諾獎得主Wilczek：填滿“空”間的粒子_風聞
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撰文 | Frank Wilczek
翻譯 | 胡風、梁丁當
中文版
在發現希格斯粒子的第10週年，我們終於可以客觀地看待它了。
為了理解希格斯粒子的重要性，讓我們想象一個海洋星球。這個星球上演化出了一種智慧魚類，它們想弄明白物體運動的規律。它們不斷地進行實驗、推導方程式，卻始終一頭霧水。其實並不奇怪，因為魚類想當然地認為它們生活的空間——海洋——是空蕩蕩的。
終於，經過幾十年的努力後，一部分魚意識到了這一點。它們發現，如果假設“真空”，也就是海洋，是一種具有質量和運動的介質的話，就可以用一個優雅的定律（牛頓定律）完美地解釋一切。於是，這些魚開始猜測海洋到底是由什麼構成的。它們把一些海水煮沸，經過複雜的光譜分析，最終確定了水分子。美妙的想象引導它們走向了具體的真理。
類似的故事也在地球上演。20世紀初，物理學家發現了支配原子核與粒子衰變的亞原子“弱力”（也稱弱相互作用，四種基本力之一）。為了描述弱力的工作機制，物理學家在最初寫下的方程中，假設了一種所謂的“W”粒子：類似光子能傳播電磁力那樣，W粒子可以傳遞弱力。不幸的是，對這些方程式一致的應用卻預測：W粒子應該和光子一樣質量為零。但事實卻不是這樣。
此時，一個看似毫無關聯的現象——超導現象——為理解弱力指明瞭一條出路。1935年，物理學家弗裏茨·倫敦（Fritz London）和海因茨·倫敦（Heinz London）兄弟二人首次提出設想：光子在超導材料中獲得了非零質量。這個質量修正了方程，使它們能正確地描述超導體中的電磁場效應。
1957年， 約翰·巴丁（John Bardeen）、 利昂·庫 珀（Leon Cooper）和約翰·羅伯特·施裏弗（John Robert Schrieffer）發現，超導體內部的電子會結合成庫珀對，併發生凝聚。這個凝聚體會阻礙光子的自由運動，使它們變得有些遲緩，這其實等效於讓光子獲得了質量。
現在，讓我們來談談彼得·希格斯（Peter Higgs）的貢獻。1964年，他和羅伯特·布魯（Robert Brout）及弗朗索瓦·恩格勒特（Francois Englert）各自獨立地發揮想象並提出：就像前文智慧魚類所生活的海洋一樣，我們以為的“空間”也並非真的空無一物，這就導致W粒子有了質量。對W粒子來説，“空間”是種流體介質——一種超級超導體。
這個大膽的假設讓弱力方程自洽了。可是，這個流體是由什麼構成的呢？什麼樣的介質能夠阻礙W粒子的自由運動，讓它獲得質量呢？當時所有已知的粒子，無論如何排列組合，都無法滿足這一點。
為了解決這個挑戰，物理學家讓質子發生碰撞，期望通過把能量匯聚到一個極小的體積，來獲得這種流體的一點碎片。2012年7月4日，在日內瓦附近的大型強子對撞機（LHC）開展的兩項各有數百名研究人員參與的實驗中，發現了一種新粒子——希格斯粒子。實驗證據顯示，希格斯粒子正是宇宙這個超級超導體的主要成分。
此後十年的反覆實驗與仔細核對完全證實了希格斯粒子具有構成我們所生存的超級超導體的正確屬性。然而，它仍然是一個謎一般的異類。縱然希格斯粒子可以使其他粒子獲得質量，但它自身的質量全然是個謎。其他已知粒子都可以被完美地納入“大統一”理論，但希格斯粒子仍然是一個無家可歸的孤兒。
這些懸而未決的問題意味着故事還沒完結。對希格斯粒子更深入的研究可能會幫我們打開一扇通向新世界的大門。這樣看來，10歲的希格斯粒子還很年輕。
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.
Frank Wilczek
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