More than six thousand light years from the surface of the earth, a rapidly spinning neutron star called the Black Widow pulsar blasts its companion brown dwarf star with radiation as the two orbit each other every 9 hours. Standing on our own planet, you might think you’re just an observer of this violent ballet. But in fact, both stars are pulling you towards them. And you’re pulling back, connected across trillions of kilometers by gravity.
距離地球表面六千多萬光年, 有一急速旋轉的中子星 稱為黑寡婦脈衝星, 它的輻射使得它的伴星── 褐矮星產生爆炸, 兩顆星每 9 小時互繞一次。 在地球上, 可能你以為自己只是 這場暴力芭蕾的旁觀者, 但事實上,這兩顆星 正把你拉向它們, 同時你在往回拉, 通過萬有引力(重力) 連接數兆公里。 萬有引力是兩個有質量的 物體的相互吸引力量──
Gravity is the attractive force between two objects with mass— any two objects with mass. Which means that every object in the universe attracts every other object: every star, black hole, human being, smartphone, and atom are all constantly pulling on each other. So why don’t we feel pulled in billions of different directions? Two reasons: mass and distance.
任何兩個有質量的物體。 即是宇宙中任何兩個物體 都具有互相吸引的性質: 每一個星球、黑洞、 人類、手機、與原子 都不斷有互相吸引的力量。 為什麼我們沒有感受到 從四方八面而來的拉力呢? 兩個原因:質量與距離。
The original equation describing the gravitational force between two objects was written by Isaac Newton in 1687. Scientists’ understanding of gravity has evolved since then, but Newton’s Law of Universal Gravitation is still a good approximation in most situations. It goes like this: the gravitational force between two objects is equal to the mass of one times the mass of the other, multiplied by a very small number called the gravitational constant, and divided by the distance between them, squared. If you doubled the mass of one of the objects, the force between them would double, too. If the distance between them doubled, the force would be one-fourth as strong.
最早描述兩個物體引力的方程式 是出於 1687 的艾薩克·牛頓。 從那時起,科學家 對引力的了解更進化了, 但牛頓的萬有引力定律 在大部分情況下 仍然是相當好的粗略估算。 萬有引力是這樣的: 兩個物體間的引力 相等於一個物體的質量 乘以另外一個物體的質量, 再乘以一個小數目, 叫做引力常數, 然後除以兩個物體距離的平方。 如果你將其中一個物體的質量加倍, 它們中間的引力會變為雙倍, 如果將兩者距離增加增一倍, 引力強度就會等於之前的四分之一。
The gravitational force between you and the Earth pulls you towards its center, a force you experience as your weight. Let’s say this force is about 800 Newtons when you’re standing at sea level. If you traveled to the Dead Sea, the force would increase by a tiny fraction of a percent. And if you climbed to the top of Mount Everest, the force would decrease— but again, by a minuscule amount.
你與地球之間的引力 將你拉向地球的中心點, 就是你所體驗到的重量。 舉個例,假設你站在 海平面上時的引力是 800 牛頓 。 如果你到死海, 引力的增加很微小, 假如你爬到珠穆朗瑪峰山頂上, 引力將會減少, 減弱的幅度仍然很小。
Traveling higher would make a bigger dent in gravity’s influence, but you won’t escape it. Gravity is generated by variations in the curvature of spacetime— the three dimensions of space plus time— which bend around any object that has mass. Gravity from Earth reaches the International Space Station, 400 kilometers above the earth, with almost its original intensity. If the space station was stationary on top of a giant column, you’d still experience ninety percent of the gravitational force there that you do on the ground. Astronauts just experience weightlessness because the space station is constantly falling towards earth. Fortunately, it’s orbiting the planet fast enough that it never hits the ground.
走到高一些的地方 會削弱多一點引力, 但你不能逃避引力。 重量是由時空曲率變異所產生的── 空間的三個維度加上時間── 會因任何具有質量的物體而彎曲。 來自地球的重力到達距離地表 400 公里的國際太空站, 幾乎是原來的強度。 如果太空站固定在巨大的柱上, 仍可感受到地表引力的 90%。 太空人感受不到重力, 是因為太空站一直向地球方向墜落。 幸運的是,它繞地球飛行的速度 快到足以讓它永遠不會掉落地面。 月球的表面
By the time you made it to the surface of the moon, around 400,000 kilometers away, Earth’s gravitational pull would be less than 0.03 percent of what you feel on earth. The only gravity you’d be aware of would be the moon’s, which is about one sixth as strong as the earth’s. Travel farther still and Earth’s gravitational pull on you will continue to decrease, but never drop to zero.
離地球大概是 40 萬公里遠, 在那裡的地球引力 只剩下不到你在地球上 感受的 0.03%。 你只會察覺到來自月球的引力, 約是地球引力的六分之一。 離得更遠一些, 地球的引力會繼續減少, 但不會是零。
Even safely tethered to the Earth, we’re subject to the faint tug of distant celestial bodies and nearby earthly ones. The Sun exerts a force of about half a Newton on you. If you’re a few meters away from a smartphone, you'll experience a mutual force of a few piconewtons. That’s about the same as the gravitational pull between you and the Andromeda Galaxy, which is 2.5 million light years away but about a trillion times as massive as the sun.
即使牢牢地拴在地球上, 我們仍受到遙遠天體 和鄰近地球天體的微弱影響。 太陽對你施加大概 0.5 牛頓的力量。 若你距離手機幾米遠,你會感受到 彼此間有一兆分之一牛頓的拉力。 這樣就好似仙女座星系 與我們之間的引力, 該星系在 2.5 百萬光年遠處, 但其質量大約是太陽的一兆倍。
But when it comes to escaping gravity, there’s a loophole. If all the mass around us is pulling on us all the time, how would Earth’s gravity change if you tunneled deep below the surface, assuming you could do so without being cooked or crushed? If you hollowed out the center of a perfectly spherical Earth— which it isn’t, but let’s just say it were— you’d experience an identical pull from all sides. And you’d be suspended, weightless, only encountering the tiny pulls from other celestial bodies. So you could escape the Earth’s gravity in such a thought experiment— but only by heading straight into it.
談到擺脫重力時 有個漏洞。 如果所有環繞我們的質量, 整天地拉向我們, 地球的引力如何改變呢?, 如果你深入地下, 假設你不會被煮熟或壓碎? 如果你在地心 掏空了一個完美的球形── 這只是個理論上的假設── 你就會感受到來自四面八方 同樣大的拉力。 因此你就會懸浮及失重, 只會感受到其他天體的微少牽引。 你能在這樣的假想實驗中 避開地球的引力── 但也只能一頭鑽進去想想罷了。