【诺奖得主Wilczek科普专栏】延迟宇宙热寂

■ 作者：FrankWilczek

■ 翻译：胡风、梁丁当

Frank Wilczek

弗兰克·维尔切克是麻省理工学院物理学教授、量子色动力学的奠基人之一。因发现了量子色动力学的渐近自由现象，他在2004年获得了诺贝尔物理学奖。

中文版

物理定律预言宇宙最终会停止运转与变化，这样的命运能被推迟吗？

在今年夏天某个酷热的上午，当我正在游泳时，我的思绪突然飘到了另一个关于“气候变化”的问题上：宇宙的“热死亡”，也就是我们常说的“热寂”。这很可能是宇宙的终极命运。讽刺的是，比起我们这个越来越热的星球，宇宙死亡的话题反而没有那么令人焦虑。

在19世纪，随着科学家对“热”的科学理解逐渐加深，产生了“宇宙热寂”的假说。它的核心思想很简单：物理系统总是会趋于平衡。比如，热量总是从较热的物体自发流入较冷的相邻物体；随着热量的交换，前者被冷却，后者被加热，当二者达到相同的温度后，热量交换就会终止。

我们可以通过给对应的系统注入能量，从而延迟到达最终的平衡与静止的时间，但这也只是暂时的。比如，我们可以给发动机加油，给动物喂食，给电池充电。但是发动机会磨损，动物会死亡，电池也会老化而无法充电。

这些日常现象可以在热力学中被进一步概括和强化。在热力学中，所谓的拱顶石就是热力学第二定律。1865年，鲁道夫 · 克劳修斯（ Rudolf Clausius ）首次给出了热力学第二定律的数学表达式。它指出：系统的无序性由熵来度量，随着时间的流逝，熵会不断增加，这也意味着系统的有序结构在逐渐消失。最终，系统会演化到一个完全无序且毫无特征的平衡态——熵最大的状态。

根据热力学第二定律的无情逻辑，宇宙最终将演化到一个毫无变化与生机的死寂状态，即“宇宙热寂”。现代宇宙学进一步充实了这一说法。

引力作用导致物质倾向于聚集。但在早期宇宙中，物质分布极为均匀，所以引力处于远离平衡的状态。随着时间的推移，引力试图达到平衡，将稀薄的星云凝聚成恒星。恒星内部的高密度和高压会点燃核反应。核燃烧释放的热将为宇宙的运动提供能量，但这只是缓刑。几百亿年后，所有的恒星的燃料都将燃烧殆尽。

要想对抗宇宙热寂，我们的子孙后辈或者宇宙中的其他智慧生命，或许可以采用以下的一些方法。下面这些方法是我在游泳的时候逐步构想出来的：

第一，在恒星耗尽燃料之后，或许可以进一步燃烧物质。因为恒星发生核聚变时，质子和中子被重新组合，但总数保持不变。继续燃烧这些粒子可以获得数百倍的能量。还有一种无可名状的燃料是“暗物质”。目前还没有人知道它是什么，但宇宙中有很多这样的暗燃料。它们或许可以给我们的后辈或者其他智慧生命留下一线希望。

第二，在未来，工程师或许可以操控行星或者死亡星球的碰撞，这种碰撞很有可能进一步释放出大量能量。而在未来的宇宙中，应该有很多这样的材料可以使用。

第三，未来的文明或许只需要很有限的能量就可以维持运转。最近关于可逆计算机与量子计算机、以及时间晶体的理论研究表明，进一步发展——至少是维持运动——所需要的能量是没有下限的。

第四，由于我们并不真正地了解到底是什么机制触发了宇宙大爆炸，可以想象，或许有那么一天，我们能够设计出类似的机制，未来也将有机会重新引爆宇宙。

这次游泳很尽兴。我脑洞大开，想象着那些可以拯救宇宙的技术手段，备感愉悦。宇宙的终极命运固然让人绝望，但或许不那么严重。

从湖里出来的时候，我似乎比之前更热了，这让我颇为郁闷。而在眼下，地球上的气候问题确实已经变得非常糟糕了，尽管可能还没有发展到无可救药的地步。

英文版

The laws of physics say that in the distant future, all change and activity in the cosmos will come to an end. Can that fate be postponed?

One very hot day this summer, during a morning swim, my mind wandered to a different version of climate change: the “heat death” of the universe. Ironically, though it remains a plausible outcome for cosmic history, it’s a less distressing subject than our own warming planet.

The idea of heat death arose with the scientific understanding of heat itself in the 19th century. The core idea is simple: Physical systems tend to settle toward equilibrium. For example, heat will tend to flow from a hot body into an adjacent cold body, cooling the former and warming the latter, until both reach the same intermediate temperature, after which heat is no longer exchanged.

Ultimate equilibrium and stasis can be postponed by the injection of energy, but only temporarily. Engines can be refueled, animals fed, batteries recharged; but engines wear down, animals die, and batteries lose their juice.

These common observations are generalized and sharpened in the science of thermodynamics. The capstone of thermodynamics is its so-called Second Law, first formulated mathematically by Rudolf Clausius in 1865, which states that entropy, a measure of disorder, increases over time—distinctive structure erodes. Featureless equilibrium is the state of maximum entropy, toward which the Second Law drives us.

The inexorable logic of the Second Law leads, in the long run, to a bland universe wherein nothing changes-that is, heat death. Modern physical cosmology has fleshed out that conclusion. Gravity wants matter to clump, but in the early universe, matter’s distribution was extremely uniform, so gravity was way out of equilibrium. Over time, gravity has sought to come into equilibrium, notably by condensing stars out of gas clouds. The high density and pressure found inside stars ignites nuclear fuel.

Nuclear burning injects heat and powers a dynamic universe. But this is a temporary reprieve. After a few tensof billions of years, stars everywhere will have exhausted their fuel and winked out.

There are several ways that our distant descendants, or other embodiments of mind in the universe, might resist heat death. Here are some ideas that occurred to me as I swam:

First, it is probably possible to burn matter further than stars do. Stars rearrange protons and neutrons but do not change their overall number. Burning those particles would give access to hundreds of times more energy. Another (barely) conceivable form of fuel is “dark matter.” At present, nobody knows what it is, but there’s lots of it in the universe.

Second, future engineers also might be able to arrange controlled collisions of planets or dead stars, to tap into the energy the crashes liberate.

Third, future minds themselves might be able to run on very limited power. Recent theoretical work on reversible and quantum computers, and on time crystals, has shown that there’s no lower limit to how little energy they need to keep making progress, or at least to keep moving.

Fourth, since we don’t really understand what triggered the Big Bang, it’s conceivable that someday we’ll be able to engineer something similar, and thereby rejuvenate the universe.

It was a good swim. I had fun saving the universe by inventing speculative technological fixes and adaptations, spiced up with wishful thinking. The long-term future of mind in the universe is desperate, but not serious.

Unfortunately, when I emerged from the lake, it was even hotter than before. Here and now on Earth the situation is dead serious—though maybe not yet utterly desperate.

本文2022年10月01日发表于微信公众号 蔻享学术《【诺奖得主Wilczek科普专栏】延迟宇宙热寂》，风云之声获授权转载。