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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 ("a solid block of paper").

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 ("a solid block of paper").

'''<font color="#ff8000"> 时间之箭The arrow of time</font>'''，是一种概念，假定时间是“单向方向”或“不对称”的。此概念于1927年由英国天体物理学家亚瑟·爱丁顿提出的一个普通物理学问题，至今尚未解决。按照爱丁顿的说法，研究原子、分子和物体的组织可以确定时间的方向，也可以绘制在一张四维的相对论世界地图上(“一整块纸”)。  

'''<font color="#ff8000"> 时间之箭The arrow of time</font>'''是一种概念，假定时间是“单向方向”或“不对称”的。此概念于1927年由英国天体物理学家亚瑟·爱丁顿提出的一个普通物理学问题，至今尚未解决。按照爱丁顿的说法，研究原子、分子和物体的组织可以确定时间的方向，也可以绘制在一张四维的相对论世界地图上(“一整块纸”)。  

<ref>{{cite book|title=The scientist as philosopher: philosophical consequences of great scientific discoveries

<ref>{{cite book|title=The scientist as philosopher: philosophical consequences of great scientific discoveries

<ref>{{cite book|title=The scientist as philosopher: philosophical consequences of great scientific discoveries

<ref>{{cite book|title=The scientist as philosopher: philosophical consequences of great scientific discoveries

【The scientist as philosopher: philosophical consequences of great scientific discoveries作为哲学家的科学家: 伟大科学发现的哲学后果】

【书籍：The scientist as philosopher: philosophical consequences of great scientific discoveries《作为哲学家的科学家: 伟大科学发现的哲学后果》】

|first1=Friedel

|first1=Friedel

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.<ref>{{cite journal

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.<ref>{{cite journal

时间的对称性(t 对称性)可以简单地理解为: 如果时间是完全对称的，那么一段真实事件的视频，无论播放多长前后，都会显得很真实。例如，重力是一种时间可逆的力。一个球被抛起，减速到停止，然后下降的情况下，录音将看起来同样现实的前进和后退。系统为 t 型对称系统。然而，球反弹并最终停止的过程是不可逆的。在前进过程中，动能耗散，熵增加。熵可能是少数几个不可逆的过程之一。根据熵增的统计概念，时间的“箭头”被确定为自由能的减少。 文献{ cite journal

'''<font color="#ff8000"> 时间对称性The symmetry of time</font>'''可以简单地理解为: 如果时间是完全对称的，那么一段真实事件的视频，无论从前往后播放还是从后往前播放，都会显得很真实。例如，重力是一种时间可逆的力。一个球被抛起，减速到停止，然后下降，在这种情况下正放和倒放看起来同样真实。该系统具有'''<font color="#ff8000"> 时间对称性The symmetry of time</font>'''。然而，球反弹并最终停止的过程是不可逆的。在前进过程中，动能耗散，熵增加。熵可能是少数几个处于时间不可逆的过程之一。根据熵增的统计概念，时间的“箭头”被确定为自由能的减少。

| author=Tuisku, P. |author2=Pernu, T.K. |author3=Annila, A. | title=In the light of time

<ref>{{cite journal| author=Tuisku, P. |author2=Pernu, T.K. |author3=Annila, A. | title=In the light of time

| author=Tuisku, P. |author2=Pernu, T.K. |author3=Annila, A. | title=In the light of time

<ref>{{cite journal| author=Tuisku, P. |author2=Pernu, T.K. |author3=Annila, A. | title=In the light of time

 

| journal=Proceedings of the Royal Society A | volume=465 |issue=2104 | pages=1173–1198 | year=2009

| journal=Proceedings of the Royal Society A | volume=465 |issue=2104 | pages=1173–1198 | year=2009

| journal=Proceedings of the Royal Society A | volume=465 |issue=2104 | pages=1173–1198 | year=2009

| journal=Proceedings of the Royal Society A | volume=465 |issue=2104 | pages=1173–1198 | year=2009

2009年11月17日，《皇家学会报告志 a 辑第465卷第2104期

2009年11月17日，《皇家学会学报》 A 辑第465卷第2104期

|doi=10.1098/rspa.2008.0494|bibcode = 2009RSPSA.465.1173T | doi-access=free }}</ref>

|doi=10.1098/rspa.2008.0494|bibcode = 2009RSPSA.465.1173T | doi-access=free }}</ref>

在1928年出版的《物理世界的本质》一书中，爱丁顿推广了这个概念，他说:

在1928年出版的《物理世界的本质》一书中，爱丁顿推广了这个概念，他说:

<blockquote>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.</blockquote>

<blockquote>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.</blockquote>

<blockquote>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.</blockquote>

<blockquote>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.</blockquote>

让我们任意画一个箭头。如果我们跟着箭头走，我们在世界的状态中发现越来越多的随机元素，那么箭头就指向未来; 如果随机元素减少了箭头指向过去的指向。这是物理学已知的唯一区别。如果我们的基本论点得到承认，随机性的引入是唯一不可撤销的事情，那么这一点就立刻得到了证实。我将用“时间的箭头”这个短语来表达时间的这种单向性质，这种性质在空间上没有类似的东西

“让我们任意画一个箭头。如果我们跟着箭头走，我们在世界的状态中发现越来越多的随机元素，那么箭头就指向未来; 如果随机元素减少了，那么箭头指向过去。这是物理学中已知的唯一区别。如果我们的基本论点得到承认，随机性的引入是唯一不可撤销的事情，那么这一点就立刻得到了证实。我会用“时间箭头”这个短语来表达时间的这种单向性质，这种性质在空间上没有类似的东西”

Eddington then gives three points to note about this arrow:

Eddington then gives three points to note about this arrow:

关于这个箭头，Eddington 给出了三点注意事项:

关于这个箭头，爱丁顿给出了三点注意事项:

  It is vividly recognized by consciousness.

  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 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. (By which he means that it is only observed in entropy, a statistical mechanics phenomenon arising from a system.)

  It makes no appearance in physical science except in the study of organization of a number of individuals. (By which he means that it is only observed in entropy, a statistical mechanics phenomenon arising from a system.)

除了研究一些个体的组织结构外，它在自然科学中没有出现。(他的意思是只有在熵中才能观察到，熵是一个系统产生的统计力学现象。)

除了研究一些个体的组织结构外，它在自然科学中没有出现。(他的意思是只有在熵中才能观察到，熵是一个系统产生的统计力学现象。

 

 

 

According to Eddington the arrow indicates the direction of progressive increase of the random element. Following a lengthy argument upon the nature of thermodynamics he concludes that, so far as physics is concerned, time's arrow is a property of entropy alone.

According to Eddington the arrow indicates the direction of progressive increase of the random element. Following a lengthy argument upon the nature of thermodynamics he concludes that, so far as physics is concerned, time's arrow is a property of entropy alone.

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.

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.

时间之箭是时间的“单向方向”或“不对称”。热力学的时间箭头是由热力学第二定律提供的，它说在一个孤立的系统中，熵倾向于随着时间而增加。熵可以被认为是一种微观无序的度量; 因此，第二定律意味着在一个孤立的系统中，相对于有序的数量而言，时间是不对称的: 当一个系统随着时间推进时，它会变得更加无序。这种不对称性可以被经验性地用来区分未来和过去，尽管测量熵并不能准确地测量时间。而且，在一个开放的系统中，熵会随着时间而减少。

时间之箭是指时间的“单向”或“不对称”。热力学的时间箭头是依据'''<font color="#ff8000"> 热力学第二定律the second law of thermodynamics'''，在一个孤立的系统中，熵倾向于随着时间而增加。熵可以看作是一种对微观无序的度量; 因此，第二定律意味着在一个孤立的系统中，相对于有序的数量而言，时间是不对称的: 当一个系统随着时间推进时，它会变得更加无序。尽管测量熵并不能准确地测量时间，但这种不对称性可以用作区分未来和过去的经验。而且，在一个开放的系统中，熵会随着时间而减少。

 

 

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.

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.

英国物理学家 Alfred Brian Pippard 爵士写道: “因此，经常油嘴滑舌地重复的观点是没有正当理由的，即热力学第二定律理论只在统计上是正确的，即微观违反反反复发生，但从未发生任何严重程度的违反。相反，从来没有任何证据表明第二定律在任何情况下失效。”然而，有一些关于违反热力学第二定律的悖论，其中一个是由于庞加莱始态复现定理。

英国物理学家阿尔弗雷德·布莱恩·派帕德爵士写道: “经常油嘴滑舌地重复的观点是没有依据的，尽管热力学第二定律理论只在统计学上是正确的，而在微观世界中的违反重复发生，但从未发生任何严重程度的违反。相反，从来没有任何证据表明第二定律在任何情况下失效。”然而，一些关于违反热力学第二定律的悖论的确存在，其中一个是由于'''<font color="#ff8000">庞加莱始态复现定理the Poincaré recurrence theorem</font>'''。

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.

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 "explored the relationship between time's arrow (the second law of thermodynamics) 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.

Harold Blum's 1951 book Time's Arrow and Evolution "explored the relationship between time's arrow (the second law of thermodynamics) 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.

Harold Blum 在1951年出版的《时间之箭与进化》一书中“探索了时间之箭(热力学第二定律)与有机进化之间的关系。”这个有影响力的文本探索“不可逆性和方向的进化和秩序，负熵，和进化。”布鲁姆认为，进化遵循特定的模式预先确定的无机性质的地球及其热力学过程。

哈罗德·布鲁姆（音译）在1951年出版的《时间之箭与进化》一书中“探索了时间之箭(热力学第二定律)与有机进化之间的关系。”这个有影响力的文本探索“不可逆性和方向的进化和秩序，负熵，和进化。”布鲁姆认为，进化遵循特定的模式，这种模式由地球的无机性质及其热力学过程预先确定。

 

 

 

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 (Big Chill) as the amount of usable energy becomes negligible. Alternatively, it may be an artifact of our place in the universe's evolution (see the Anthropic bias), with this arrow reversing as gravity pulls everything back into a Big Crunch.

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 (Big Chill) as the amount of usable energy becomes negligible. Alternatively, it may be an artifact of our place in the universe's evolution (see the Anthropic bias), with this arrow reversing as gravity pulls everything back into a Big Crunch.

宇宙的时间箭头指向宇宙膨胀的方向。它可能与热力学箭头有关，当可用能量变得微不足道时，宇宙正走向热死亡(Big Chill)。或者，它可能是我们在宇宙演化中所处位置的人工制品(见人择偏差) ，随着重力将一切拉回大坍缩，这个箭头发生了逆转。

'''<font color="#ff8000"> 宇宙时间之箭The cosmological arrow of time</font>'''指向宇宙膨胀的方向。它可能与热力学箭头有关，当可用能量变得微不足道时，宇宙正走向热死亡（'''<font color="#ff8000"> 大冷寂Big Chill</font>'''）。或者，它可能是我们在宇宙演化中所处位置的人工制品(见'''<font color="#ff8000"> 人择偏差the Anthropic bias</font>''') ，随着重力将一切拉回大坍缩，这个箭头发生了逆转。

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.

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.

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.

=== Radiative arrow of time ===

=== Radiative arrow of time ===

Waves, from [[radio waves]] to [[sound waves]] to those on a pond from throwing a stone, expand outward from their source, even though the [[wave equation]]s accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves,<ref>{{cite web |url=http://www4.ncsu.edu/~fouque/fink.pdf|archive-url=https://web.archive.org/web/20051231022842/http://www4.ncsu.edu/~fouque/fink.pdf |archive-date=31 December 2005 |url-status=dead |title=Time-Reversed Acoustic |author=Mathias Fink |date=30 November 1999 |access-date=27 May 2016 |author-link=Mathias Fink }}</ref> so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it,{{Citation needed|date=May 2010}}<!-- ref>{{cite book |author1=Nikolai Chernov |author2=Roberto Markarian |year=2006 |title=Chaotic Billiards |publisher=American Mathematical Soc. |page=207 |isbn=978-0-8218-4096-2}}</ref --> making the latter contradictory to the second law of thermodynamics in usual circumstances.

Waves, from [[radio waves]] to [[sound waves]] to those on a pond from throwing a stone, expand outward from their source, even though the [[wave equation]]s accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves,<ref>{{cite web |url=http://www4.ncsu.edu/~fouque/fink.pdf|archive-url=https://web.archive.org/web/20051231022842/http://www4.ncsu.edu/~fouque/fink.pdf |archive-date=31 December 2005 |url-status=dead |title=Time-Reversed Acoustic |author=Mathias Fink |date=30 November 1999 |access-date=27 May 2016 |author-link=Mathias Fink }}</ref> so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it,{{Citation needed|date=May 2010}}<!-- ref>{{cite book |author1=Nikolai Chernov |author2=Roberto Markarian |year=2006 |title=Chaotic Billiards |publisher=American Mathematical Soc. |page=207 |isbn=978-0-8218-4096-2}}</ref --> making the latter contradictory to the second law of thermodynamics in usual circumstances.

Waves, from radio waves to sound waves to those on a pond from throwing a stone, expand outward from their source, even though the wave equations accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves, so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it,<!-- ref></ref --> making the latter contradictory to the second law of thermodynamics in usual circumstances.

Waves, from radio waves to sound waves to those on a pond from throwing a stone, expand outward from their source, even though the wave equations accommodate solutions of convergent waves as well as radiative ones. This arrow has been reversed in carefully worked experiments that created convergent waves, so this arrow probably follows from the thermodynamic arrow in that meeting the conditions to produce a convergent wave requires more order than the conditions for a radiative wave. Put differently, the probability for initial conditions that produce a convergent wave is much lower than the probability for initial conditions that produce a radiative wave. In fact, normally a radiative wave increases entropy, while a convergent wave decreases it,<!-- ref></ref --> making the latter contradictory to the second law of thermodynamics in usual circumstances.

波，从无线电波到声波，再到池塘里的石头，从它们的源头向外扩展，即使波动方程包含了会聚波和辐射波的解。这个方向在精心设计的产生收敛波的实验中被颠倒过来，所以这个方向可能来自热力学方向，因为满足产生收敛波的条件比产生辐射波的条件需要更多的次序。换句话说，产生会聚波的初始条件的概率远低于产生辐射波的初始条件的概率。事实上，正常情况下，辐射波会增加熵，而会聚波会减少熵，! -- ref -- 使后者在通常情况下与热力学第二定律相矛盾。

虽然波动方程包含了会聚波和辐射波的解，但所有的波，从无线电波到声波，再到石头扔进池塘里产生的波，都是从源头向外扩展。这个方向在精心设计的产生收敛波的实验中被颠倒过来，所以这个方向可能来自热力学方向，因为满足产生收敛波的条件比产生辐射波的条件需要更多的次序。换句话说，产生会聚波初始条件的概率远低于产生辐射波初始条件的概率。事实上，正常情况下，辐射波会增加熵，而会聚波会减少熵，<!-- ref></ref -->使后者在通常情况下与热力学第二定律相矛盾。

=== Causal arrow of time ===

=== Causal arrow of time ===

A [[causality|cause]] precedes its effect: the causal event occurs before the event it causes or affects. Birth, for example, follows a successful conception and not vice versa. Thus causality is intimately bound up with time's arrow.

A [[causality|cause]] precedes its effect: the causal event occurs before the event it causes or affects. Birth, for example, follows a successful conception and not vice versa. Thus causality is intimately bound up with time's arrow.

— Omar Khayyám (translation by Edward Fitzgerald).

— Omar Khayyám (translation by Edward Fitzgerald).

ー Omar khayy m (翻译: Edward Fitzgerald)。

— Omar Khayyám (翻译: 爱德华·菲茨杰拉德)。

</blockquote>

</blockquote>

</blockquote>

</blockquote>

/ blockquote

</blockquote>

== Further reading ==

== Further reading ==

* {{cite journal | last1 = Lebowitz | first1 = Joel L. | authorlink = Joel L. Lebowitz | year = 2008| title = Time's arrow and Boltzmann's entropy | journal = [[Scholarpedia]] | volume = 3 | issue = 4| page = 3448 | doi = 10.4249/scholarpedia.3448 |bibcode = 2008SchpJ...3.3448L | doi-access = free }}

* {{cite journal | last1 = Lebowitz | first1 = Joel L. | authorlink = Joel L. Lebowitz | year = 2008| title = Time's arrow and Boltzmann's entropy | journal = [[Scholarpedia]] | volume = 3 | issue = 4| page = 3448 | doi = 10.4249/scholarpedia.3448 |bibcode = 2008SchpJ...3.3448L | doi-access = free }}