Consider the spot where you’re sitting. Travel backwards in time and it might’ve been submerged at the bottom of a shallow sea, buried under miles of rock, or floating through a molten, infernal landscape. But go back far enough— about 4.6 billion years, and you’d be in the middle of an enormous cloud of dust and gas orbiting a newborn star. This is the setting for some of the biggest, smallest mysteries of physics: the mysteries of cosmic dust bunnies.
想像你現在所坐的這個點。 時間旅行回到過去, 這個點有可能是位在 一片淺海的底部, 被數英里的岩石給掩埋, 或者漂浮在熱熔的地獄環境。 但,如果回到夠久的過去—— 大約四十六億年前, 你會身處在塵埃和氣體 構成的巨型雲霧中, 繞著新生的星體運轉。 這是最大、最小物理學之謎 所發生的環境,這個謎就是: 宇宙塵兔之謎。
Seemingly empty regions of space between stars actually contain clouds of gas and dust, usually blown there by supernovas. When a dense cloud reaches a certain threshold called the Jeans mass, it collapses in on itself. The shrinking cloud rotates faster and faster, and heats up, eventually becoming hot enough to burn hydrogen in its core. At this point a star is born. As fusion begins in the new star, it sends out jets of gas that blow off the top and bottom of the cloud, leaving behind an orbiting ring of gas and dust called a protoplanetary disk. This is a surprisingly windy place; eddies of gas carry particles apart, and send them smashing into each other. The dust consists of tiny metal fragments, bits of rock, and, further out, ices.
在星體之間,看似 空無一物的宇宙區域, 其實有著氣體和塵埃所形成的雲, 通常是被超級新星炸到那裡去的。 當雲的密度高到達 「金斯質量」這個門檻值時, 它自己就會崩塌。 縮小的雲會越轉越快速, 溫度跟著上升, 最終熱到足以燃燒 它核心區域的氫。 在這個時點,星體就誕生了。 當新星中的融合開始, 它就會噴射出氣體, 將雲的頂部和底部吹掉, 留下繞行的環狀氣體和塵埃, 稱為「原行星盤」。 這個地方風大得驚人; 氣體旋渦會把粒子帶開, 讓它們互相碰撞。 塵埃包含了小型的 金屬碎片、小堆岩石, 在更外圍還有冰。
We’ve observed thousands of these disks in the sky, at various stages of development as dust clumps together into larger and larger masses.
我們已經觀察到天空中 有數千個這種星盤, 處在不同的發展階段, 塵埃結在一起,質量越來越大。
Dust grains 100 times smaller than the width of a human hair stick to each other through what’s called the van der Waals force. That’s where a cloud of electrons shifts to one side of a molecule, creating a negative charge on one end, and a positive charge on the other. Opposites attract, but van der Waals can only hold tiny things together.
比人類頭髮的寬度還要小一百倍的 塵埃粒子會彼此黏在一起, 讓它們相黏的力量 叫做「凡得瓦爾力」。 此時,電子雲就會 轉移到分子的一邊, 在一端創造出負電荷, 另一端創造出正電荷。 異性相吸,但凡得瓦爾力只能夠 將微小的東西結合在一起。
And there’s a problem: once dust clusters grow to a certain size, the windy atmosphere of a disk should constantly break them up as they crash into each other. The question of how they continue to grow is the first mystery of dust bunnies.
有一個問題:一旦塵埃群 成長到了某個大小之後, 星盤大氣中的風很大, 經常使這些塵埃群彼此相撞而散開。 它們如何持續成長, 是塵兔的第一個謎。
One theory looks to electrostatic charge to answer this. Energetic gamma rays, x-rays, and UV photons knock electrons off of gas atoms within the disk, creating positive ions and negative electrons. Electrons run into and stick to dust, making it negatively charged. Now, when the wind pushes clusters together, like repels like and slows them down as they collide. With gentle collisions they won’t fragment, but if the repulsion is too strong, they’ll never grow. One theory suggests that high energy particles can knock more electrons off of some dust clumps, leaving them positively charged. Opposites again attract, and clusters grow rapidly.
有個從靜電電荷來解答的理論。 高能加馬射線、X 光、UV 光子 在星盤中將電子 從氣體原子中打掉, 創造出正離子和負電子, 電子碰到塵埃就會黏住塵埃, 讓它帶負電。 當風把塵埃群推到一起時, 同性相斥,且當它們 相撞時會讓它們慢下來。 輕輕碰撞不會讓它們破碎, 但如果相斥太強, 它們就永遠不會成長。 有一種理論指出,高能粒子 能把更多電子從塵埃團中給除去, 讓塵埃團帶正電。 異性相吸,塵埃群就會快速成長。
But before long we reach another set of mysteries. We know from evidence found in meteorites that these fluffy dust bunnies eventually get heated, melted and then cooled into solid pellets called chondrules. And we have no idea how or why that happens. Furthermore, once those pellets do form, how do they stick together? The electrostatic forces from before are too weak, and small rocks can’t be held together by gravity either. Gravity increases proportionally to the mass of the objects involved. That’s why you could effortlessly escape an asteroid the size of a small mountain using just the force generated by your legs. So if not gravity, then what? Perhaps it’s dust. A fluffy dust rim collected around the outside of the pellets could act like Velcro. There’s evidence for this in meteors, where we find many chondrules surrounded by a thin rim of very fine material– possibly condensed dust.
但,不久,我們又 碰到了另一組謎。 從隕石找到的證據顯示, 這些蓬鬆的塵兔最終 會被加溫、熔化, 接著冷卻,成為固態顆粒, 叫做隕石球粒。 我們不知道這是怎麼發生的, 也不知道為什麼發生。 此外,一旦那些顆粒真的形成了, 它們要如何黏在一起? 之前的靜電力太弱了, 小岩石無法靠引力就結合在一起。 物體間的引力 和物體的質量成正比。 那就是為什麼你可以毫不費力, 光靠雙腿產生的力量, 就脫離小山大小的小行星。 如果不是引力,那是什麼? 也許是塵埃。 在顆粒外部集結的蓬鬆塵埃外緣 可能會有魔鬼沾的功能。 在隕石中有找到相關證據, 我們發現許多隕石球粒被薄薄一層 非常好的材質給包住—— 有可能是高密度的塵埃。
Eventually the chondrule pellets get cemented together inside larger rocks, which at about 1 kilometer across are finally large enough to hold themselves together through gravity. They continue to collide and grow into larger and larger bodies, including the planets we know today.
最終,隕石球粒的顆粒會在 更大的岩石中被黏結起來, 這些岩石大約有一公里寬, 終於,這些顆粒因此就夠大, 足以用引力持續結合在一起了。 它們繼續碰撞, 成長成更大的物體, 包括我們現今所知的行星。
Ultimately, the seeds of everything familiar– the size of our planet, its position within the solar system, and its elemental composition– were determined by an uncountably large series of random collisions. Change the dust cloud just a bit, and perhaps the conditions wouldn’t have been right for the formation of life on our planet.
最終,我們熟悉的一切—— 我們行星的大小, 在太陽系中的位置, 以及組成的元素—— 其根源是由一大連串 隨機撞擊所決定的。 若塵雲有稍微改變一點點, 也許就不會有對的條件 讓我們的星球上出現生命了。