In medieval times, alchemists tried to achieve the seemingly impossible. They wanted to transform lowly lead into gleaming gold. History portrays these people as aged eccentrics, but if only they'd known that their dreams were actually achievable. Indeed, today we can manufacture gold on Earth thanks to modern inventions that those medieval alchemists missed by a few centuries. But to understand how this precious metal became embedded in our planet to start with, we have to gaze upwards at the stars. Gold is extraterrestrial. Instead of arising from the planet's rocky crust, it was actually cooked up in space and is present on Earth because of cataclysmic stellar explosions called supernovae. Stars are mostly made up of hydrogen, the simplest and lightest element. The enormous gravitational pressure of so much material compresses and triggers nuclear fusion in the star's core. This process releases energy from the hydrogen, making the star shine. Over many millions of years, fusion transforms hydrogen into heavier elements: helium, carbon, and oxygen, burning subsequent elements faster and faster to reach iron and nickel. However, at that point nuclear fusion no longer releases enough energy, and the pressure from the core peters out. The outer layers collapse into the center, and bouncing back from this sudden injection of energy, the star explodes forming a supernova. The extreme pressure of a collapsing star is so high, that subatomic protons and electrons are forced together in the core, forming neutrons. Neutrons have no repelling electric charge so they're easily captured by the iron group elements. Multiple neutron captures enable the formation of heavier elements that a star under normal circumstances can't form, from silver to gold, past lead and on to uranium. In extreme contrast to the million year transformation of hydrogen to helium, the creation of the heaviest elements in a supernova takes place in only seconds. But what becomes of the gold after the explosion? The expanding supernova shockwave propels its elemental debris through the interstellar medium, triggering a swirling dance of gas and dust that condenses into new stars and planets. Earth's gold was likely delivered this way before being kneaded into veins by geothermal activity. Billions of years later, we now extract this precious product by mining it, an expensive process that's compounded by gold's rarity. In fact, all of the gold that we've mined in history could be piled into just three Olympic-size swimming pools, although this represents a lot of mass because gold is about 20 times denser than water. So, can we produce more of this coveted commodity? Actually, yes. Using particle accelerators, we can mimic the complex nuclear reactions that create gold in stars. But these machines can only construct gold atom by atom. So it would take almost the age of the universe to produce one gram at a cost vastly exceeding the current value of gold. So that's not a very good solution. But if we were to reach a hypothetical point where we'd mined all of the Earth's buried gold, there are other places we could look. The ocean holds an estimated 20 million tons of dissolved gold but at extremely miniscule concentrations making its recovery too costly at present. Perhaps one day, we'll see gold rushes to tap the mineral wealth of the other planets of our solar system. And who knows? Maybe some future supernova will occur close enough to shower us with its treasure and hopefully not eradicate all life on Earth in the process.
在中世纪, 炼金术士们总是梦想着 完成一些不可能的事情。 他们想把锈迹斑斑的铁块 变成亮闪闪的金子。 历史学家们觉得这些人是怪人, 但是他们不知道这些梦想, 事实上是现实的。 如今,我们可以大量生产金块, 多亏了那些在炼金术士时代, 尚未出现的现代发明。 但是想要了解这稀有金属 是如何点缀我们的星球的, 我们得先仰望星空, 探索宇宙的奥秘。 金存在在外星球上。 它并不是直接形成于地壳上, 而是来自宇宙, 由于超新星爆炸, 而来到地球。 恒星由宇宙中最简单、 最轻的元素氢构成。 由于巨大的引力, 氢被压缩并激发横行的核聚变。 这一过程会释放能量, 令其发光。 几百万年来, 核聚变将氢元素转化成 一些更沉的元素: 如氦、碳、氧, 并支持它们快速燃烧, 最终变为铁或镍。 然而,当核聚变不再释放能量时, 核中的气压也会逐渐降低。 外层因此破裂崩塌,向心坠入, 恒星从被突然注入的能量恢复之后, 爆炸形成超新星。 爆裂的恒星的气压非常大, 使得质子和电子被贴在一起, 在核中形成中子。 中子不带电荷, 所以它们很容易被铁元素吸引, 很多中子促进了更沉的元素的形成, 使得在一般情况下的恒星, 不能从银变为金、铅和铀。 由氢到氦的形成大概要几百万年, 然而超新星中最沉的元素的形成 只需要仅仅几秒钟。 那么爆炸后的结果是什么呢? 不断扩展的超新星冲击波 用它的星际间介质 推进它的残骸, 使其带动周围的气体和尘埃旋转 最后又凝缩成心的恒星和行星。 地球上的金子 很可能就是这么形成的, 在被开发成金矿前。 几十亿年后, 我们开采这稀有的物质。 因为它的稀有, 所以开采过程变得很昂贵。 事实上,人类史上开采过的所有金矿 仅仅能填满 三个奥林匹克级别的游泳馆而已, 尽管它们质量很高, 因为金比水重 20 倍。 所以,我们可以生产 更多的这令人垂涎的金子吗? 答案是可以的。 我们可以使用粒子加速器来 模仿复杂的核反应, 进而产生宇宙中的金。 但这些机器 只能通过原子来制造金、 制造一克金所需的时间 像宇宙形成一样漫长, 同时花费也远超制成的金的价值, 所以这并不是一个明智的选择。 不过如果假设 我们能够开采世界上所有的金, 那么就有其他的地方供我们选择。 海洋里大概包含 两千万吨未溶解的金, 不过因为它们集中在很小的区域, 开采所需花费巨大。 也许有一天, 我们可以看到淘金热 在太阳系的其它行星上流行起来。 谁知道呢? 也许在将来超新星会距离我们越来越近, 洒落大批黄金, 同时不要毁灭地球上的其它物质。