諾獎得主Wilczek：奇蹟般的繆子實驗_風聞
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撰文 | Frank Wilczek
翻譯 | 胡風、梁丁當
中文版
繆子是一種基本粒子。它具有一些與人們熟悉的電子類似的基本性質。比如，它們帶有完全相同的電荷。但二者又有兩點很大的區別：繆子比電子重約200倍，而且很不穩定，其平均壽命只有約2微秒。
相比其他奇異粒子，對繆子的探測異常容易。它能在高能加速器中大量產生。儘管一微秒聽上去很短，但是快速運動的繆子在消失前能夠穿越很長的距離，從而留下易被探測的軌跡。當人們在談到繆子特定的質量和磁矩時，往往覺得理所當然。但在實際測量中，我們需要對數百萬個不同的粒子進行取樣，而測到的數據居然幾乎相同，這個事實是極為深刻與神奇的。迄今為止的精密測量讓我們更加確信，所有的繆子，就像所有電子一樣，具有完全相同的性質。
繆子永遠在旋轉。正如物理學家所説，它們有“自旋”，這是解釋它們許多行為的關鍵。如果繆子處於磁場中，它的旋轉軸就會繞磁場旋轉，類似於一個傾斜的陀螺繞着豎直方向旋轉。這種類似陀螺的運動叫做進動。繆子在磁場中的進動率是磁場強度、一些已知的物理常數和一個被稱為磁矩的物理量的乘積。
對繆子的磁矩進行粗略的估計是比較容易的（如果你學了一學期相對論量子場論的話）。但是，如果要達到超過千分之一的精度，就必須引入被稱為虛粒子的奇異量子效應，更別提超過十億分之一了。根據量子理論，看上去虛無的空間實則充滿了生機——各種不同的粒子在極短的時間內產生與湮滅。這些所謂的虛粒子壽命太短，所以無法被探測器直接記錄下來，但它們卻會改變真實粒子的行為。
很多虛粒子在空間中分佈得很稀疏。要精確地計算它們對繆子磁矩的影響，是一項耗時而複雜的工作。但經過幾十年的發展，物理學家已經非常精通此道。而其他的虛粒子，如夸克、反夸克和膠子，卻完全不同。它們在空間中密集分佈，會產生強烈的相互作用。我們並不擅長計算它們的影響，這個瓶頸制約了目前理論預測的精度。
目前，解決這個問題的辦法通常有兩個。老辦法是從真實粒子的實驗數據進行外推，來估算需要的信息。新方法則是用超級計算機進行人力無法完成的第一性原理計算。這兩種方法照理應該得到相同的結果，但目前它們並不一致。計算機算出來的繆子磁矩與最新的實驗測量一致，而利用實驗數據外推得到的結果卻有十億分之一的差異。
這個差異若被證實，就意味着有一種迄今為止未被發現的粒子在起作用，嶄新的物理現象可能就在眼前。這個可能性太讓人興奮了，物理學家們蜂擁而上，提出各種猜想以搶佔先機。新的實驗結果剛一公佈，一夜之間就冒出了幾十個相互競爭的提議。物理學家知道將要宣佈一個新的發現，所以有備而來。但正如計算機的結果所顯示的那樣，這個“差異”仍然有可能只是空歡喜一場。
但無論如何，對於物理學家來説，這是一個奇蹟、一件幸事，因為我們竟然能夠在如此微小的差異上發揮我們的聰明才智。精度所傳達的最深刻的信息是 ：物體的真實世界和數學構造的理想世界，在令人難以置信的精度上，是同一個世界。
英 文 版
On April 7, the physics world was startled into glorious confusion by two announcements of a new measurement and a new calculation of the magnetic moment of the muon, published in the journals Physical Review Letters and Nature. The new results, accurate to the level of one part per billion, are the product of multiyear collaborations by large groups of physicists at institutions around the world. You might think that the work of making such precise measurements and calculations is as dull as science gets, but it can make magic happen.
Muons are elementary particles that in several fundamental ways resemble the more familiar electrons; for example, both carry exactly the same amount of electric charge. But there are two big differences: Muons are roughly 200 times heavier than electrons, and they are unstable, with a mean lifetime of roughly two microseconds.
As exotic particles go, muons are uncommonly user-friendly. They are easy to produce in large numbers at high-energy accelerators. And though a microsecond may not sound like a long time, fast-moving muons can travel a long way before they expire, leaving easily detectable tracks. Though it’s often taken for granted, the fact that we can talk about “the” mass and “the” magnetic moment of the muon, when in practice we sample millions of different particles, is both profound and amazing. Precision measurements so far reinforce our confidence that all It is painstaking and intricate work to calculate the influence of virtual particles precisely, but after decades of practice, physicists have gotten very good at it. muons, like all electrons, have exactly the same properties.
Muons are forever rotating—as physicists say, they have “spin”—which is key to many aspects of their behavior. If a muon is exposed to a magnetic field, its rotation axis circles around that field’s direction, similarly to how the axis of a tilted, spinning top circles around the vertical. This top-like motion is called precession. The rate of a muon’s precession in a magnetic field is equal to the product of the strength of the magnetic field, some known physical constants and a number called the magnetic moment.
Making a rough prediction of the magnetic moment of a muon is child’s play (for children who’ve taken a semester of relativistic quantum field theory). But to make it accurate beyond one part in a thousand, let alone one part in a billion, you’ve got to bring in the weird quantum effect known as virtual particles. According to quantum theory, apparently empty space is actually alive with activity, as particles of all different kinds briefly fluctuate into and out of existence. These so-called virtual particles are too fleeting to register directly in practical detectors, but they modify the calculated behavior of real particles.
Many kinds of virtual particles are distributed sparsely in space. It is painstaking and intricate work to calculate their influence on the muon’s magnetic moment precisely, but after decades of practice, physicists have gotten very good at it. Other virtual particles like quarks, antiquarks and gluons are a different story. They are dense in space and they interact strongly with one another. We are nowhere near so skillful at calculating their effects, and at present this bottleneck limits the precision of theoretical predictions.
Two different approaches to this problem are currently on the market. The older approach extrapolates from data about real particles to estimate the required information. The newer approach uses supercomputer technology to do superhuman, first-principles calculations. Those two approaches should give the same answer, but presently they don’t. The computer calculations give an answer for the muon magnetic moment that agrees with the new experimental measurement, while the data extrapolations leave a part-in-a-billion discrepancy.
A genuine discrepancy would signal that there are hitherto undiscovered particles at work. If so, other new phenomena could be right around the corner. That tantalizing possibility has triggered a gold rush of speculation and stake-claiming by physicists. Immediately after the new experimental result was announced, dozens of rival proposals materialized overnight. Physicists knew an announcement was coming, and they were prepared. But the “discrepancy” still might turn out to be fool’s gold, as the computer calculations suggest.
In any case, it’s a miracle and a blessing for physicists that we get to exercise our wits over the mere possibility of such minute discrepancies. For the deepest message of precision is that the real world of physical objects and the ideal world of mathematical constructions turn out, with mind-boggling accuracy, to be the same world.
Frank Wilczek
弗蘭克·維爾切克是麻省理工學院物理學教授、量子色動力學的奠基人之一。因發現了量子色動力學的漸近自由現象，他在2004年獲得了諾貝爾物理學獎。
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