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We're all familiar with gravity as 'the reason objects fall', but there's much, much more to gravity than that.
我们都知道引力是让物体下落的原因，但是引力的意义远不仅如此。
Gravity is a long range attractive force between all objects with mass:
引力是所有有质量的物体之间的一种互相吸引力：
it's what keeps us from falling off the earth, it's what keeps the earth in orbit around the sun,
它使我们不至于坠出地球；让地球按绕日轨道运行，
and it's what caused the sun itself to form four and a half billion years ago!
并且它在45亿年前促使了太阳的形成！
It's amazing to think that every massive object attracts every other in the universe –
想到宇宙中每一个巨大的物体都相互吸引，真是不可思议—
that means that your dog, the earth, and a black hole in the Andromeda galaxy 2.5 million light years away
这意味着你的狗狗、地球以及距离地球250万光年的仙女座中的黑洞
are all gravitationally attracted to you, and you to them.
都被你所吸引，你也被它们吸引着。
In the 17th century, Isaac Newton discovered that the strength of the gravitational force decreases by the square of the distance between two objects –
在17世纪，艾萨克·牛顿发现，引力的强度随着两个物体之间距离的平方而减小—
so if you're twice as far away, gravity is only one fourth as strong!
所以，如果距离增加一倍，那么引力就减弱到四分之一！
He also discovered that the strength of gravity is proportional to the mass of the objects in question:
他还发现引力的强度与物体的质量成正比：
the more massive an object, the stronger the gravitational force.
一个物体质量越大，产生的引力也就越强
That's why we can all feel the Earth pulling on us, but don't really notice the pull of the moon – it's smaller, and farther away!
因为我们都能感觉到地球对自己的吸引力，但是很难感受到月球的吸引力—因为月球更小且更远！
(the moon's gravity is strong enough to cause the tides, though!)
（虽然月球的引力足够影响潮汐！）
And when I said earlier that gravity is an attraction between objects with mass, I lied.
我之前说，引力是具有质量的物体之间的吸引力，我撒谎了。
I meant objects with Energy. Because in addition to massive objects, gravity also attracts light and other massless (but energetic) particles,
我指的是具有能量的物体。因为除了质量物体，引力也吸引光和其他无质量（但有能量）的粒子，
so that a photon of light can be bent slightly passing the sun,
所以光子可以在经过太阳时轻微弯曲，
or trapped completely by a black hole.
或者完全被黑洞困住。
Now do you understand the Gravity of the situation?
现在你了解重力的分量了吗？
