The Directions of Time: how physics develops its temporal asymmetries

物理学的基本定律没有提到过去和未来的区别。如果世界的某些状态序列可以解决基本物理方程，那么同样的序列也可以反过来解决。在大规模物理的偶然情况下（想想行星的运动，或者乒乓球的运动），同样是（近似地）正确的。但是在科学研究的大多数情况下——以及在日常世界中遇到的大多数情况下——过去和未来之间存在明显的区别。饮料中的冰块会融化；它们不是自发形成的。火吞噬东西；他们不会自发地创造它们。热咖啡冷却下来。由于我们通常有充分的理由认为，宏观世界的事实最终可以用微观世界的规律来解释，因此这里存在一个悖论。如果过去/未来的区别在小尺度上已经存在，它怎么可能在大尺度上出现呢？这让20世纪初发展了统计力学的物理学家们深感困惑，统计力学是物理学的一个分支，它使我们能够计算出控制大规模现象（如热和压力）的定律，而只给出了控制微观粒子的定律。尽管统计力学在实践上取得了成功，但它仍然困扰着今天的物理学家和哲学家：对于过去和未来之间的差异从何而来，还没有一个被普遍接受的解释。统计力学的哲学，这个项目的目的是贡献，首先是关注这个持续的难题。它对物理学的影响是深远而广泛的，它的影响远远超出了物理学，影响着我们处理哲学问题的方法，比如因果之间的区别，或者记忆和预期之间的区别。然而，太多的统计力学哲学研究都把注意力集中在这个难题上，以至于忽视了统计力学的积极成就，这些成就是巨大的。无论这门学科在概念上有多么困难，它的方法在预测大规模现象的结果方面都取得了成功，具有很高的可靠性和准确性。这在很大程度上被一个学科所忽略，这个学科的问题是“微观物理学在原则上如何导致时间的方向？”而不是“我们如何理解、证明、理解物理学家从缺乏方向性的起点构建时间导向定律所使用的实际方法？”该项目旨在纠正这一错误。它的目标是详细研究物理学家在统计力学中用来进行计算和预测的定量方法是什么，并确切地看到必须添加或假设什么才能使这些方法在哲学上合理。我们希望这样做能给我们提供真正成功理解统计力学所需的洞察力，并通过它来了解时间的方向。

The fundamental laws of physics do not make reference to the difference between past and future. If some sequence of states of the world solves the equations of fundamental physics, so does the very same sequence, run backwards. In occasional situations in large-scale physics (think of the motion of the planets, or of a table-tennis ball) the same is (approximately) true. But in most situations studied in science - and most situations encountered in the everyday world - there is a clear distinction between past and future. Ice cubes in a drink melt; they do not spontaneously form. Fires consume things; they do not spontaneously create them. Hot coffee cools down.Since we normally think - for good reason - that the facts of the macroscopic world can ultimately be explained by the laws of the microscopic world, there is a paradox here. How can the past/future distinction emerge at the large scale unless it is already present at the small? This profoundly puzzled the physicists who, at the start of the 20th century, developed statistical mechanics, the branch of physics that allows us to work out the laws governing large-scale phenomena - like heat and pressure - given only the laws governing microscopic particles. Despite the practical success of statistical mechanics, it continues to puzzle physicists and philosophers today: there is no generally-accepted explanation of where the difference between past and future originates. The philosophy of statistical mechanics, to which this project aims to contribute, is concerned above all else with this continuing puzzle. Its implications in physics are profound and widespread, and it matters well beyond physics, influencing our approach to philosophical problems such as the distinction between cause and effect, or between memory and anticipation.Too much research in philosophy of statistical mechanics, though, has fixated on the puzzle to the extent that it ignores the positive achievements of statistical mechanics, which are enormous. Whatever the conceptual difficulties of the subject, its methods succeed, with great reliability and great accuracy, at predicting just how large-scale phenomena turn out. This has been largely missed by a subject whose question has been 'how in principle could microscopic physics lead to a direction of time?' and not 'how can we understand, justify, make sense of the actual methods used by physicists to construct time-directed laws from a starting point which lacks that direction?'This project aims to rectify this mistake. Its goal is to study, in some detail, just what the quantitative methods are that physicists use to perform calculations and make predictions in statistical mechanics, and to see exactly what must be added or assumed to make those methods philosophically justified. The hope is that doing so will give the insight that is needed to really succeed in understanding statistical mechanics, and through it, the direction of time.