A Theory of Everthing?

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Today, Einstein's goal of combining the physical laws of the universe in one theory that explains it all is the Holy Grail of modern physics.
running time 6:21

Newton's Embarrassing Secret

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Although Newton discovered the law of gravity nearly 300 years ago, until Einstein came along, scientists had no idea how gravity actually worked.
running time 6:04

A New Picture of Gravity

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Einstein's success in explaining gravity as warps and curves in the fabric of space and time set him on a quest to unify gravity with electricity and magnetism.
running time 7:11

A Strange New Wold

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As Einstein struggled to unite the weak force of gravity with the much stronger force of electromagnetism, physics moved on, examining the bizarre way tiny bits of matter interact with one another inside the atom.
running time 4:30

The Quantum Cafe

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According to quantum mechanics, at the tiny scale of atoms and particles, the world is a game of chance.
running time 8:24

Gravity-The Odd Man Out

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For decades, no one could figure out how gravity operates when you get down to the quantum world of atoms and subatomic particles.
running time 4:25

Strings to the Rescue

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Now string theory—the idea that everything is made of tiny, vibrating strands of energy—holds out the hope of unifying the world of the very large and the world of the very small.
running time 7:58

Science or Philosophy?

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String theorists have a problem: strings, if they exist, are so small that there's little hope of ever seeing one, so how can the theory be tested?
running time 3:35

Two Conflicting Sets of Laws

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In order to solve some of the deepest mysteries of the universe, the rules that govern large objects like galaxies must be combined with the rules that govern small objects like subatomic particles.
running time 8:26

One Master Equation

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Many physicists now believe that strings—miniscule vibrating strands of energy thought to make up all matter—hold the key to uniting the world of the large and the world of the small in a single theory.
running time 4:40

The Birth of String Theory

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In the 1960s, physicists caught a glimpse of what appeared to be strange, string-like objects hidden beneath the abstract symbols of a 200-year-old equation.
running time 4:12

The Standard Model

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Meanwhile, mainstream science was embracing particles as points, not strings, and the Standard Model was born, uniting the strong force, the weak force, and electromagnetism.
running time 6:36

Wrestling with String Theory

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By the 1970s, a few young physicists worked on taming the unruly equations of string theory and succeeded in describing how gravity works in the subatomic world, a key element missing from the Standard Model.
running time 7:18

The Theory of Everthing

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A revised version of string theory, free of mathematical inconsistencies, seemed capable of describing all the building blocks of nature, and it launched a hot new field of physics.
running time 5:29

Multiple Dimensions

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Despite our perception that we live in a universe with four dimensions—three spatial and one temporal—string theory demands that our universe has 11 dimensions.
running time 6:57

Five Flavors of String Theory

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By the mid-1980s physicists had developed five different versions of string theory, raising the question of whether it would prove to be a theory of everything or a theory of nothing.
running time 4:59

The Wild West Of Physics

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String theory is radically changing our ideas about the nature of space, opening up the possibility that extra dimensions, rips in the fabric of space, and parallel universes actually exist.
running time 10:02

The Potential of Strings

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Strings provide a unified framework for viewing the universe, but for a while, confusingly, there were five different versions of string theory.
running time 4:09

Getting to One Theory

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What physicists thought were five different theories turned out to be five different ways of looking at the same thing. String theory was unified at last.
running time 5:21

Parallel Universes

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The extra dimension of space required to unify string theory suggests that we may be trapped on just one tiny slice of a higher-dimensional universe.
running time 5:03

Escaping Gravity

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The weakness of gravity compared to the other forces has confounded physicists for decades, but now string theorists believe that gravity may be leaking into parallel universes.
running time 5:58

Riddle of the Big Bang

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The origin of the universe has always been a mysterious event in which the laws of physics appear to break down. Could it be that the big bang was caused by the collision of two parallel universes?
running time 7:37

Signs of Strings

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The hunt is on for evidence that supports string theory—for example, extra dimensions or supersymmetry.
running time 6:29

Too Elegant to be Wrong?

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Will string theory turn out to be a dead end? Most string theorists believe that such an elegant and mathematically beautiful idea couldn't be completely wrong.
running time 3:53

弦理論是一門理論物理學上的學說。理論裡的物理模型認為組成所有物質的最基本單位是一小段「能量弦線」，大至星際銀河，小至電子， 質子，夸克一類的基本粒子都是由這佔有二度空間的「能量線」所組成。中文的翻譯上，一般是譯作「弦」或「絃」。

較早時期所建立的粒子學說則是認為所有物質是由只佔ㄧ度空間的「點」狀粒子所組成，也是目前廣為接受的物理模型，也很成功的解釋和預測相當多的物理現象和問題，但是此理論所根據的「粒子模型」卻遇到一些無法解釋的問題。比較起來，「弦理論」的基礎是「波動模型」，因此能夠避開前一種理論所遇到的問題。更深的弦理論學說不只是描述「弦」狀物體，還包含了點狀、薄膜狀物體，更高維度的空間，甚至平行宇宙。值得注意的是，弦理論目前尚未能做出可以實驗驗證的準確預測，關於這一點，以下內文會說明。

弦理論的雛形是在1968年由Gabriele Veneziano發現。他原本是要找能描述原子核內的強作用力的數學公式，然後在一本老舊的數學書裡找到了有200年之久的尤拉公式（Euler's Function），這公式能夠成功的描述他所要求解的強作用力。然而進一步將這公式理解為一小段類似橡皮筋那樣可扭曲抖動的有彈性的「線段」卻是在不久後由李奧納特·蘇士侃所發現，這在日後則發展出「弦理論」。

雖然弦理論最開始是要解出強作用力的作用模式，但是後來的研究則發現了所有的最基本粒子，包含正反夸克，正反電子，正反中微子等等，以及四種基本作用力「粒子」（強、弱作用力粒子，電磁力粒子，以及重力粒子），都是由一小段的不停抖動的能量弦線所構成，而各種粒子彼此之間的差異只是這弦線抖動的方式和形狀的不同而已。

另外，「弦理論」這一用詞所指的原本包含了26度空間的玻色弦理論，和加入了超對稱性的超弦理論。在近日的物理界，「弦理論」一般是專指「超弦理論」，而為了方便區分，較早的「玻色弦理論」則以全名稱呼。1990年代，愛德華·維頓提出了一個具有11 度空間的M理論，他和其他學者找到強力的證據，證明了當時許多不同版本的超弦理論其實是M理論的不同極限設定條件下的結果。這些發現帶動了第二次超弦理論革新

弦理論會吸引這麼多注意，大部分的原因是因為它很有可能會成為大一統理論。弦理論也可能是量子重力的解決方案之一。除了重力之外，它很自然的成功描述了各式作用力，包含了電磁力和其他自然界存在的各種作用力。超弦理論還包含了組成物質的基本粒子之一的費米子。至於弦理論能不能成功的解釋基於目前物理界已知的所有作用力和物質所組成的宇宙，這還是未知數。

在未獲實驗證實之前，弦理論是屬於哲學的範疇，不能完全算是物理學。無法獲得實驗證明的原因之一是目前尚沒有人對弦理論有足夠的了解而做出正確的預測，另一個則是目前的高速粒子加速器還不夠強大。

科學家們使用目前的和正在籌備中的新一代的高速粒子加速器試圖尋找超弦理論裡主要的超對稱性學說所預測的超粒子。但是就算是超粒子真的找到了，這仍不能算是可以證實弦理論的強力證據，因為那也只是找到一個本來就存在於這個宇宙的粒子而已，不過這至少表示研究方向是正確的。

Greene, Brian. The Elegant Universe（中文譯本《優雅的宇宙》）. W.W. Norton and Co. New York,NY. c1999

The Elegant Universe Online (http://www.pbs.org/wgbh/nova/elegant/program.html) (含3小時的影片)