【诺奖得主Wilczek科普专栏】填满“空”间的粒子
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Frank Wilczek
作者 | Frank Wilczek
翻译 | 胡风、梁丁当
中文版
10 年前发现的希格斯粒子仍是揭开宇宙运行奥秘的希望。
在发现希格斯粒子的第10周年，我们终于可以客观地看待它了。
为了理解希格斯粒子的重要性，让我们想象一个海洋星球。这个星球上演化出了一种智慧鱼类，它们想弄明白物体运动的规律。它们不断地进行实验、推导方程式，却始终一头雾水。其实并不奇怪，因为鱼类想当然地认为它们生活的空间——海洋——是空荡荡的。
终于，经过几十年的努力后，一部分鱼意识到了这一点。它们发现，如果假设“真空”，也就是海洋，是一种具有质量和运动的介质的话，就可以用一个优雅的定律（牛顿定律）完美地解释一切。于是，这些鱼开始猜测海洋到底是由什么构成的。它们把一些海水煮沸，经过复杂的光谱分析，最终确定了水分子。美妙的想象引导它们走向了具体的真理。
类似的故事也在地球上演。20世纪初，物理学家发现了支配原子核与粒子衰变的亚原子“弱力”（也称弱相互作用，四种基本力之一）。为了描述弱力的工作机制，物理学家在最初写下的方程中，假设了一种所谓的“W”粒子：类似光子能传播电磁力那样，W粒子可以传递弱力。不幸的是，对这些方程式一致的应用却预测：W粒子应该和光子一样质量为零。但事实却不是这样。
此时，一个看似毫无关联的现象——超导现象——为理解弱力指明了一条出路。1935年，物理学家弗里茨·伦敦（Fritz London）和海因茨·伦敦（Heinz London）兄弟二人首次提出设想：光子在超导材料中获得了非零质量。这个质量修正了方程，使它们能正确地描述超导体中的电磁场效应。
1957年， 约翰·巴丁（John Bardeen）、 利昂·库 珀（Leon Cooper）和约翰·罗伯特·施里弗（John Robert Schrieffer）发现，超导体内部的电子会结合成库珀对，并发生凝聚。这个凝聚体会阻碍光子的自由运动，使它们变得有些迟缓，这其实等效于让光子获得了质量。
现在，让我们来谈谈彼得·希格斯（Peter Higgs）的贡献。1964年，他和罗伯特·布鲁（Robert Brout）及弗朗索瓦·恩格勒特（Francois Englert）各自独立地发挥想象并提出：就像前文智慧鱼类所生活的海洋一样，我们以为的“空间”也并非真的空无一物，这就导致W粒子有了质量。对W粒子来说，“空间”是种流体介质——一种超级超导体。
这个大胆的假设让弱力方程自洽了。可是，这个流体是由什么构成的呢？什么样的介质能够阻碍W粒子的自由运动，让它获得质量呢？当时所有已知的粒子，无论如何排列组合，都无法满足这一点。
为了解决这个挑战，物理学家让质子发生碰撞，期望通过把能量汇聚到一个极小的体积，来获得这种流体的一点碎片。2012年7月4日，在日内瓦附近的大型强子对撞机（LHC）开展的两项各有数百名研究人员参与的实验中，发现了一种新粒子——希格斯粒子。实验证据显示，希格斯粒子正是宇宙这个超级超导体的主要成分。
此后十年的反复实验与仔细核对完全证实了希格斯粒子具有构成我们所生存的超级超导体的正确属性。然而，它仍然是一个谜一般的异类。纵然希格斯粒子可以使其他粒子获得质量，但它自身的质量全然是个谜。其他已知粒子都可以被完美地纳入“大统一”理论，但希格斯粒子仍然是一个无家可归的孤儿。
这些悬而未决的问题意味着故事还没完结。对希格斯粒子更深入的研究可能会帮我们打开一扇通向新世界的大门。这样看来，10岁的希格斯粒子还很年轻。
英文版
This year marks the 10th anniversary of the discovery of the Higgs particle.Now we can see it in perspective.
To understand its significance, imagine an ocean planet where intelligent fish evolve and start to make theories of how things move. They do experiments and deduce equations but it is a messy hodgepodge, because the fish, taking their ever-present environment for granted, think of their ocean as“empty space.” After decades of work, though, some realize that by postulating that “empty space” is a medium-ocean-that has mass and motion of its own, you can account for everything using simple, elegant laws (namely, Newton’s laws). Next, the fish start to wonder what their hypothetical ocean is made of. They boil some ocean, do some sophisticated spectroscopy, and ultimately identify water molecules. Imagined beauty guided them to concrete truth.
A broadly similar story played out here on Earth. When physicists in the early 20th century discovered the subatomic “weak force” that governs many transformations of nuclei and particle decays, they first arrived at imperfect equations to try to describe how it works. Those equations postulated particles called “W,” which spur the weak force in the same way that photons spur the electromagnetic force. Unfortunately, consistent application of those equations predicts that W particles, like photons, should have mass equal to zero, which they don’t.
A seemingly far-removed phenomenon,superconductivity, suggested a way out.As first envisioned in 1935 by the physicist brothers Fritz and Heinz London, photons acquire non-zero mass inside superconducting material. That mass modifies the equations in just such a way that they correctly describe how electrodynamics works inside superconductors. In 1957, John Bardeen,Leon Cooper and J. Robert Schrieffer showed that electrons inside superconductors condense into a cohesive ocean of two-electron molecules that impedes the free motion of photons and renders them a bit sluggish, in effect giving them mass.
Now we come to the role of Peter Higgs.In 1964 he, and independently Robert Brout and Francois Englert, had the imagination to suggest that W particles have their mass because what we perceive as “empty space” is no emptier than the ocean of our imagined intelligent fish. As far as W particles are concerned, “empty space” is a fluid medium: a super-dupersuperconductor.
That audacious hypothesis made the equations consistent. But what makes up this medium, invisible yet pervasive, that impedes the free motion of W particles and gives them their mass? No combination of the known ingredients of matter was up to the job.
To address that challenge, physicists banged protons together,concentrating energy into a very small volume, hoping to break off little pieces of the fluid. On July 4, 2012, two experimental collaborations, each involving hundreds of researchers at the Large Hadron Collider near Geneva, presented evidence that a new particle, named after Higgs, is the main constituent of this cosmic super-duper-superconductor.
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Ten years of intense experimentation and scrutiny have confirmed that the Higgs particle has the right properties to make the super-duper-superconductor we inhabit. Yet it remains an enigmatic outlier. While the influence of Higgs particles gives mass to other particles, its own mass remains totally mysterious. And while all the other known particles fit beautifully into an overarching “grand unified” theory, the Higgs particle remains a stranded orphan. Those loose ends suggest that there should be more to the story, and that closer study of the Higgs particle might open a portal into new and otherwise inaccessible worlds. Thus, the Higgs particle is 10 years young.
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编辑：王亚琨
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