The ancient Greeks had a great idea: The universe is simple. In their minds, all you needed to make it were four elements: earth, air, fire, and water. As theories go, it's a beautiful one. It has simplicity and elegance. It says that by combining the four basic elements in different ways, you could produce all the wonderful diversity of the universe. Earth and fire, for example, give you things that are dry. Air and water, things that are wet. But as theories go, it had a problem. It didn't predict anything that could be measured, and measurement is the basis of experimental science. Worse still, the theory was wrong. But the Greeks were great scientists of the mind and in the 5th century B.C., Leucippus of Miletus came up with one of the most enduring scientific ideas ever. Everything we see is made up of tiny, indivisible bits of stuff called atoms. This theory is simple and elegant, and it has the advantage over the earth, air, fire, and water theory of being right. Centuries of scientific thought and experimentation have established that the real elements, things like hydrogen, carbon, and iron, can be broken down into atoms. In Leucippus's theory, the atom is the smallest, indivisible bit of stuff that's still recognizable as hydrogen, carbon, or iron. The only thing wrong with Leucippus's idea is that atoms are, in fact, divisible. Furthermore, his atoms idea turns out to explain just a small part of what the universe is made of. What appears to be the ordinary stuff of the universe is, in fact, quite rare. Leucippus's atoms, and the things they're made of, actually make up only about 5% of what we know to be there. Physicists know the rest of the universe, 95% of it, as the dark universe, made of dark matter and dark energy. How do we know this? Well, we know because we look at things and we see them. That might seem rather simplistic, but it's actually quite profound. All the stuff that's made of atoms is visible. Light bounces off it, and we can see it. When we look out into space, we see stars and galaxies. Some of them, like the one we live in, are beautiful, spiral shapes, spinning gracefully through space. When scientists first measured the motion of groups of galaxies in the 1930's and weighed the amount of matter they contained, they were in for a surprise. They found that there's not enough visible stuff in those groups to hold them together. Later measurements of individual galaxies confirmed this puzzling result. There's simply not enough visible stuff in galaxies to provide enough gravity to hold them together. From what we can see, they ought to fly apart, but they don't. So there must be stuff there that we can't see. We call that stuff dark matter. The best evidence for dark matter today comes from measurements of something called the cosmic microwave background, the afterglow of the Big Bang, but that's another story. All of the evidence we have says that dark matter is there and it accounts for much of the stuff in those beautiful spiral galaxies that fill the heavens. So where does that leave us? We've long known that the heavens do not revolve around us and that we're residents of a fairly ordinary planet, orbiting a fairly ordinary star, in the spiral arm of a fairly ordinary galaxy. The discovery of dark matter took us one step further away from the center of things. It told us that the stuff we're made of is only a small fraction of what makes up the universe. But there was more to come. Early this century, scientists studying the outer reaches of the universe confirmed that not only is everything moving apart from everything else, as you would expect in a universe that began in hot, dense big bang, but that the universe's expansion also seems to be accelerating. What's that about? Either there is some kind of energy pushing this acceleration, just like you provide energy to accelerate a car, or gravity does not behave exactly as we think. Most scientists think it's the former, that there's some kind of energy driving the acceleration, and they called it <i>dark energy</i>. Today's best measurements allow us to work out just how much of the universe is dark. It looks as if dark energy makes up about 68% of the universe and dark matter about 27%, leaving just 5% for us and everything else we can actually see. So what's the dark stuff made of? We don't know, but there's one theory, called <i>supersymmetry</i>, that could explain some of it. Supersymmetry, or SUSY for short, predicts a whole range of new particles, some of which could make up the dark matter. If we found evidence for SUSY, we could go from understanding 5% of our universe, the things we can actually see, to around a third. Not bad for a day's work. Dark energy would probably be harder to understand, but there are some speculative theories out there that might point the way. Among them are theories that go back to that first great idea of the ancient Greeks, the idea that we began with several minutes ago, the idea that the universe must be simple. These theories predict that there is just a single element from which all the universe's wonderful diversity stems, a vibrating string. The idea is that all the particles we know today are just different harmonics on the string. Unfortunately, string theories today are, as yet, untestable. But, with so much of the universe waiting to be explored, the stakes are high. Does all of this make you feel small? It shouldn't. Instead, you should marvel in the fact that, as far as we know, you are a member of the only species in the universe able even to begin to grasp its wonders, and you're living at the right time to see our understanding explode.
古代希臘人有一個偉大的想法: 宇宙是簡單的。 在他們心中, 要建構宇宙只須要四種元素: 土、 空氣、 火、 還有水。 就像它的理論一樣, 這是美麗的。 既簡單、又優雅。 理論中認為 藉由四種元素的不同組合, 可以產生宇宙萬物。 比如說土加火, 就會有一些乾的東西。 而空氣加水,就會得到濕的東西。 但依照這理論, 會有一個問題。 它沒辦法預測出任何 可以測量的事物, 而測量又是實驗科學的基礎。 更糟的是,這理論是錯的。 但希臘人仍是 思想上偉大的科學家, 在西元前 5 世紀, 米勒都的留基伯(Leucippus of Miletus) 提出有史以來 最經得起考驗的科學思想之一。 我們看到的每樣東西 都由微小、看不見的東西組成, 這東西稱作原子。 這個理論既簡單又優雅, 而它超越 土、空氣、水、火的理論 而它超越 土、空氣、水、火的理論 更接近事實。 幾世紀以來的 科學思想及實驗 證實了實體的物質, 像是氫氣、 碳、 還有鐵, 都可以分解成原子。 在留基伯的理論中,原子是最小的、 看不見的物質, 但仍然可以作區分, 像是氫原子、 碳原子、 鐵原子。 留基伯的想法裡, 唯一有問題的是 事實上原子也還可以再細分。 而且,他的原子說最終 只解釋了宇宙組成 的一小部份。 只解釋了宇宙組成 的一小部份。 平常所見到的物質, 實際上在宇宙中 非常稀少。 留基伯討論的原子、 還有它們組成的物質, 事實上只佔了 我們知道的物質 的大約 5%。 物理學家認為其餘的宇宙, 就是那 95%, 是暗宇宙, 由暗物質和暗能量組成。 我們怎麼知道? 嗯,我們會知道是因為 我們有觀察到。 這聽起來蠻簡單的, 但實際上有點複雜。 所有原子組成的物質 是看得見的。 它會反射光線, 然後我們就看得到。 當我們觀察宇宙時, 我們會看到星星和銀河。 有些就像我們居住的地方一樣, 很美麗、有螺旋的結構、 優雅地在太空中旋轉。 當科學家在 1930 年代 第一次 觀測一群星系時, 並同時估算它們的質量, 科學家們大為震驚。 他們發現 並沒有足夠的可見物質 讓這些星系群 靠在一起。 之後對單獨星系的觀察 證實了這個謎樣的結果。 星系裡就是沒有足夠的 可見物質 來提供足夠的重力 讓它們靠在一起。 從我們的觀測之中, 它們應該向外飛走, 但並沒有。 所以一定有什麼東西 是我們看不到的。 我們把這東西稱作 暗物質。 現今關於暗物質 最有力的證據 來自於對於一種叫作 宇宙背景微波輻射的觀測, 也就是大爆炸後 留下的餘光, 但這是另一段故事了。 所有我們掌握的證據 都說明了暗物質的存在, 而在天空中 那些美麗的螺旋星系中 它佔據了大部份的質量。 這告訴我們什麼? 長久以來我們知道 天空並不是繞著我們旋轉, 而我們只是 一顆平凡行星上的居民, 繞著一顆平凡的恆星、 座落在一個平凡星系的螺旋臂上。 暗物質的發現 帶領我們 更進一步了解 物質的本質。 這告訴我們 組成我們的物質 只是組成宇宙的物質 其中的一小部份。 但是不只這樣。 在這世紀初, 研究宇宙外部的科學家 證實了不只是 物質與物質之間不斷遠離, 就如同你想像 宇宙誕生於 高溫、高密度的大爆炸一樣, 這宇宙向外的擴張 似乎正在加速。 這代表什麼? 要不就是有某種能量 促成這樣的加速, 就像是你提供能量 讓一輛車加速一樣; 要不就是重力 和我們想的不完全一樣。 大多數科學家相信 前者是對的, 也就是有某種能量 促成這種加速, 他們把它叫作 暗物質。 現今最好的觀測 讓我們可以計算 宇宙有多少部份 是屬於暗的。 看似暗能量佔了 大約 68% 的宇宙, 而暗物質佔了 27%, 剩的 5% 才是 所有其它我們看得到的東西。 所以這些黑暗的東西 是由什麼組成的? 我們並不知道, 但有一個理論, 叫作「超對稱理論」, 可以解釋一些。 超對稱(supersymmetry), 或簡稱 SUSY, 預測了一系列新的粒子, 其中的一些 可以組成暗物質。 如果我們找到 超對稱的證據, 我們就可以從 對宇宙那 5% 的認識, 也就是我們看得到的那些, 進展到 大約 1/3。 還算不錯。 暗能量可能 更難去了解, 但有些猜測的理論 也許指出了方向。 其中有些 可以追溯到 古希臘人那第一個偉大的想法, 也就是幾分鐘前 我們起頭的想法, 這想法說明宇宙必須是簡單的。 這些理論 預測所有的宇宙萬物 只由單一一種東西組成, 一條振動的絃。 這想法是說 我們今天知道的所有粒子 只是這條絃上不同的波。 不幸的是,絃論現今 還沒辦法證實。 然而,宇宙中還有 這麼多事物等著去探索, 這賭注太大了。 這所有的東西 讓你覺得藐小嗎? 不應該這樣。 相反地, 你應該讚嘆 ──至少到目前為止── 你是這宇宙中 唯一一個開始了解 宇宙奧秘的物種 其中的一員, 而且你生對時代 來見證這知識的爆炸。