More than six thousand light years from the surface of the earth, a rapidly spinning neutron star called the Black Widow pulsar
距离地球表面六千光年的地方，有一个快速旋转的中子星，它名叫黑寡妇脉冲星，
blasts its companion brown dwarf star with radiation as the two orbit each other every 9 hours.
它向互绕彼此旋转的棕矮星每隔9个小时发出电波。
Standing on our own planet, you might think you're just an observer of this violent ballet.
当我们身处于地球时，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, connected across trillions of kilometers by gravity.
而你正在努力挣脱，相距几十万亿公里的你们被引力所联系起来。
Gravity is the attractive force between two objects with mass -- any two objects with mass.
引力是两个有质量的物体之间所产生的吸引力--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, human being, smartphone, and atom are all constantly pulling on each other.
每一颗星星、黑洞、人类、智能手机和原子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 was written by Isaac Newton in 1687.
最初描述两个物体之间引力的公式是由艾萨克·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 is still a good approximation in most situations.
但是牛顿的万有引力定律，在多数情况下仍是有效的粗略衡量方法。
It goes like this: the gravitational force between two objects is equal to the mass of one times the mass of the other,
这个公式这样的：两个物体间的万有引力等于一个物体的质量乘以另一个物体的质量，
multiplied by a very small number called the gravitational constant, and divided by the distance between them, squared.
再乘以一个很小的数值叫做万有引力常数，再除以两个物体间距离的平方。
If you doubled the mass of one of the objects, the force between them would double, too.
若将其中一物体的质量变成两倍，那么它们之间的引力也会变成两倍。
If the distance between them doubled, the force would be one-fourth as strong.
如果它们间的距离变成两倍，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, a force you experience as your weight.
你和地球间的万有引力把你拉扯向它的中心，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 when you're standing at sea level.
假设这个力量有800牛顿，这是当你站在海平面上的情况。
If you traveled to the Dead Sea, the force would increase by a tiny fraction of a percent.
如果你去死海旅游，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 -- but again, by a minuscule amount.
如果你爬上了珠穆朗玛峰顶端，重力会减少微不足道的一点点。

Traveling higher would make a bigger dent in gravity's influence, but you won't escape it.
你爬得越高，重力影响会减弱一些，但它不可能消失。
Gravity is generated by variations in the curvature of spacetime -- the three dimensions of space plus time
引力是由时空扭曲所产生，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, 400 kilometers above the earth, with almost its original intensity.
地球的引力能延续到国际空间站，虽然空间站距地球400公里之外，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 of the gravitational force there that you do on the ground.
你还是会感受到与在地面上相比百分之九十的引力。
Astronauts just experience weightlessness because the space station is constantly falling towards earth.
宇航员之所以感受到失重，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, around 400,000 kilometers away,
当你去到月球表面，距离地球40万公里外，
Earth's gravitational pull would be less than 0.03 percent of what you feel on earth.
你所感受到的地球万有引力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, which is about one sixth as strong as the earth's.
你唯一能感觉到的引力来自于月球，而月球引力只有地球的六分之一。
Travel farther still and Earth's gravitational pull on you will continue to decrease, but never drop to zero.
当你离得更远时，地球对你的引力会持续减少，但不会降为0。
Even safely tethered to the Earth, we're subject to the faint tug of distant celestial bodies and nearby earthly ones.
虽然我们被安全地栓在地球上，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 a mutual force of a few piconewtons.
如果你在距离你的手机几米之外，你会感受到相互间几皮牛顿的引力。
That's about the same as the gravitational pull between you and the Andromeda Galaxy,
相当于你与仙女座星座之间的引力，
which is 2.5 million light years away but about a trillion times as massive as the sun.
仙女星座距离我们2.5百万光年，但它却比太阳庞大一万亿倍。
But when it comes to escaping gravity, there's a loophole.
其实，想要逃避引力是有方法的。
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 if you tunneled deep below the surface, assuming you could do so without being cooked or crushed?
若我们挖洞进入地球深处，地心引力会如何变化呢，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 -- which it isn't, but let's just say it were
如果你在地球的中心挖一个，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, only encountering the tiny pulls from other celestial bodies.
你会悬在空中，进入失重状态，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 -- but only by heading straight into it.
所以只有在这样的思维实验当中--钻到地心，才能逃离地球万有引力。