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此词条暂由彩云小译翻译，未经人工整理和审校，带来阅读不便，请见谅。{{Other uses}}

{{hatnote|This article is an overview of the subject. For a more technical discussion and for information related to current research, see [[Entropy (arrow of time)]].}}

{{hatnote|This article is an overview of the subject. For a more technical discussion and for information related to current research, see [[Entropy (arrow of time)]].}}

=== Thermodynamic arrow of time ===

=== Thermodynamic arrow of time ===

{{Main|Entropy (arrow of time)}}

{{Main|Entropy (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.

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。

时间之箭是指时间的“单向”或“不对称”。热力学的时间之箭是依据'''<font color="#ff8000"> 热力学第二定律the second law of thermodynamics'''，它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，这就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，熵会随着时间增加而减少。

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>'''。

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没有依据的观点是没有说服力的，尽管热力学第二定律理论只在统计学上是正确的，然而在微观世界中的相悖时常发生，但从未发生任何严重程度的相悖。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，一些违反热力学第二定律的悖论的确存在，其中一个是'''<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.

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.

'''<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。

'''<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，宇宙正走向热灭亡（'''<font color="#ff8000"> 和大冷寂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。

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.

'''<font color="#ff8000"> 热力学时间箭头The thermodynamicarrow of time</font>'''和'''<font color="#ff8000"> 热力学第二定律the second law of thermodynamics</font>'''被视为宇宙早期初始条件下的结果。因此，最终她们是宇宙学的设置引发的。

'''<font color="#ff8000"> 热力学时间箭头The thermodynamicarrow of time</font>'''和'''<font color="#ff8000"> 热力学第二定律the second law of thermodynamics</font>'''被视为宇宙早期初始条件下的结果。因此，最终它们是宇宙学的设置引发的。

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.

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。

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 -->而后者在通常情况下与热力学第二定律相矛盾。