How far would you have to go to escape gravity? - Rene Laufer

翻译人员: Yifan L 校对人员: Carol Wang

More than six thousand light years

距离地球表面

from the surface of the earth,

六千光年的地方，

a rapidly spinning neutron star

有一个快速旋转的中子星，

called the Black Widow pulsar

它名叫黑寡妇脉冲星，

blasts its companion brown dwarf star with radiation

它向互绕彼此旋转的棕矮星

as the two orbit each other every 9 hours.

每隔 9 个小时发出电波。

Standing on our own planet,

当我们身处于地球时，

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

你会觉得自己只是 星星狂放芭蕾舞的观测者，

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

其实，这两颗星都向你施加引力，

And you’re pulling back,

而你正在努力挣脱，

connected across trillions of kilometers

相距几十万亿公里的你们

by gravity.

被引力所联系起来。

Gravity is the attractive force between two objects with mass—

引力是两个有质量的 物体之间所产生的吸引力——

any two objects with mass.

任何两个有质量的物体间都存在。

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

即宇宙任一物体都吸引其它物体：

every star, black hole,

每一颗星星、黑洞、

human being, smartphone, and atom

人类、智能手机和原子

are all constantly pulling on each other.

都在持续不断地吸引其他物体。

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

为何我们觉察不到 自己被拉向几十亿个不同方向呢？

Two reasons: mass and distance.

有两个原因：质量和距离。

The original equation describing the gravitational force between two objects

最初描述两个物体之间引力的公式

was written by Isaac Newton in 1687.

是由艾萨克·牛顿在 1687 年所写。

Scientists’ understanding of gravity has evolved since then,

此后，科学家们对引力的了解 又深刻了许多。

but Newton’s Law of Universal Gravitation

但是牛顿的万有引力定律，

is still a good approximation in most situations.

在多数情况下 仍是有效的粗略衡量方法。

It goes like this:

这个公式这样的：

the gravitational force between two objects

两个物体间的万有引力

is equal to the mass of one

等于一个物体的质量

times the mass of the other,

乘以另一个物体的质量，

multiplied by a very small number

再乘以一个很小的数值

called the gravitational constant,

叫做万有引力常数，

and divided by the distance between them, squared.

再除以两个物体间距离的平方。

If you doubled the mass of one of the objects,

若将其中一物体的质量变成两倍，

the force between them would double, too.

那么它们之间的引力也会变成两倍。

If the distance between them doubled,

如果它们间的距离变成两倍，

the force would be one-fourth as strong.

它们间的引力就只有 原来的四分之一强度了。

The gravitational force between you and the Earth pulls you towards its center,

你和地球间的万有引力 把你拉扯向它的中心，

a force you experience as your weight.

你所感知到的这种力量 就是你的体重。

Let’s say this force is about 800 Newtons

假设这个力量有 800 牛顿，

when you’re standing at sea level.

这是当你站在海平面上的情况。

If you traveled to the Dead Sea,

如果你去死海旅游，

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

重力会增加一点点百分比。

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

如果你爬上了珠穆朗玛峰顶端，

but again, by a minuscule amount.

重力会减少微不足道的一点点。

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

你爬得越高，重力影响会减弱一些，

but you won’t escape it.

但它不可能消失。

Gravity is generated by variations in the curvature of spacetime—

引力是由时空扭曲所产生，

the three dimensions of space plus time—

时空，即是 3 维空间加上时间——

which bend around any object that has mass.

它在拥有质量的物体周围弯曲。

Gravity from Earth reaches the International Space Station,

地球的引力能延续到国际空间站，

400 kilometers above the earth,

虽然空间站距地球 400 公里之外，

with almost its original intensity.

但地球对其引力几乎是原本的强度。

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

如果空间站被固定在一根 巨大的柱子上，

you’d still experience ninety percent

你还是会感受到 与在地面上相比

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

百分之九十的引力。

Astronauts just experience weightlessness

宇航员之所以感受到失重，

because the space station is constantly falling towards earth.

是因为空间站 被地球的引力持续拉扯。

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

所幸它一直在快速旋转， 所以不至于撞向地球。

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

当你去到月球表面，

around 400,000 kilometers away,

距离地球 40 万公里外，

Earth’s gravitational pull would be

你所感受到的地球万有引力

less than 0.03 percent of what you feel on earth.

已经不足原来的百分之 0.03 了。

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

你唯一能感觉到的引力来自于月球，

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

而月球引力只有地球的六分之一。

Travel farther still

当你离得更远时，

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

地球对你的引力会持续减少，

but never drop to zero.

但不会降为 0。

Even safely tethered to the Earth,

虽然我们被安全地栓在地球上，

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

但还是会被遥远天体 和我们周围的物体所吸引，

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

太阳对你产生 0.5 牛顿的引力。

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

如果你在距离你的手机几米之外，

a mutual force of a few piconewtons.

你会感受到相互间几皮牛顿的引力，

That’s about the same as the gravitational pull

相当于你与仙女座星座之间的引力，

between you and the Andromeda Galaxy,

仙女星座

which is 2.5 million light years away

距离我们 2.5 百万光年，

but about a trillion times as massive as the sun.

但它却比太阳庞大一万亿倍。

But when it comes to escaping gravity,

其实，想要逃避引力

there’s a loophole.

是有方法的。

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

如果周围所有的质量都在拉我们，

how would Earth’s gravity change

若我们挖洞进入地球深处，

if you tunneled deep below the surface,

地心引力会如何变化呢，

assuming you could do so without being cooked or crushed?

假设你在地下不被烤焦或被压垮的话？

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

如果你在地球的中心挖一个，

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

假定地球是一个完美球形， 虽然它不是——

you’d experience an identical pull from all sides.

你会感受到来自各个方同等的引力，

And you’d be suspended, weightless,

你会悬在空中，进入失重状态，

only encountering the tiny pulls from other celestial bodies.

其他天体对你产生的作用 则几乎可以忽略不计。

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

所以只有在这样的思维实验当中——

but only by heading straight into it.

钻到地心，才能逃离地球万有引力。