Top Ten Ideas of Physics: Foundations for Understanding the Universe

徐一鴻著

普林斯頓大學出版社

讀 者 福 利

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Editor's Note

在追逐“最熱最新”科研熱點的時代，著名物理學家徐一鴻（Anthony Zee）選擇回到起點，書寫一部聚焦于學科基礎思想的作品。

在新作

Top Ten Ideas of Physics: Foundations for Understanding the Universe

徐教授自敘，他在近三、四十年來反復思索這份“Top10理念清單”，盡管排序有變，其中的核心內容卻歷經(jīng)時間考驗，始終穩(wěn)固。這些思想不僅關乎物理學本身，對不同背景的讀者同樣具有啟發(fā)價值。正如作者所言：這本書適合所有真正意義上的“智識讀者”——無論年長年少，只要你渴望一睹宇宙真理的本質與真容。

作者簡介

徐一鴻

Anthony Zee

徐一鴻教授是著名美籍華裔物理學家、作家，籍貫上海，出生于昆明，后移民到巴西。在普林斯頓大學取得學士學位，在哈佛大學取得博士學位。現(xiàn)任美國加州大學圣塔芭芭拉分校物理系教授。

恒久而冷清

我在此務請讀者首先閱讀這篇序言，以了解本書的宗旨。若讀者是來尋找物理學“最熱最新”的課題，還請上網(wǎng)搜索爆款熱帖，本書恐怕會令你失望。

我構思這本書已至少三十年，也許有四十年了，早在我寫下第一本學術專著《自然力的統(tǒng)一》（譯者注：原著為 Unity of Forces in the Universe，無中譯本，為行文通暢，特添加譯名）以及第一本通俗科普《可畏的對稱》（Fearful Symmetry）之后不久。1（譯者注：如無特別說明，本文所有注釋均為作者自注）在寫作這些書時，我會思索“統(tǒng)一”（unity）和“對稱”（symmetry）如何成為了理論物理學的兩大基石理念。此后多年間，我陸續(xù)寫下了自己心目中“理論物理十大理念”的清單。這本書的部分靈感可說源于上世紀八十年代美國深夜電視節(jié)目中主持人宣布“十大榜單”的傳統(tǒng)，盡管那些榜單常?；恼Q不經(jīng)（譯者注：例如“最不受貓歡迎的十大貓糧口味”、“十大跡象表明你可能就是班里最傻的學生”）。于是我自己也會戲謔性地列出榜單，比如“凝聚態(tài)物理中最難看的十大術語”，掛在辦公室門口以娛樂同事。

直到近年，當我向普林斯頓大學出版社提交本書計劃，并著手動筆時，我翻出多年來零星記錄下的五六份理論物理的“十大理念”清單。頗為有趣也令我欣喜的是，縱然細節(jié)略有變動，排序有所調整，但核心內容幾乎一致。這讓我深感安慰：經(jīng)歷了理論物理研究的潮流變遷，這些念茲在茲的思想到底是有恒久價值，經(jīng)得起歲月淘洗。

簡而言之，這本書聚焦于理論物理的智識根基，而非學界風潮。即便是學識淵博、對物理頗有研究興趣的讀者，也常被一類“哇哦”、聳人聽聞的新發(fā)現(xiàn)所吸引——可這些發(fā)現(xiàn)對物理學家而言，有的不過乏味，有的則所幸能很快被遺忘。顯然，這背后不乏流行媒體的推波助瀾——炒作那些只有極少數(shù)物理學家相信或關心的狂想更有經(jīng)濟效益，翻翻目錄讀者應能發(fā)現(xiàn)，本書不談“最熱最新”，只論“恒久而冷清”。

物理學的基石性理念

本書每一章會探討一個基本思想，簡要闡述其精義，下面我將用一兩句話分別對書中十章進行小結：

1. 世界是可以被理解的。

人類所懷的最不可思議的信念，正是宇宙是可以被理解的。而大自然也一次又一次對理論物理學家投以她的仁慈。

2. 物理定律無所不在、恒古不變。

若物理的存在并非普遍而永恒，我們就無法理解宇宙，亦無法追溯其“嬰兒時期”。

3. 世界的本質是量子的。

我們所處日常世界之下，竟?jié)摬刂粋€神秘的量子世界——在這里，常識和傳統(tǒng)意義上的“現(xiàn)實”分崩離析——堪稱是人類最震撼的發(fā)現(xiàn)。

4. 量子場，恒久不息——愛因斯坦的至愛。

宇宙是一場量子場的交織之舞。這些永恒之場激發(fā)孕育出我們熟知的粒子：有些稍縱即逝，有些看似不朽——但也許并非如此。

5. 可畏的對稱：宇宙處處是對稱。

為何越深入本質，物理越對稱、越簡潔、越“優(yōu)雅”？這是個真正深奧的謎題。

6. 愛因斯坦：用相對論終結的“相對性”，編排時空之舞。

物理定律不應物理學家的視角而變——真理并不“相對”，這與某些哲學家向你宣揚的恰恰相反。

7. 自然力的統(tǒng)一。

物理曾由繁雜定律構成，而20世紀幾乎不可思議的成就，是將其簡化為四種基本相互作用，其中三種已統(tǒng)一為“大一統(tǒng)理論”，惟獨引力仍拒絕入列共舞。

8. 造物主說的是數(shù)學的語言。

維格納（Wigner）曾質疑數(shù)學之于物理的高效是否過于“不合理”。我和多數(shù)（但非全部）理論物理學家都對此深感困惑。

9. 熵與熱漲落：一切關乎“分享”。

從兩個粒子的交互中無從知曉億萬粒子的集體行動——熵與時間之箭由此而生。這正是“多者異也“的體現(xiàn)——整體多于部分之和。

10. 物理關乎“作用量”。

從費馬（Fermat）的“最短時間原理”開始，我們離開了牛頓的方程體系，轉而以“作用量”構建自然的演化。這是物理最深層的語言。

以上就是十章的內容，每章探討一個重大理念。我曾想半開玩笑地加上與破產相關的第十一章，題為：《物理危機：道德破產的警世故事》2，但最終還是刪去了——不過，誰知道呢，也許將來它又會在另一本新書中冒出來。

“十大”中“十”這個數(shù)字，當然只是我們這個地球上進化的巧合。未來也許會增減。

不只是代入計算，也不是細枝末梢

物理不只是“代入公式，算出結果”。然而，中學乃至大學、研究生階段的“題海戰(zhàn)術”，讓學生難免得出這種印象。追求“結果”確是部分物理學家的動力，但絕非全部。迪克·費曼（譯者注：即著名物理學家理查德·費曼的昵稱，原文為 Dick Feynman）或許以其獨特的文采說得最為精辟：“物理就像性愛：當然，它可能會產生某些實在的結果，但這可不是我們做它的原因?！弊分鸾Y果或可贏得名聲與財富，但許多人，包括我自己，更多的是被理解宇宙運作之奧秘的渴望所驅使。我和一些朋友沉醉于理念之間的邏輯關聯(lián)，驚嘆于這些理念竟讓我們人類如此渺小的存在，得以理解從微觀到宏觀、幾乎匪夷所思的復雜宇宙！“匪夷所思！”這句話似乎就是為物理學量身定制的。

愛因斯坦說：“我想知道上帝如何創(chuàng)造這個世界。我不關心這個或那個現(xiàn)象。我想知道的是祂的思想，其余皆為細枝末梢?！闭f得多好！但恐怕只有他這種地位，才敢一口否定物理學中的大部分知識為細枝末節(jié)。我當年在《可畏的對稱》（Fearful Symmetry）中引用這句話，頗為驚艷，卻也收到些“決斗狀”般的郵件。有人一輩子鉆研“這個”，因此憤怒非常；也有人是“那個”方面的全球權威，卻從未聽說“這個”。愛因斯坦已故多年，無從為這份“狂妄”買單。若他尚在，或許會調皮地建議我讓“這個博士”與“那個教授”直接聯(lián)系。

放輕松。其實所有物理學家都在研究“這個或那個現(xiàn)象”，包括愛因斯坦自己。除了探討時空與量子，他還研究過濕沙的粘滯性3以及冰箱的熱力學原理——讀來妙趣橫生。事實上，幾乎所有物理學家4一生研究的都不過是愛因斯坦眼中的“這個或那個”。

而確然，幾乎所有重大突破也都源于對“這個或那個”的研究。比如，年輕的馬克思·普朗克（Max Planck）開創(chuàng)量子革命，并非因為哲思宇宙，而是緣于鉆研加熱腔體中的電磁波；又如，暗能量的發(fā)現(xiàn)，建立于對超新星爆炸機制的深入理解。我常說，物理有如大馬戲團，萬象并存：馴獸師、空中飛人到雜技小丑5，皆有其位。物理學的前沿在諸多領域推進，因此關于空間和時間本質的根本性洞見也必然——與許多物理學愛好者的想象大相徑庭——千載難逢。是的，我理解“多者異也”的概念，程度或許比你認為的還要深。

盡管如此，你手中的這本書所談的正是物理學的基礎理念，不特別關注愛因斯坦口中的“這個或那個”，除非作為說明需要。所以，在讀者想向我發(fā)送憤怒郵件前，請先記住這一點。固然我知道“細節(jié)決定成敗”（devil is in the weeds），學會鉆研細節(jié)很重要，但本書宗旨是提供全面概覽。

免責聲明

首先說明幾點：這既不是一般的通俗讀物，也不是教材。哪怕只講其中一個思想，展開來也夠寫一本書。比如第四章所講的量子場論，我就寫過一本科普書、一本專業(yè)教材和一本半科普半專業(yè)讀物。我常需在簡潔與“不讓讀者完全懵圈”之間尋找平衡。因此許多必要主題被省略或一筆帶過6。可想而知有些學界的同行會因此不滿，但我會鼓足勇氣直面他們的顧慮和怒火。

我的目標是傳達這些思想的意義，并略述其來龍去脈。有時我會集中講某一專題。例如，要講量子物理，面面俱到絕無可能，于是我選了貝爾不等式（Bell’s inequality）來切入，也因為寫書時恰逢三位驗證該理論的實驗物理學家獲得諾貝爾獎。而為解釋貝爾不等式，我必須介紹電子自旋——這或許是最詭異的物理概念之一，但我們當今的科技社會正高度依賴它。

再如，熵與信息兩個概念在大眾認知中常被誤解，我便幾近教科書式地詳加闡釋。

我提過自己出版了多本物理科普讀物和教科書，因此本書內難免會重復舊作。為什么要重復？費曼早年自問此事時給出了我十分認同的回答：“因為人類的每天里都有新的一代誕生。因為人類的歷史上總有偉大理念誕生，而這些理念若不刻意、清晰地傳給新的一代，就無法長存?！?7

無疑，不少專家會對書中的泛泛論述頗有微詞。但若我列舉所有保留條件和詳細說明，要寫的恐怕就不是一本書，而是一整座圖書館了。普通讀者肯定會不堪重負。

我同樣確定，若請別的物理學家列出物理學“十大理念”，他們的清單必然和我的不同。對此，我只能說：這份清單源于我個人在理論物理上的探索旅程；而這種書必然、且最好是：理直氣壯地帶有鮮明的個人觀點。若你極力反對，我誠摯地邀請你寫一本自己的書。

本書并非線性敘述，章節(jié)安排基于我自己的邏輯。就像前文所說，這些理念的排序亦經(jīng)調整。若讀者看完某章節(jié)未盡理解，請不妨繼續(xù)讀下去。因為你并不需要這章的前置知識才能理解后一章。然而，各章的基礎理念不可避免地交織在一起。例如，“規(guī)律越深入越顯對稱”（第五章）與“看似不同力的最終統(tǒng)一”（第七章）必然相關。熟悉我其他作品的讀者知道，我偏愛大量使用尾注——有些或可發(fā)人深思，有些或只為引人發(fā)噱。首次閱讀時也不妨忽略尾注，稍后再回看。這純屬個人喜好問題。

最后聲明，我并非歷史學家，書中敘事當然不保證歷史嚴謹性。

此書為誰而作

我在自己的數(shù)本通俗科普書的前言中都包含了這一節(jié)：“本書為誰而寫？”毫無疑問，如果有中學生、大學生因本書而立志投身物理，我將無比欣慰。而在受眾的另一端，我知道還有許多在職或退休的職場人士，熱衷閱讀通俗物理書。此書也獻給他們，致敬他們追求宇宙真理的勇氣與熱情。

當然，讀者若能略知愛因斯坦的時空觀及量子物理的基礎，那自是更佳；畢竟這些革命性的思想已有百年歷史，本書只能粗略點到。

總之，這本書獻給所有真正意義上的“智識讀者”：無論年長年少，只要你渴望一識物理的本質與真容。

*強烈建議讀者，特別是年輕人，細讀部分尾注，那里可能有更多數(shù)學討論，以及一些職業(yè)建議——如果你決定進入物理學領域的話。例見第三章中海森堡（Heisenberg）評論克羅寧（Kronig）的尾注。

注釋：

1. 該書最初由麥克米倫出版，后由普林斯頓大學出版社在Princeton Science Library系列中重新出版。我自豪地說，它已被翻譯成德語、日語、韓語、法語、西班牙語、土耳其語和中文（包括繁體和簡體）。多年來，尤其是在旅行講學途中，我非常高興能巧遇許多在其早期物理學生涯中讀過此書的物理學家們。

2. 供非美國地區(qū)讀者一哂，在美國，“破產法第十一章”含義通常等同于破產。

3. 很久以前，一家石油勘探公司甚至支付酬勞讓我閱讀愛因斯坦這項鮮為人知的貢獻。多謝了，喬！

4. 那些不這么做的，至少在我看來，應歸為數(shù)學家或哲學家。

5. 可悲的是，在我們當下的墮落時代，小丑似乎常常主宰了舞臺。

6. 這部分內容是為了預防亞馬遜網(wǎng)站上那些常把教科書和科普書混為一談的讀者。

7. R. P. 費曼（R. P. Feynman）著，《這個不科學的年代》(The Meaning of It All ），1963年出版。

章節(jié)試讀

Chapter 1

THE PHYSICAL WORLD IS COMPREHENSIBLE

The most incomprehensible thing about the world is that it is comprehensible.1

• Why is the world comprehensible?

Of Einstein’s many insightful sayings, this is my favorite. Indeed, this apparent miracle, that the world is comprehensible, is what motivates physicists. In a lawless universe behaving with neither rhyme nor reason, physics would be impossible.

I should hasten to say what hardly needs to be said, but with silly nit-pickers lurking about I have to say it. What Einstein meant by the world is the physical world, the world studied by physicists. Plenty of questions are beyond the comprehension and grasp of physics, of course. What is the meaning of life? Why are we here at all? Why does the universe exist? Is there another level of existence we know nothing about? You could make up your own list.

We have made stupendous progress since our days in the caves when nearly everything was incomprehensible. But even within almost living memory, the progress has been so vast that it almost beggars imagination. Two hundred years ago, physicists did not know what light was. One hundred and fifty years ago, physicists had no inkling that space-time was curved. One hundred years ago, physicists struggled to grasp the laws governing the realm of the quantum underlying our world. It is true that progress in theoretical physics has slowed nowadays. Nevertheless, progress is being made.

Why should Nature be comprehensible by the mind of these creatures that evolved recently on a dust mite of a planet orbiting an insignificant star drifting in a far-from-prestigious neighborhood of a mediocre average-looking galaxy? Why should the laws of physics be simple and beautiful? We could have found ourselves living in an intrinsically ugly universe, a chaotic world in no way graspable through thinking, as Einstein, he again, put it.

• A smashingly revolutionary idea

Some readers might be surprised by my choice of the idea that tops my list of top ideas. The world is comprehensible! We tend to take for granted that physics is possible, that the physical world is comprehensible. But as far as I know, no ancient civilization, East or West, harbored the idea that the world was comprehensible, except, arguably, for a handful of Greeks. Even in the West, this enormous and profound idea surfaced only in the last four or five centuries, and the first inkling that the world could be understood without invoking the divine was met with ferocious resistance. It was a slow, gradual awakening threatened with excommunication, torture, and death.

The sun god pulling his chariot daily across the sky from east to west, and then somehow teleporting himself and his entire conveyance back to the east at night—this was our “understanding” not all that long ago. Yet now, for the first time in human history, a reasonably advanced student of physics could open a book on astrophysics and learn how to calculate the temperature and pressure inside a star, a particularly simple homework exercise for a garden-variety star like the sun, neither a red giant nor a white dwarf, without too many peculiar features.

• Why versus how

I cannot tell you why the world is comprehensible. It is important to understand that physics does not answer the why questions. Physicists strive to steadily turn the whys into hows. We cannot tell you why the apple falls, but we can tell you how it falls, and reduce a variety of phenomena into an ever smaller set of underlying phenomena. Along the way, many whys were replaced by fewer whys, and whys by hows. Indeed, a crucial step in making physics possible is to separate the how questions from the why questions.

We can tell you why the sky is blue. It has to do with how light scatters off the air molecules, and after a heroic struggle we have reduced that to how two quantum particles, the photon and the electron, interact with each other. We can tell you that every massive object in the universe attracts every other massive object. We can tell you a lot about this force called gravity, and how it is equivalent to curved spacetime. But we cannot tell you why our universe contains such a force. Of course, were this force absent, then our universe would look quite different from the one we know.

The preceding sentence gives a capsule summary of the anthropic principle. It has been abused to answer every why question with the existence of humans. You know the jingle; various physical constants must have the value they have, since otherwise humans would not exist. A fascinating subject perhaps, but I will stay away from it in this book. Only the tried and true foundational ideas, as I said.

We do not know why there are four fundamental interactions in the universe, instead of two or six, but physics can tell us how three of these four interactions could be unified into a grand unified theory, as we will see in chapter 7.

• That the physical world is comprehensible is miraculous

Physics as a subject wouldn’t exist were the world not comprehensible.

Yet, if you think about it, the very comprehensibility of the world is miraculous. Sad that we have grown blasé in this enlightened age and take so much for granted. But flash yourself back a few millennia when no one even presumed to imagine the possibility of comprehending the world, let alone to attempt the task of actually comprehending the world.

You need only look at ancient cultures around the world. In school, we learned about the realm of rationality that is ancient Greece. Even in Greek myths, we could discern the urge to understand. But the notion that we could comprehend the physical world was far from universal. For example, the philosophers of ancient China envisaged man in harmony with Nature and emphasized understanding human behavior rather than the physical world. The West emphasized the control of Nature. Perhaps controlling is but one small step from understanding. To some extent, the entire Greco-Roman Judeo-Christian conception of Nature to be conquered and ravaged was missing in ancient China.2

Noticing regularity alone is not enough. Certainly, Asians recorded regularity with as much diligence as the Greeks. But the notion that there might be laws governing these regularities was mostly absent. That the world was comprehensible was far from obvious to every thinker in every civilization.

Nor should we assume that the development of physics as it happened on our planet is typical. For example, a civilization could develop in a binary star system, on one of the nineteen planets orbiting the two stars, which in turn waltz around each other in a highly eccentric ellipse. The regularity of the heavens, as well as the usual law of gravitation, might remain hidden from this particular civilization for a long time, perhaps longer than the natural lifetime of the civilization.

Or imagine us in a civilization on a planet much like Terra, except that it is completely covered by a single deep ocean. Electricity might be known as an epiphenomenon associated with some peculiar fish regarded as far inferior to Pisces sapiens, which is what we call ourselves scientifically. Magnetism is however totally unknown and light is merely an exotic phenomenon near the edge of the habitable world. Physicists would have developed an extensive understanding of water waves.3 Some of our dead sink to a nether region where none of us are able to roam due to the crushing pressure. Others among our dead would float upward, and from observing these lightweight dead, politicians, celebrities, and such, some bright youngster eventually proposed the existence of a force named buoyancy. Later, a fishy Einstein would have the brilliant insight that buoyancy is due to a more fundamental force called gravity pulling the water around these lightweight objects down.?

• We alone could understand the universe?

I do not doubt for a second what my computer science friends tell me, that with massive data analysis, artificial intelligence would in the foreseeable future allow us to communicate with animals. At that point, perhaps we could ask them what they understand of the physical world. Yet I also have no doubt that no other lifeforms on earth can understand nonabelian gauge theories (which we will encounter in chapter 7), let alone invent them. But then why is the physical world comprehensible to humans (at least thus far)?

Einstein was absolutely right that it is a profound mystery. It could well be that this particular primate species, after eons of evolution, is no more capable of fully understanding the physical world than one of the gophers in my backyard. (The emphasis is on “fully”!) It is certainly possible, of course, that we will eventually hit a brick wall. The pessimists in the physics community would say that we have already hit that proverbial wall.

I want to be optimistic because I need that optimism to buoy me professionally. Einstein expressed his profound wonderment more than a hundred years ago. He and his contemporaries could also feel, when confronted with the mysteries of the quantum world, that the striking progress in understanding of the physical world gained in the 19th century may come to a grinding halt. But they didn’t, and pressed on. And progress has been made for a century and more. Of course, in deciphering the quantum world, that generation of physicists was pushed along by an almost excessive wealth of experiments, which are now sorely lacking, no matter what the cheerleading boosters try to convince us otherwise. Needless to say, I am talking about landmark experiments that reveal some deep truths, not routine experiments measuring the decay rate of an absurdly named particle or the conductivity, super or not, of some newly minted alloy.

I am among those who believe that the universe, with its zillions of galaxies each with zillions of stars, is teeming with intelligent life. I have even thought seriously about communicating with extraterrestrial intelligence.? When we finally come into contact with extraterrestrial intelligent beings, what can we talk about? Not terrestrial biology, which may or may not be similar to their biology?. And definitely not Tang dynasty poetry, nor Shakespearean plays, which so many scholars spent, are spending, and will spend entire lifetimes on. But surely, if the extraterrestrials have mastered enough technology to communicate with us, they could discuss the classification of Lie algebras and gauge theories with us, perhaps teaching us a thing or two. Whatever, but I could hardly doubt that they have also come to realize that the universe is comprehensible.

Notes

1 Albert Einstein “Physics and Reality,” Franklin Institute Journal (March 1936).

2 Sadly, China is coming late to this highly useful idea, and now a common attitude goes “You ravaged the world, now it is our turn, so don’t act holier than thou and preach to us.”

3 But not the surface water waves we commonly observe and teach to some undergraduates. See FbN, chapters VIII.1 and 2, in particular page 282.

4 An early example of unification that I will talk about in chapter 7.

5 See my SETI talk and the published papers mentioned therein. https://www.youtube.com/watch?v=MHuXlJzqKqs.

6 See, for example, A. Kershenbaum, The Zoologist’s Guide to the Galaxy: What Animals on

Earth Reveal about Aliens–and Ourselves, and A. Weir, Project Hail Mary.

PUP 內容速遞

本書章節(jié)目錄

Prologue

1. The physical world is comprehensible

2. The laws of physics are the same here, there, and everywhere, the same yesterday, today, and tomorrow

3. The world is quantum

4. Quantum fields forever: Einstein’s total love

5. Fearful symmetry: a universe full of symmetries

6. Einstein, the exterminator of relativity and the choreographer of spacetime

7. Unity of forces in the universe

8. The Creator speaks the language of mathematics

9. Entropy and thermal agitation: all about sharing

10. Physics is where the action is

Parting comments

Acknowledgments

Bibliography

Timeline

Index

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《物理》50年精選文章