测量定义时间 | 诺奖得主Wilczek专栏

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时间是什么？时间在根本上到底是连续的，还是由某种尚未探测到的离散单元组成的？尽管我们对时间的测量越来越精确，但我们依然不理解时间的本质。

作者 | Frank Wilczek（麻省理工学院教授、2004年诺贝尔奖得主）

翻译 | 梁丁当、胡风

时间的精确测量是物理学的一项辉煌成就，现在仍是个激动人心的前沿领域。

Accurate measurement of time is one of the glories of physics, and one of the most exciting frontiers of current research.

在历史的长河中，人类大部分时候都是利用太阳和月亮来计时的。地球自转一周构成我们的“一天”，绕着太阳公转一圈便是我们的“一年”，而地球和月球的华尔兹则给了我们“月”的概念。日晷利用物体在阳光下的投影来测定时辰，但它也有明显的缺陷，比如阴天多云的时候，它就不管用了。

For most of history, people measured time using the sun and moon. Earth’s rotation gives us the day, its revolution around the sun gives us the year, and its dance with the moon gives us the month. Sundials used the shadow cast by the sun to track the hours, but they had drawbacks; for one thing, they didn’t work on cloudy days.

对于更短的时间，人们在数个世纪中都是用沙漏和水钟来计量的，但它们的精确度容易受到温度和湿度变化，以及振动的影响。到了中世纪，机械钟出现了，它由摆锤与精巧的齿轮系统构成，代表了当时的技术巅峰。到了20世纪初，这些基于摆锤和弹簧的时钟进一步演变成各种做工精妙的钟表，但它们仍然摆脱不了摩擦的影响。摩擦会妨碍钟摆移动，还会导致零件损耗。

To measure briefer periods of time, hourglasses and water clocks served for many centuries, even though changes in temperature, humidity and vibration could throw them off. Mechanical clocks using falling weights and elaborate systems of gears were a high point of medieval technology. By the early 20th century, clocks with pendulums and springs had evolved into remarkable, beautifully contrived instruments. But even these clocks were subject to friction, which affects their movements and wears away their parts.

而对电磁场和物质的更深入理解，让人们能够制造更精准的时钟。电子的流动取代了流沙或滴水，电磁场取代了齿轮，微小的石英取代了弹簧。目前最先进的时钟利用原子或分子的振动来测量时间。最精确的原子钟即便经过了相当于宇宙年龄那么长的时间，误差都不会超过1秒。

Increased understanding of electromagnetism and of matter in general made even better clocks possible. Flows of electrons replaced flows of sand or water; electromagnetic fields replaced gears; tiny quartz crystals replaced springs. Today’s cutting-edge clocks are based on measuring the vibrations of atoms and molecules. The most precise atomic clocks are so accurate that, over a period of time equivalent to the whole history of the universe to date, they would be off by less than one second.

当今这些精通量子力学的钟表匠还在追寻着更高的计时精度。比如，原子核的振动比原子整体的振动更快，因此有可能利用原子核的振动来计时，制造出比原子钟更准的原子核钟。精度更高的时钟可以帮助物理学家判断时间在根本上到底是连续的，还是由某种尚未探测到的离散单元组成的。更精密的时钟有着广泛的用途，比如开发更好的GPS系统，或是探测引力波。

Today’s quantum horologists are looking to improve things further. For instance, it might be possible to monitor atomic nuclei, which vibrate faster than whole atoms. More accurate clocks would help physicists determine whether time is truly continuous or fundamentally granular, made from units yet to be detected. More tangible applications of better timekeeping abound as well, from developing better GPS systems to detecting passing gravitational waves.

虽然这些时钟千差万别，它们给出的时间却是一致的。我们很容易把这个事实当成理所当然，但其实它是非常不可思议的。人的情绪不同时，感受到的时间流逝速度也不同，可能是白驹过隙，也可能是度日如年。但事实上时间在极为严格地均匀流动，这是一个现实世界的客观性质，不受人主观意识的影响。

It is an amazing fact, too easily taken for granted, that all these radically different sorts of clocks agree with one another. In different moods we might feel time’s flow to be a surge or an ooze, but its rigorous uniformity is an objective feature of physical reality.

尽管我们对时间的测量越来越精确，但我们依然不理解时间的本质。在古罗马神学家、哲学家奥古斯丁（Augustine）的名著《忏悔录》（Confessions，撰写于397-398年）的第11章中，我们可以找到一段关于时间的最佳哲学讨论。奥古斯丁是一位基督教主教，一位信徒问了他一个让人头疼的问题：“上帝创造世界之前在做什么？” 奥古斯丁曾考虑过类似这样的回答：上帝在为那些窥探天机的人准备地狱。然而他最终决定认真对待这个问题，并触及了它的根源。

Even as we get better at measuring time, however, its nature remains hard to grasp. One of the best philosophical discussions of time can be found in chapter 11 of St. Augustine’s “Confessions,” written in 397-398. Augustine was a Christian bishop, and one of his parishioners asked him a vexing question: “What was God doing before creation?” Augustine considered, but rejected, the answer “He was preparing hell for those who pry into mysteries.” Instead, he took the question seriously—and got to its root.

奥古斯丁指出，每当我们使用“以前”或者“以后”这样的术语时，我们都在谈论时间，可它的本质却似乎很神秘：“时间是什么？如果没人问我，我知道，可如果我试图去解释它，却又做不到。”但是奥古斯丁发现了一个道理，其中蕴含了后来相对论和量子理论的一个核心思想：如果要理解一样事物，你就必须考虑它是如何测量的。正如他所说：“如果我们不能测量，那它就什么都不是。”

Whenever we use terms like before and after, Augustine reasoned, we are speaking of time, whose essence seems mysterious: “What is time? If no one asks me, I know, but if I try to explain it, I cannot.” But he discovered a principle that anticipates a major theme of relativity and quantum theory: To understand what something is, you must think about how it is measured. As Augustine puts it, “What we measure not, is not.”

按照这个逻辑，奥古斯丁关于“什么是时间”这个问题的答案很简单：“时间就是时钟所测量的东西。”根据这样的理解，那个信徒的问题就解决了：因为在上帝创世之前没有时钟，所以就没有时间。在当今物理宇宙学中，类似的问题依然存在，比如“大爆炸之前发生了什么？”也许奥古斯丁的回答仍然是我们能给出的最好答案：在无法测量时间的背景下讨论什么是“以前”，是毫无意义的。

Following this logic, Augustine’s answer to “What is time?” is simple: “Time is what clocks measure.” With that understanding, the parishioner’s question dissolves. Before creation there were no clocks, and without clocks there could be no time. Today, in physical cosmology, the parishioner’s question is still with us, in the form “What happened before the Big Bang?” I suspect that Augustine’s answer might still be the best one we have: To ask about “before,” in that timeless context, is literally senseless.

作者简介

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。因在夸克粒子理论（强作用）方面所取得的成就，他在2004年获得了诺贝尔物理学奖。

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