H. D. Zeh
Conceptual problems regarding the arrow of time in classical physics, quantum physics, cosmology, and quantum gravity are discussed. Particular attention is paid to the dynamical role of the quantum indeterminism, and to various concepts of timelessness.
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I address the question whether the origin of the observed arrow of time can be derived from quantum cosmology. After a general discussion of entropy in cosmology and some numerical estimates, I give a brief introduction into quantum geometrodynamics and argue that this may provide a sufficient framework for studying this question. I then show that a natural boundary condition of low initial entropy can be imposed on the universal wave function. The arrow of time is then correlated with the size of the Universe and emerges from an increasing amount of decoherence due to entanglement with unobse…
Quantum gravity, the initial low entropy state of the Universe, and the problem of time are interlocking puzzles. In this article, we address the origin of the arrow of time from a cosmological perspective motivated by a novel approach to quantum gravitation. Our proposal is based on a quantum counterpart of the equivalence principle, a general covariance of the dynamical phase space. We discuss how the nonlinear dynamics of such a system provides a natural description for cosmological evolution in the early Universe. We also underscore connections between the proposed non-perturbative quantum…
Source of the observed thermodynamic arrow
热力学时间箭头之谜归结为宇宙在过去如何具有较低熵的问题。我表明，没有特殊的熵降低机制（或波动）是必要的。作为膨胀的结果，在宇宙历史上的一个特定时期，由于新的主导长程作用力的影响，在主导短程作用力下接近最大熵的状态变得非常不可能。这发生在脱钩的时候，在此之前，我没有对箭头做任何声明。宇宙学在热力学中的作用是由T.黄金
Singularities, Entropy and the Arrow of Time, {\it or} Is CRT a Gauge Symmetry in Quantum Gravity?
探讨离散变换CRT是否为‘量子引力’的规范对称性，先定义具体框架，发现一般答案为否，且分析了在不同空间中CRT的情况及相关限制条件。
Arrow of Time in String Theory
暴胀使得时间之箭的问题可以被理解为一个关于时空结构的问题：为什么最早的空间部分的内禀曲率比它可能的表现要好得多？这实际上只是对一个更熟悉的问题的补充：是什么机制阻止了最早的空间截面的外在曲率发散，就像经典的广义相对论所建议的那样？我们认为，由乌古里、瓦法和韦林德所描绘的“从无到有”的弦式版本，同时解决了这两个问题。这个论证虽然非常简单，但却依赖于整体微分几何中一些最深刻的定理。这些结果意味着，当一个空的toral时空从无到有被创造出来时，最早的空间截面被迫是[准经典的]精确的局部各向同性的。这种局部的各向同性反过来又迫使暴胀子进入它的最小熵状态。这个理论解释了为什么箭在黑洞或宇宙收缩中（如果有的话）不会反转。
Theoretical achievements, as well as much controversy, surround multiverse theory. Various types of multiverses, with an increasing amount of complexity, were suggested and thoroughly discussed in literature by now. While these types are very different, they all share the same basic idea: our physical reality consists of more than just one universe. Each universe within a possibly huge multiverse might be slightly or even very different from the others. The quilted multiverse is one of these types, whose uniqueness arises from the postulate that every possible event will occur infinitely many …
Scientists continue to wrestle with the enigma of time. Is time a dynamic or a fundamental property of spacetime? Why does it have an arrow pointing from past to future? Why are physical laws time-symmetric in a universe with broken time-reversal symmetry? These questions remain a mystery. The hope has been that an understanding of the selection of the initial state for our universe would solve such puzzles, especially that of time's arrow. In this article, I discuss how the birth of the universe from the multiverse helps to unravel the nature of time and the reasons behind the time-reversal s…
Quantum cosmology offers a unique stage to address questions of time related to its underlying (and perhaps truly quantum dynamical) meaning as well as its origin. Some of these issues can be analyzed with a general scheme of quantum cosmology, others are best seen in loop quantum cosmology. The latter's status is still incomplete, and so no full scenario has yet emerged. Nevertheless, using properties that have a potential of pervading more complicated and realistic models, a vague picture shall be sketched here. It suggests the possibility of deriving a beginning within a beginningless theor…
Our observed Universe has a very strong arrow of time rooted in its low entropy starting point. This low entropy start can be related to various "tuning puzzles" about the early state of the Universe. Here we explore the relationship between the arrow of time and the emergence of classical from quantum in the hopes of ultimately gaining insights into cosmological initial conditions. Our focus is on einselection, the process whereby interactions with an environment select preferred states for a quantum system. This process plays an essential role in the emergence of classical from quantum. Stud…
The Decoherent Arrow of Time and the Entanglement Past Hypothesis
本文提出纠缠过去假说，认为宇宙初始量子态具有极低纠缠熵，以此解释退相干时间之箭，并与热力学过去假说对比，指出未来研究挑战。
Entanglement between pair-created twin universes with opposite time arrows should leave a birthmark on CMB spectrum
本研究提出宇宙与时间箭头相反的孪生宇宙成对创生并相互纠缠，通过欧几里得虫洞模型证明纠缠选择唯一全局真空，增强长波长CMB功率谱，留下可检验的出生印记。
A New Approach to Quantising Space-Time: I. Quantising on a General Category
提出了一种新的方法来量化因果集或拓扑或其他时空（或空间）模型的问题。我们的出发点是观察到这种类型的实体可以被视为一个类别中的对象，其箭头是结构保持映射。这激发了研究量化系统的一般问题，该系统的“配置空间”（或历史-理论模拟）可以被视为一个范畴中的对象集合。在这第一个一系列的文件，我们研究这个问题的一般和发展计划的基础上建设一个类似的组，用于规范量化的系统，其配置空间是一个流形$Q\simeq G/H$，其中G和H是李群。特别是，我们选择作为G的类似物的幺半群的“箭头字段”的范畴。从物理上讲，这意味着该类别中两个物体之间的箭头被视为某种动量的模拟。在找到“类量化幺半群”，我们展示了如何合适的表示可以使用一束希尔伯特空间的一组对象。
Gauge symmetry and the arrow of time: How to count what counts
本论文探讨物理学哲学中理论最小物理内容与时间之箭两大问题，论证宇宙整体大小非预测所需，将其移除后得出时间之箭为局部现象，从而解释了时间方向的体验。
Cosmology and Local Physics
这篇文章是为了纪念丹尼斯·西亚马。它重新审视了一系列问题，他投入了大量的时间和精力，关于局部物理学和宇宙的大尺度结构之间的关系-特别是奥尔伯的悖论，马赫的原则，和各种箭头的时间。因此，重点是局部物理学受到宇宙本身影响的各种方式。
Constraints on Reversing the Thermodynamic Arrow of Time from Black Hole Thermodynamics, Wormholes, and Time-Symmetric Quantum Mechanics
该研究通过黑洞热力学、虫洞和时间对称量子力学探讨单一宇宙中逆转热力学时间箭头的限制，发现熵只能在不同部分重新分配而无法真正逆转。
Structural morphology and the gravitational arrow of time
研究牛顿N体问题中，全局尺度不变聚类对比度V相近的两个N=5000中心构型，局部各向异性差异显著，揭示V无法唯一确定形态，需结合局部关系可观测量补充描述。
Cosmology, Time's Arrow, and That Old Double Standard
人们普遍认为，时间不对称性在很大程度上是一个宇宙学问题;解释时间不对称性的任务主要是解释早期宇宙条件的一个方面。然而，讨论这些问题的宇宙学家经常犯类似于困扰19世纪热力学统计基础讨论的错误。特别是，他们往往犯了适用各种时间“双重标准”的错误-例如，没有认识到某些统计学论点在任何时间方向上都具有同等的效力。本文旨在澄清的问题，什么才算是宇宙学时间不对称的充分解释。一个特别令人关注的问题是，是否有可能解释为什么熵在大爆炸附近很低，而不表明它在大坍缩附近也一定很低，如果宇宙坍缩的话。我批评了对这种可能性提出的一些反对意见，表明这些反对意见往往依赖于时间上的双重标准。我还简要地讨论了如果我们认真对待这一观点会出现的一些问题。（例如，我们能观察到时间逆转的未来吗？）
Restoring Classical Energy Conditions At Microscopic Level
提出一种基于量子不等式和量子利息猜想的机制，在量子化层面恢复经典能量条件，并解释时间之箭、引力微弱性、量子概率性及物质-反物质不对称等问题，支持暴胀理论。
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