There are a number of different varieties of quarks: there are six “flavors,” which we call up, down, strange, charmed, bottom, and top. The first three flavors had been known since the 1960s but the charmed quark was discovered only in 1974,the bottom in 1977, and the top in 1995. Each flavor comes in three “colors,” red, green, and blue. (It should be emphasized that these terms are just labels: quarks are much smaller than the wavelength of visible light and so do not have any color in the normal sense. It is just that modern physicists seem to have more imaginative ways of naming new particles and phenomena - they no longer restrict themselves to Greek!) A proton or neutron is made up of three quarks, one of each color. A proton contains two up quarks and one down quark; a neutron contains two down and one up. We can create particles made up of the other quarks (strange, charmed, bottom, and top), but these all have a much greater mass and decay very rapidly into protons and neutrons.
We now know that neither the atoms nor the protons and neutrons within them are indivisible. So the question is: what are the truly elementary particles, the basic building blocks from which everything is made? Since the wavelength of light is much larger than the size of an atom, we cannot hope to “look” at the parts of an atom in the ordinary way. We need to use something with a much smaller wave-length. As we saw in the last chapter, quantum mechanics tells us that all particles are in fact waves, and that the higher the energy of a particle, the smaller the wavelength of the corresponding wave. So the best answer we can give to our question depends on how high a particle energy we have at our disposal, because this determines on how small a length scale we can look. These particle energies are usually measured in units called electron volts. (In Thomson’s experiments with electrons, we saw that he used an electric field to accelerate the electrons. The energy that an electron gains from an electric field of one volt is what is known as an electron volt.) In the nineteenth century, when the only particle energies that people knew how to use were the low energies of a few electron volts generated by chemical reactions such as burning, it was thought that atoms were the smallest unit. In Rutherford’s experiment, the alpha-particles had energies of millions of electron volts. More recently, we have learned how to use electromagnetic fields to give particles energies of at first millions and then thousands of millions of electron volts. And so we know that particles that were thought to be “elementary” thirty years ago are, in fact, made up of smaller particles. May these, as we go to still higher energies, in turn be found to be made from still smaller particles? This is certainly possible, but we do have some theoretical reasons for believing that we have, or are very near to, a knowledge of the ultimate building blocks of nature.
Flavor 味
Disposal 處理
Volt 伏特
存在有幾種不同類型的夸克——至少有六種以上的“味”，這些味我們分別稱之為上、下、、魅、底和頂。每種味都帶有三種“色”，即紅、綠和藍。（必須強調，這些術語僅僅是記號：夸克比可見光的波長小得多，所以在通常意義下沒有任何顏色。這只不過是現代物理學家更富有想像力地去命名新粒子和新現象而已——他們不再將自己限制於只用希臘文！）一個質子或中子是由三個夸克組成，每個一種顏色。一個質子包含兩個上夸克和一個下夸克；一箇中子包含兩個下夸克和一個上夸克。我們可用其他種類的夸克（、魅、底和頂）構成粒子，但所有這些都具有大得多的質量，並非常快地衰變成質子和中子。
現在我們知道，不管是原子還是其中的質子和中子都不是不可分的。問題在於什麼是真正的基本粒子——構成世界萬物的最基本的構件？由於光波波長比原子的尺度大得多，我們不能期望以通常的方法去“看”一個原子的部分，而必須用某些波長短得多的東西。正如我們在上一章 所看到的，量子力學告訴我們，實際上所有粒子都是波動，粒子的能量越高，J則其對應的波動的波長越短。所以，我們能對這個問題給出的最好的回答，取決於我們的設想中所能得到多高的粒子能量，因為這決定了我們所能看到的多小的尺度。這些粒子的能量通常是以稱為電子伏特的單位來測量。（在湯姆遜的電子實驗中，我們看到他用一個電場去加速電子，一個電子從一個伏特的電場所得到的能量即是一個電子伏特。）19世紀，當人們知道如何去使用的粒子能量只是由化學反應——諸如燃燒——產生的幾個電子伏特的低能量時，大家以為原子即是最小的單位。在盧瑟福的實驗中，α粒子具有幾百萬電子伏特的能量。更近代，我們知道使用電磁場給粒子提供首先是幾百萬然後是幾十億電子伏特的能量。這樣我們知道，20年之前以為是“基本”的粒子，原來是由更小的粒子所組成。如果我們用更高的能量時，是否會發現這些粒子是由更小的粒子所組成的呢？這一定是可能的。但我們確實有一些理論的根據，相信我們已經擁有或者說接近擁有自然界的終極構件的知識。
