Pippard wrote: "There is thus no justification for the view, often glibly repeated, that the Second Law of Thermodynamics is only statistically true, in the sense that microscopic violations repeatedly occur, but never violations of any serious magnitude. On the contrary, no evidence has ever been presented that the Second Law breaks down under any circumstances." However, there are a number of paradoxes regarding violation of the second law of thermodynamics, one of them due to the Poincaré recurrence theorem.
This arrow of time seems to be related to all other arrows of time and arguably underlies some of them, with the exception of the [|weak arrow of time].
Harold Blum's 1951 book Time's Arrow and Evolution discusses "the relationship between time's arrow and organic evolution." This influential text explores "irreversibility and direction in evolution and order, negentropy, and evolution." Blum argues that evolution followed specific patterns predetermined by the inorganic nature of the earth and its thermodynamic processes.
Cosmological arrow of time
The cosmological arrow of time points in the direction of the universe's expansion. It may be linked to the thermodynamic arrow, with the universe heading towards a heat death as the amount of Thermodynamic free energy becomes negligible. Alternatively, it may be an artifact of our place in the universe's evolution, with this arrow reversing as gravity pulls everything back into a Big Crunch.If this arrow of time is related to the other arrows of time, then the future is by definition the direction towards which the universe becomes bigger. Thus, the universe expands—rather than shrinks—by definition.
The thermodynamic arrow of time and the second law of thermodynamics are thought to be a consequence of the initial conditions in the early universe. Therefore, they ultimately result from the cosmological set-up.