Instead, a red quark has to be joined to a green and a blue quark by a “string” of gluons (red + green + blue = white).
Such a triplet constitutes a proton or a neutron. Another possibility is a pair consisting of a quark and an antiquark (red+ anti red, or green + anti green, or blue + antiblue = white).
Such combinations make up the particles known as mesons, which are unstable because the quark and antiquark can annihilate each other, producing electrons and other particles.
Similarly, confinement prevents one having a single gluon on its own, because gluons also have color. Instead, one has to have a collection of gluons whose colors add up to white. Such a collection forms an unstable particle called a glueball.
The fact that confinement prevents one from observing an isolated quark or gluon might seem to make the whole notion of quarks and gluons as particles somewhat metaphysical.
However, there is another property of the strong nuclear force, called asymptotic freedom, that makes the concept of quarks and gluons well defined. At normal energies, the strong nuclear force is indeed strong, and it binds the quarks tightly together.
However, experiments with large particle accelerators indicate that at high energies the strong force becomes much weaker, and the quarks and gluons behave almost like free particles.
Fig. 5.2 shows a photograph of a collision between a high-energy proton and antiproton. The success of the unification of the electromagnetic and weak nuclear forces led to a number of attempts to combine these two forces with the strong nuclear force into what is called a grand unified theory(or GUT). This title is rather an exaggeration: the resultant theories are not all that grand, nor are they fully unified, as they do not include gravity. Nor are they really complete theories, because they contain a number of parameters whose values cannot be predicted from the theory but have to be chosen to fit in with experiment. Nevertheless, they may be a step toward a complete, fully unified theory. The basic idea of GUTs is as follows: as was mentioned above, the strong nuclear force gets weaker at high energies. On the other hand, the electromagnetic and weak forces, which are not asymptotically free, get stronger at high energies. At some very high energy, called the grand unification energy, these three forces would all have the same strength and so could just be different aspects of a single force. The GUTs also predict that at this energy the different spin-1/2 matter particles, like quarks and electrons, would also all be essentially the same, thus achieving another unification.
Meson 介子
Glueball 膠球
Metaphysical 形而上學的
asymptotic 漸進的
resultant 由此導致的
反之，一個紅夸克必須用一串膠子和一個綠夸克以及一個藍夸克聯結在一起（紅＋綠＋藍＝白）。這樣的三胞胎構成了質子或中子。其他的可能性是由一個夸克和一個反夸克組成的對（紅＋反紅，或綠＋反綠，或藍＋反藍＝白）。這樣的結合構成稱為介子的粒子。介子是不穩定的，因為夸克和反夸克會互相湮滅而產生電子和其他粒子。類似地，由於膠子也有顏色，色禁閉使得人們不可能得到單獨的膠子。相反地，人們所能得到的膠子的團，其迭加起來的顏色必須是白的。這樣的團形成了稱為膠球的不穩定粒子。
色禁閉使得人們觀察不到一個孤立的夸克或膠子，這事實使得將夸克和膠子當作粒子的整個見解看起來有點玄學的味道。然而，強核力還有一個叫做漸近自由的性質，它使得夸克和膠子成為定義得很好的概念。在正常能量下，強核力確實很強，它將夸克很緊地捆在一起。但是，大型粒子加速器的實驗指出，在高能下強作用力變得弱得多，夸克和膠子的行為就像自由粒子那樣。圖5.2是張一個高能質子和一個反質子碰撞的照片。
統一電磁和弱力的成功，使許多人試圖將這兩種力和強核力合併在所謂的大統一理論（或GUT）之中。這名字相當誇張，所得到的理論並不那麼輝煌，也沒能將全部力都統一進去，因為它並不包含引力。它們也不是真正完整的理論，因為它們包含了許多不能從這理論中預言而必須人為選擇去適合實驗的參數。儘管如此，它們可能是朝著完全的統一理論推進的一步。GUT的基本思想是這樣：正如前面提到的，在高能量時強核力變弱了；另一方面，不具有漸近自由性質的電磁力和弱力在高能量下變強了。在非常高的叫做大統一能量的能量下，這三種力都有同樣的強度，所以可看成一個單獨的力的不同方面。在這能量下，GUT還預言了自旋為1/2的不同物質粒子（如夸克和電子）也會基本上變成一樣，這樣導致了另一種統一。
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