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.
中世紀時期 煉金術士進行了一項空前的實驗 他們想讓低鉛變成金燦燦的黃金 歷史稱這些人為「老古怪」 只怪他們身不逢時吧 因為現在在地球上煉金已不是夢想 這還要多虧現代的發明 這對以前的術士來說可是天方夜譚 不過想了解此貴金屬 是怎麼出現在地球上的 我們還得仰望星空尋求答案 黃金是外來物質 它並非來自岩石地殼 而是來自太空 隨著恆星大爆炸飛來了地球 這種大爆炸的恆星叫作超新星 恆星主要由最簡單最輕的氫元素組成 眾多物質產生的巨大重力 會壓縮並引起星核的核聚變 這過程氫會釋能 使星星更明亮 數百萬年以來 聚變反應將氫轉變成更大質量的元素 例如氦、碳、和氧 並在高溫下快速結合氫,生成鐵和鎳 但這時核聚變不再釋放能量 核心壓力逐漸消失 外層向核心迅速收縮 核心因突然有能量注入而出現了反彈 最終星體爆炸,形成了超新星 坍縮星產生的壓力極大 使得亞原子質子和電子在核心聚集 形成中子 由於中子不產生電荷排斥 因此很容易被鐵族元素捕獲 積聚成更大質量的元素 這在正常情況下是不可能形成的 元素從銀變成金 從鉛變成鈾 相較于氫變成氦所經歷的百萬年 超新星中最重元素的形成過程 只需幾秒 那爆炸後是什麼變成了黃金呢 持續擴大的超新星產生衝擊波 推動元素碎片,四散在星際介質中 引起氣體和塵埃的流動漩渦 最終壓縮成新恆星和行星 地球上的黃金可能就是這樣而來的 最初,地熱活動將黃金擠壓分散在地脈中 數十幾億年後,採礦技術才讓它重見光明 由於黃金的稀有性,採礦過程十分昂貴 事實上,人類至今開採的黃金 只能填滿三個奧運大小的游泳池 即使真能填滿,這些黃金的總質量極大 因為黃金的密度是水的20倍 這種夢寐以求的商品是否可以人工製造? 其實是可以的 通過粒子加速器,我們可以模擬複雜的核反應 這和在恆星中產生黃金一樣 但這些機器只能用逐顆原子來製成黃金 製造一克黃金就要花近5千9百萬年 大大超出了現有黃金的價值 所以這並非良策 但如果我們做個假設 地球上所有的黃金已開採殆盡 那還有哪些地方值得我們深究呢 海洋估計有2千萬噸溶解黃金 但因為其濃度太低,目前想製成黃金代價大 也許有一天,挖掘礦產資源的淘金熱 會出現在太陽系其他的星球上 誰又知道呢? 搞不好哪天會有個超新星出現 向地球展示其財富 希望這過程不會毀滅地球