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.
古希腊人有个很好的想法: 宇宙很简单。 在他们看来, 宇宙的一切 都由四种元素组成: 土壤、空气、火, 还有水。 这个美妙的理论继续发展, 它既简单,又优雅。 它说,只要用不同的方式 组合这四种元素, 就能得到宇宙中 多样的一切。 比如说,土壤和火 能合成干燥的东西。 空气和水能合成湿的。 但是随着理论发展, 问题来了。 它的预测里, 没有能被测量的东西, 而测量是实验科学的基础。 更糟的是, 这个理论根本不对。 不过,希腊人是伟大的哲学家 而且在公元前5世纪, 留基伯提出了 有史以来最久经考验的理论。 我们能看到的一切 都由一种叫原子的 看不见的微粒组成。 这个理论简单而优美, 而且比起四元素说 它更有希望被证实。 几个世纪的科学思考和实验 证实了真实的物质, 比如氢、碳、或者铁, 都能被分解成原子。 留基伯的理论中, 原子是物质最小的组成部分, 就像氢、碳、或者铁。 留基伯理论中唯一的错误, 就是原子其实仍然可分。 而且人们发现,他的理论 只能解释宇宙组成的一小部分。 宇宙中看似常见的物质, 事实上相当稀少。 留基伯的原子, 以及它们的组成 其实只占了宇宙的5%。 物理学家知道, 宇宙剩下的95% 都是暗宇宙, 即暗物质和暗能量, 我们是怎么知道的? 因为如果我们看着什么, 我们就会看见什么。 看起来很简单, 其实非常深奥。 所有组成原子的物质 都是可见的。 它们反射光,从而被我们看见。 当我们仰望星空, 就能看见星星和星系。 其中有一些, 和银河系一样, 美丽,螺旋状, 在宇宙中优雅地旋转。 科学家在1930年代 第一次测量了星系群的运动 和它们包含物质的重量, 他们非常惊讶。 他们发现 没有足够的可见物质 来让物质聚在一起。 接下来对各个星系的单独测量 也证实了 这个令人困惑的结果。 星系中没有 足够的可见物质 来产生足够支持它们的引力。 在我们看来,它们会飞散开, 可是它们没有。 所以一定有什么物质 是我们看不见的。 我们叫它们:暗物质。 如今,暗物质存在的最佳证明 是测量一种 叫“宇宙微波背景辐射”的余辉, 产生在宇宙大爆炸中, 那又是另一个故事了。 我们所有的证据 都表明了暗物质存在, 并且是布满天穹的美丽星系的 主要成分。 所以我们要怎样呢? 我们早就知道 地心说是错的, 我们生活的星球, 其实相当普通, 它的恒星也很普通, 位于一个普通星系的旋臂。 发现暗物质 使我们又揭开了 物质的一层面纱。 我们发现,组成我们的物质 只是宇宙极小的一部分。 剩下的还有很多。 本世纪初, 科学家研究了宇宙的外围 并证实,所有东西 都在互相原理, 想想开天辟地的大爆炸 这并不令人意外, 而且,宇宙膨胀的速度 似乎也在加快。 这是怎么回事? 要么有一种能量 推动了这种加速, 就像你让汽车加速一样, 要么就是引力的作用 和我们想的不完全一样。 大多数科学家相信前者, 即有一种能量,推动了加速, 他们叫它:暗能量。 当下最好的测量 使我们能够算出 宇宙中暗部分有多大。 看起来,暗能量 组成了宇宙的68%, 暗物质则占27%, 我们,和我们能看见的 只有5%。 那暗物质和暗能量的组成是什么? 我们不知道, 但有一个叫“超对称”的理论 可以解释其中的一部分。 超对称简称SUSY, 预测了很多新粒子, 其中的一些也许可以组成暗物质。 如果能证明SUSY, 我们就能从理解宇宙的5%, 也就是我们能看见的东西, 到理解大约三分之一。 对一天的工作来说,并不赖。 理解暗能量也许会更难, 但也有一些投机理论 也许能指出道路。 其中一些理论 能追溯到 古希腊人的最早的伟大想法, 就是我们几分钟前讲过的 宇宙一定很简单的想法。 这些理论预言,宇宙的多样 全都来自一个元素, 也就是“弦 ”。 我们知道的一切微粒 都是弦的不同谐波。 不幸的是, 弦论至今都无法被证明。 不过,可以探索的宇宙如此广袤, 风险很高。 这一切是否让你觉得自己很渺小? 不用这样 相反,你应该惊叹 我们已经发现的一切, 你是宇宙中能思考 自己这个奇迹的 唯一物种的一员, 而且你生逢其时, 能看到我们的探索。