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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。
作者 | Frank Wilczek
时间反演对称性可以被打破，而这种看似突兀的“皱纹”也给了我们一个认识物理世界的窗口。
自17世纪末现代物理学诞生以来，我们对自然界运行规律的认识已经经历了数次重大的拓展与革新。我们引入了电磁场、时空曲率和量子革命，使现代物理学发展地愈加丰富与强大。然而，无论历经多少发展与骚动，有一个自然规律的基本特征，始终完好无损。它简单美丽，却又神秘莫测。这个特征就是时间反演对称性，简称T对称。比如，当你拍摄了一部记录一系列事件的影片后，T对称意味着，如果把影片从后往前倒着放映，我们所看到的事件和现实世界所遵循的基本定律还是一样的。
无论是牛顿的运动方程和引力定律、麦克斯韦的电动力学方程、爱因斯坦的相对论，还是量子力学的基石——薛定谔方程与狄拉克方程，它们都满足时间反演对称。事实上，直到1964年以前，科学家观察到的所有物理现象似乎都满足时间反演对称性。
但是，在1964年，詹姆斯·克罗宁（James Cronin）、范伦丁·菲奇（Valentine Fitch）以及他们在美国长岛布鲁克黑文国家实验室的合作者发现，在K介子的衰变过程中，它的一些细微特征十分值得注意。因为在类似倒放电影的逆向过程中，它的行为看上去有些许不同，可能违背了T对称。在这项研究发表前不久，物理学家理查德·费曼（Richard Feynman）已经出版了那本著名的《费曼物理学讲义》（Lectures on Physics）。在其中，他还诡异地预言了这项研究的进展。费曼在书中提到了位于日本内冈市（Neiko）的一扇门。在这座被誉为全日本最美丽的门上，点缀着精巧的装饰。这些装饰几乎是完美对称的——除了一个上下颠倒的小图案。
“传说中，为了避免天神嫉妒人类太过完美，人们故意制造了这个瑕疵，”费曼接着写道 ：“为什么大自然的对称性只是接近完美的？或许我们可以换个角度来解释 ：上帝故意创造了几乎完全对称的定律，于是我们便不会心生嫉妒。”
关于T对称破坏，有一种更加物理的解释。1973年，日本物理学家小林诚和益川敏英发现，对大部分物理过程而言，T对称源自三个更深层次的原理 ：相对论、量子力学与高度对称性，后者是我们描述宇宙四大基本力的理论的核心。小林-益川矩阵理论可以解释为什么克罗宁和菲奇观测的物理过程违背了T对称。但这个理论并不完整，因为在很多物理过程中，T对称表现得比理论预期的要好。
这段传奇故事似乎很快会迎来了一个辉煌的结局。1977年，两位美国物理学家罗伯托 · 佩切伊（Roberto Peccei）和海倫 · 奎恩（Helen Quinn）证明，如果进一步在方程中加入对称性，就可以完全解释在什么情况下满足或者违背T对称。史蒂文 · 温伯格（Steven Weinberg）和我各自独立地意识到，这个证明意味着存在一种全新的粒子。我把它命名为轴子。它本是一个洗衣液的名字，在这很是应景，因为轴子的引入可以清理掉一个棘手的问题。
后来我和其他人的研究表明，如果在宇宙大爆炸中产生了轴子，就可以解释另一个谜团，即“暗”物质的形成。一直以来，暗物质的存在都是根据它对普通（可见）物质的引力影响间接推测的。
从理论上讲，轴子很难探测。但世界各地的物理学家们正在设计与制造能够探测它的仪器。如果轴子真能被探测到，那么对于为什么时间反演对称会被破坏的终极解释或许是另一个版本——与费曼的玩笑不同，它或许会更贴合爱因斯坦的一句话 ：“上帝有很多小心思，但他并不坏！”
In most cases, physics follows the same rules whether things run forward or backward-but not quite alwaysSince the dawn of recognizably modern physics in the late 17th century, our understanding of nature’s basic operating system has undergone major expansions and renovations. It got vastly more detailed and powerful. We brought in electromagnetic fifields, space-time curvature and the quantum revolution. Through all that growth and tumult, a simple and weirdly beautiful, yet enigmatic and seemingly gratuitous, feature of the laws remained intact. It is known as time reversal symmetry. Time reversal symmetry-or T for short-says that if you take a movie of events in the physical world and run it backward, what you see will obey the same basic laws.Newton’s laws of motion and gravity, Maxwell’s equations of electrodynamics, Einstein’s relativity theories and the Schrödinger and Dirac equations that implement quantum theory all obey that principle. So did everything scientists observed in the physical world-until 1964. That year James Cronin, Valentine Fitch and collaborators at Brookhaven National Laboratory on Long Island discovered that some subtle features of the behavior of highly unstable particles called K mesons would look a little difffferent in a time-reversed movie.Richard Feynman’s famous “Lectures on Physics,” written shortly before, eerily antici pated this development. He cited a gate in Neiko that is often described as Japan’s most beautiful. Its delicate, otherwise symmetric ornamentation has one small element carved upside down. “The story is that it was carved upside down so that the gods will not be jealous of the perfection of man,” he wrote. “We might like to turn the idea around and think that the true explanation of the near symmetry of nature is this: that God made the laws only nearly symmetrical so that we should not be jealous.”There might be a more conventional explanation. In 1973 the Japanese physicists Takuru Kobayashi and Toshihide Maskawa showed that for most processes time-reversal symmetry follows unavoidably from three other, deeper principles-relativity, quantum mechanics and the high symmetry that is central to our theories of the universe’s four fundamental forces. The Kobayashi-Maskawa theory also showed why T fails in the sorts of processes that Cronin and Fitch studied. But the theory is not quite complete; T works a little better, in more instances, than Kobayashi and Maskawa described.A glorious conclusion to this saga might be emerging. In 1977 two American physicists, Roberto Peccei and Helen Quinn, showed that including further symmetry in our equations can close the remaining gap in answering where T works and doesn’t. Steven Weinberg and I discovered independently that implementing this idea brings in a wholly new kind of particle. I named this theoretical particle the axion, after a laundry detergent, because it would clean up a nasty problem. Later work by myself and others showed that axions produced in the Big Bang would have the right properties to answer another mystery: They could be the socalled “dark” matter whose existence is, so far, only inferred from its gravitational inflfluence on ordinary (visible) matter.Axions are predicted to be hard to detect, but research physicists around the world are designing and building instruments that could be up to the job. If so, the ultimate explanation for exceptions to T would be difffferent from Feynman’s joke and more in line with one of Einstein’s: “Subtle is the Lord, but malicious He is not!”