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Some processes that involve high energy particles and are governed by the weak force (such as K-meson decay) defy the symmetry between time directions. However, all known physical processes <em>do</em> preserve a more complicated symmetry (CPT symmetry), and are therefore unrelated to the second law of thermodynamics, or to the day-to-day experience of the arrow of time. A notable exception is the wave function collapse in quantum mechanics, which is an irreversible process. It has been conjectured that the collapse of the wave function may be the reason for the Second Law of Thermodynamics. However it is more accepted today that the opposite is correct, namely that the (possibly merely apparent) wave function collapse is a consequence of quantum decoherence, a process that is ultimately an outcome of the Second Law of Thermodynamics.

Some processes that involve high energy particles and are governed by the weak force (such as K-meson decay) defy the symmetry between time directions. However, all known physical processes <em>do</em> preserve a more complicated symmetry (CPT symmetry), and are therefore unrelated to the second law of thermodynamics, or to the day-to-day experience of the arrow of time. A notable exception is the wave function collapse in quantum mechanics, which is an irreversible process. It has been conjectured that the collapse of the wave function may be the reason for the Second Law of Thermodynamics. However it is more accepted today that the opposite is correct, namely that the (possibly merely apparent) wave function collapse is a consequence of quantum decoherence, a process that is ultimately an outcome of the Second Law of Thermodynamics.

一些涉及高能粒子并受弱力支配的过程(如 k 介子衰变)违背了时间方向之间的对称性。然而，所有已知的物理过程 em do / em 保留了一个更加复杂的对称性(CPT 对称) ，因此与热力学第二定律无关，或者与日常经验的时间箭头无关。一个值得注意的例外是波函数崩溃在量子力学，这是一个不可逆性。有人猜测，波函数的崩塌可能是热力学第二定律的原因。然而，今天人们更接受的是，相反的观点是正确的，即波函数崩溃(可能只是表面上的)是量子退相干的结果，这个过程最终是热力学第二定律的结果。

一些涉及高能粒子并受弱力支配的过程(如 k 介子衰变)违背了时间方向之间的对称性。然而，所有已知的物理过程都保留了一种更加复杂的对称性(CPT 对称) ，因此与热力学第二定律或时间箭头的日常经验无关。一个值得注意的例外是量子力学中波函数的坍缩是一个不可逆过程。据猜测，波函数的坍缩可能是热力学第二定律成立的原因。然而，今天被更接受的是，相反的观点是正确的，即(可能只是表面上)波函数坍缩是量子退相干的结果，这个过程最终是热力学第二定律的结果。

The universe was in a uniform, high density state at its very early stages, shortly after the Big Bang. The hot gas in the early universe was near thermodynamic equilibrium (giving rise to the horizon problem) and hence in a state of maximum entropy, given its volume. Expansion of a gas increases its entropy, however, and expansion of the universe has therefore enabled an ongoing increase in entropy. Viewed from later eras, the early universe can thus be considered to be highly ordered. The uniformity of this early near-equilibrium state has been explained by the theory of cosmic inflation.

The universe was in a uniform, high density state at its very early stages, shortly after the Big Bang. The hot gas in the early universe was near thermodynamic equilibrium (giving rise to the horizon problem) and hence in a state of maximum entropy, given its volume. Expansion of a gas increases its entropy, however, and expansion of the universe has therefore enabled an ongoing increase in entropy. Viewed from later eras, the early universe can thus be considered to be highly ordered. The uniformity of this early near-equilibrium state has been explained by the theory of cosmic inflation.

宇宙在宇宙大爆炸后不久的早期阶段，处于一个统一的、高密度的状态。早期宇宙中的热气体接近热力学平衡(引起视界问题) ，因此在其体积上处于熵最大的状态。然而，气体的膨胀增加了它的熵，因此宇宙的膨胀使得熵不断增加。从后来的时代来看，早期的宇宙可以被认为是高度有序的。这种早期近平衡态的均匀性已经用宇宙膨胀理论来解释。

在大爆炸后不久的早期阶段，宇宙处于一中均匀的高密度状态。早期宇宙中的热气体接近热力学平衡(引起视界问题) ，因此考虑体积时，宇宙处于最大熵的状态。气体的膨胀会增加它的熵，然而，宇宙的膨胀使得熵不断增加。从后来的时代来看，早期的宇宙是高度有序的。这种早期近平衡态的均匀性已经被宇宙膨胀理论解释了。

According to this theory the universe (or, rather, its accessible part, a radius of 46&nbsp;billion light years around Earth) evolved from a tiny, totally uniform volume (a portion of a much bigger universe), which expanded greatly; hence it was highly ordered. Fluctuations were then created by quantum processes related to its expansion, in a manner supposed to be such that these fluctuations are uncorrelated for any practical use. This is supposed to give the desired initial conditions needed for the Second Law of Thermodynamics.

According to this theory the universe (or, rather, its accessible part, a radius of 46&nbsp;billion light years around Earth) evolved from a tiny, totally uniform volume (a portion of a much bigger universe), which expanded greatly; hence it was highly ordered. Fluctuations were then created by quantum processes related to its expansion, in a manner supposed to be such that these fluctuations are uncorrelated for any practical use. This is supposed to give the desired initial conditions needed for the Second Law of Thermodynamics.

根据这一理论，宇宙(或者更确切地说，其可接近的部分，围绕地球460亿光年的半径)是从一个极小的、完全一致的体积(一个更大的宇宙的一部分)演化而来的，这个体积极大地膨胀，因此它是高度有序的。涨落随后由与其膨胀相关的量子过程产生，以一种假定这些涨落在任何实际应用中都是不相关的方式。这是为了给出热力学第二定律所需要的理想初始条件。

根据这一理论，宇宙(或者更确切地说，其可接近的部分，围绕地球460亿光年的半径)是从一个极小的、完全一致的体积(更大的宇宙的一部分)演化而来的，这个体积膨胀得很大，因此它是高度有序的。波动是由与宇宙膨胀相关的量子过程产生的，在某种程度上，这些波动在任何实际应用中都是不相关的。这就给出了给出热力学第二定律所需要的理想初始条件。

The universe is apparently an open universe, so that its expansion will never terminate, but it is an interesting thought experiment to imagine what would have happened had the universe been closed. In such a case, its expansion would stop at a certain time in the distant future, and then begin to shrink. Moreover, a closed universe is finite.

The universe is apparently an open universe, so that its expansion will never terminate, but it is an interesting thought experiment to imagine what would have happened had the universe been closed. In such a case, its expansion would stop at a certain time in the distant future, and then begin to shrink. Moreover, a closed universe is finite.

It is unclear what would happen to the [[Second Law of Thermodynamics]] in such a case. One could imagine at least three different scenarios (in fact, only the third one is plausible, since the first two require a smooth cosmic evolution, contrary to what is observed):

It is unclear what would happen to the [[Second Law of Thermodynamics]] in such a case. One could imagine at least three different scenarios (in fact, only the third one is plausible, since the first two require a smooth cosmic evolution, contrary to what is observed):

It is unclear what would happen to the Second Law of Thermodynamics in such a case. One could imagine at least three different scenarios (in fact, only the third one is plausible, since the first two require a smooth cosmic evolution, contrary to what is observed):

It is unclear what would happen to the Second Law of Thermodynamics in such a case. One could imagine at least three different scenarios (in fact, only the third one is plausible, since the first two require a smooth cosmic evolution, contrary to what is observed):

目前还不清楚在这种情况下，热力学第二定律会发生什么。我们可以想象至少有三种不同的情况(事实上，只有第三种情况是可信的，因为前两种情况需要宇宙平稳演化，这与我们观察到的情况相反) :

目前还不清楚在这种情况下，热力学第二定律会发生什么变化。我们可以设想至少有三种不同的情况(事实上，只有第三种情况是可信的，因为前两种情况需要宇宙平稳演化，这与我们观察到的情况相反) :

* A highly controversial view is that in such a case the arrow of time will reverse.<ref>{{Cite journal |doi = 10.1103/PhysRevD.32.2489|pmid = 9956019|bibcode = 1985PhRvD..32.2489H|title = Arrow of time in cosmology|year = 1985|last1 = Hawking|first1 = S. W.|journal = Physical Review D|volume = 32|issue = 10|pages = 2489–2495}}</ref> The quantum fluctuations—which in the meantime have evolved into galaxies and stars—will be in [[Superposition principle|superposition]] in such a way that the whole process described above is reversed—i.e., the fluctuations are erased by [[destructive interference]] and total uniformity is achieved once again. Thus the universe ends in a [[Big Crunch]], which is similar to its beginning in the [[Big Bang]]. Because the two are totally symmetric, and the final state is very highly ordered, entropy must decrease close to the end of the universe, so that the Second Law of Thermodynamics reverses when the universe shrinks. This can be understood as follows: in the very early universe, interactions between fluctuations created [[Quantum entanglement|entanglement]] ([[quantum correlation]]s) between particles spread all over the universe; during the expansion, these particles became so distant that these correlations became negligible (see [[quantum decoherence]]). At the time the expansion halts and the universe starts to shrink, such correlated particles arrive once again at contact (after circling around the universe), and the entropy starts to decrease—because highly correlated initial conditions may lead to a decrease in entropy. Another way of putting it, is that as distant particles arrive, more and more order is revealed because these particles are highly correlated with particles that arrived earlier.

* A highly controversial view is that in such a case the arrow of time will reverse.<ref>{{Cite journal |doi = 10.1103/PhysRevD.32.2489|pmid = 9956019|bibcode = 1985PhRvD..32.2489H|title = Arrow of time in cosmology|year = 1985|last1 = Hawking|first1 = S. W.|journal = Physical Review D|volume = 32|issue = 10|pages = 2489–2495}}</ref> The quantum fluctuations—which in the meantime have evolved into galaxies and stars—will be in [[Superposition principle|superposition]] in such a way that the whole process described above is reversed—i.e., the fluctuations are erased by [[destructive interference]] and total uniformity is achieved once again. Thus the universe ends in a [[Big Crunch]], which is similar to its beginning in the [[Big Bang]]. Because the two are totally symmetric, and the final state is very highly ordered, entropy must decrease close to the end of the universe, so that the Second Law of Thermodynamics reverses when the universe shrinks. This can be understood as follows: in the very early universe, interactions between fluctuations created [[Quantum entanglement|entanglement]] ([[quantum correlation]]s) between particles spread all over the universe; during the expansion, these particles became so distant that these correlations became negligible (see [[quantum decoherence]]). At the time the expansion halts and the universe starts to shrink, such correlated particles arrive once again at contact (after circling around the universe), and the entropy starts to decrease—because highly correlated initial conditions may lead to a decrease in entropy. Another way of putting it, is that as distant particles arrive, more and more order is revealed because these particles are highly correlated with particles that arrived earlier.

*It could be that this is the crucial point where the [[wavefunction collapse]] is important: if the collapse is real, then the quantum fluctuations will not be in superposition any longer; rather they had collapsed to a particular state (a particular arrangement of galaxies and stars), thus creating a [[Big Crunch]], which is very different from the [[Big Bang]]. Such a scenario may be viewed as adding [[boundary conditions]] (say, at the distant future) that dictate the wavefunction collapse.<ref>{{Cite arxiv |eprint=quant-ph/0507269 |title = Two-time interpretation of quantum mechanics g|last1 = Gruss|first1 = Eyal Y.|last2 = Aharonov|first2 = Yakir|year = 2005}}</ref>

*It could be that this is the crucial point where the [[wavefunction collapse]] is important: if the collapse is real, then the quantum fluctuations will not be in superposition any longer; rather they had collapsed to a particular state (a particular arrangement of galaxies and stars), thus creating a [[Big Crunch]], which is very different from the [[Big Bang]]. Such a scenario may be viewed as adding [[boundary conditions]] (say, at the distant future) that dictate the wavefunction collapse.<ref>{{Cite arxiv |eprint=quant-ph/0507269 |title = Two-time interpretation of quantum mechanics g|last1 = Gruss|first1 = Eyal Y.|last2 = Aharonov|first2 = Yakir|year = 2005}}</ref>

*The broad consensus among the scientific community today is that smooth initial conditions lead to a highly non-smooth final state, and that this is in fact the source of the thermodynamic arrow of time.<ref>{{Cite journal |doi = 10.4249/scholarpedia.3448|bibcode = 2008SchpJ...3.3448L|title = Time's arrow and Boltzmann's entropy|year = 2008|last1 = Lebowitz|first1 = Joel|journal = Scholarpedia|volume = 3|issue = 4|pages = 3448|doi-access = free}}</ref> Highly non-smooth [[gravity|gravitational]] systems tend to collapse to [[black hole]]s, so the [[wavefunction]] of the whole universe evolves from a [[Superposition principle|superposition]] of small fluctuations to a [[Superposition principle|superposition]] of states with many [[black hole]]s in each. It may even be that it is impossible for the universe to have both a smooth beginning and a smooth ending. Note that in this scenario the energy density of the universe in the final stages of its shrinkage is much larger than in the corresponding initial stages of its expansion (there is no [[destructive interference]], unlike in the first scenario described above), and consists of mostly black holes rather than free particles.

*The broad consensus among the scientific community today is that smooth initial conditions lead to a highly non-smooth final state, and that this is in fact the source of the thermodynamic arrow of time.<ref>{{Cite journal |doi = 10.4249/scholarpedia.3448|bibcode = 2008SchpJ...3.3448L|title = Time's arrow and Boltzmann's entropy|year = 2008|last1 = Lebowitz|first1 = Joel|journal = Scholarpedia|volume = 3|issue = 4|pages = 3448|doi-access = free}}</ref> Highly non-smooth [[gravity|gravitational]] systems tend to collapse to [[black hole]]s, so the [[wavefunction]] of the whole universe evolves from a [[Superposition principle|superposition]] of small fluctuations to a [[Superposition principle|superposition]] of states with many [[black hole]]s in each. It may even be that it is impossible for the universe to have both a smooth beginning and a smooth ending. Note that in this scenario the energy density of the universe in the final stages of its shrinkage is much larger than in the corresponding initial stages of its expansion (there is no [[destructive interference]], unlike in the first scenario described above), and consists of mostly black holes rather than free particles.

 

In the first scenario, the cosmological [[arrow of time]] is the reason for both the thermodynamic arrow of time and the quantum arrow of time. Both will slowly disappear as the universe will come to a halt, and will later be reversed.

In the first scenario, the cosmological [[arrow of time]] is the reason for both the thermodynamic arrow of time and the quantum arrow of time. Both will slowly disappear as the universe will come to a halt, and will later be reversed.

In the first scenario, the cosmological arrow of time is the reason for both the thermodynamic arrow of time and the quantum arrow of time. Both will slowly disappear as the universe will come to a halt, and will later be reversed.

In the first scenario, the cosmological arrow of time is the reason for both the thermodynamic arrow of time and the quantum arrow of time. Both will slowly disappear as the universe will come to a halt, and will later be reversed.