a rapidly spinning neutron star

有一急速旋轉的中子星

called the Black Widow pulsar, blasts its companion brown dwarf star with radiation

它的輻射使得它的伴星── 褐矮星產生爆炸，

as the two orbit each other every 9 hours.

兩顆星每 9 小時互繞一次。

Standing on our own planet,

在地球上，

you might think you're just an observer of this violent ballet.

可能你以為自己只是這場暴力芭蕾的旁觀者，

But in fact, both stars are pulling you towards them.

但事實上，這兩顆星正把你拉向它們，

And you're pulling back,

同時你在往回拉，

connected across trillions of kilometers

連接數兆公里

by gravity.

通過萬有引力（重力） 。

Gravity is the attractive force between two objects with mass—

萬有引力是兩個有質量的 物體的相互吸引力量──

any two objects with mass.

任何兩個有質量的物體。

Which means that every object in the universe attracts every other object:

即是宇宙中任何兩個物體 都具有互相吸引的性質：

every star, black hole,

每一個星球、黑洞、

human being, smartphone, and atom

人類、手機、與原子

are all constantly pulling on each other.

都不斷有互相吸引的力量。

So why don't we feel pulled in billions of different directions?

為什麼我們沒有感受到 從四方八面而來的拉力呢？

Two reasons: mass and distance.

兩個原因：質量與距離。

The original equation describing the gravitational force between two objects

最早描述兩個物體引力的方程式

was written by Isaac Newton in 1687.

是出於 1687 的艾薩克·牛頓。

Scientists' understanding of gravity has evolved since then,

從那時起，科學家 對引力的了解更進化了，

but Newton's Law of Universal Gravitation

但牛頓的萬有引力定律

is still a good approximation in most situations.

在大部分情況下 仍然是相當好的粗略估算。

It goes like this:

萬有引力是這樣的：

the gravitational force between two objects

兩個物體間的引力

is equal to the mass of one

相等於一個物體的質量

times the mass of the other,

乘以另外一個物體的質量，

multiplied by a very small number

再乘以一個小數目，

called the gravitational constant,

叫做引力常數，

and divided by the distance between them, squared.

然後除以兩個物體距離的平方。

If you doubled the mass of one of the objects,

如果你將其中一個物體的質量加倍，

the force between them would double, too.

它們中間的引力會變為雙倍，

If the distance between them doubled,

如果將兩者距離增加增一倍，

the force would be one-fourth as strong.

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,

你與地球之間的引力 將你拉向地球的中心點，

a force you experience as your weight.

就是你所體驗到的重量。

Let's say this force is about 800 Newtons

舉個例，假設你站在海平面上時的引力

when you're standing at sea level.

是 800 牛頓 。

If you traveled to the Dead Sea,

如果你到死海，

the force would increase by a tiny fraction of a percent.

引力的增加很微小，

And if you climbed to the top of Mount Everest, the force would decrease—

假如你爬到珠穆朗瑪峰山頂上，引力將會減少，

but again, by a minuscule amount.

減弱的幅度仍然很小。

Traveling higher would make a bigger dent in gravity's influence,

走到高一些的地方會削弱多一點引力，

but you won't escape it.

但你不能逃避引力。

Gravity is generated by variations in the curvature of spacetime—

重量是由時空曲率變異所產生的──

the three dimensions of space plus time—

空間的三個維度加上時間──

which bend around any object that has mass.

會因任何具有質量的物體而彎曲。

Gravity from Earth reaches the International Space Station,

來自地球的重力到達國際太空站

400 kilometers above the earth,

距離地表 400 公里

with almost its original intensity.

幾乎是原來的強度。

If the space station was stationary on top of a giant column,

如果太空站固定在巨大的柱上，

you'd still experience ninety percent

仍可感受到 90%

of the gravitational force there that you do on the ground.

來自地球表面的引力

Astronauts just experience weightlessness

太空人感受不到重力，

because the space station is constantly falling towards earth.

是因為太空站一直向地球方向墜落。

Fortunately, it's orbiting the planet fast enough that it never hits the ground.

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。

By the time you made it to the surface of the moon,

等你到了月球的表面

around 400,000 kilometers away,

離地球大概是 40 萬公里遠，

Earth's gravitational pull would be

在那裡的地球引力

less than 0.03 percent of what you feel on earth.

只剩下不到你在地球上感受的 0.03%。

The only gravity you'd be aware of would be the moon's,

你只會察覺到來自月球的引力，

which is about one sixth as strong as the earth's.

約是地球引力的六分之一。

Travel farther still

離得更遠一些，

and Earth's gravitational pull on you will continue to decrease,

地球的引力會繼續減少，

but never drop to zero.

但不會是零。

Even safely tethered to the Earth,

即使牢牢地拴在地球上，

we're subject to the faint tug of distant celestial bodies and nearby earthly ones.

我們仍受到遙遠天體和鄰近地球天體的微弱影響。

The Sun exerts a force of about half a Newton on you.

太陽對你施加大概 0.5 牛頓的力量。

If you're a few meters away from a smartphone, you'll experience

若你距離手機幾米遠，你會感受到

a mutual force of a few piconewtons.

彼此間有一兆分之一牛頓的拉力。

That's about the same as the gravitational pull

這樣就好似仙女座星系

between you and the Andromeda Galaxy,

與我們之間的引力，

which is 2.5 million light years away

該星系在 2.5 百萬光年遠處，

but about a trillion times as massive as the sun.

但其質量大約是太陽的一兆倍。

But when it comes to escaping gravity,

談到擺脫重力時

there's a loophole.

有個漏洞。

If all the mass around us is pulling on us all the time,

如果所有環繞我們的質量， 整天地拉向我們，

how would Earth's gravity change

地球的引力如何改變呢？

if you tunneled deep below the surface,

如果你深入地下，

assuming you could do so without being cooked or crushed?

假設你不會被煮熟或壓碎？

If you hollowed out the center of a perfectly spherical Earth—

如果你在地心 掏空了一個完美的球形──

which it isn't, but let's just say it were—

這只是個理論上的假設──

you'd experience an identical pull from all sides.

你就會感受到來自四面八方 同樣大的拉力。

And you'd be suspended, weightless,

因此你就會懸浮及失重，

only encountering the tiny pulls from other celestial bodies.

只會感受到其他天體的微少牽引。

So you could escape the Earth's gravity in such a thought experiment—

你能在這樣的假想實驗中 避開地球的引力──

but only by heading straight into it.

但也只能一頭鑽進去想想罷了。