So what is gravity? I bet most of you think it's: 'What goes up, must come down!' Is that right?
重力是什么？我想你们大多会想起：“向上移动的东西，一定会跌下来！”对吗？
Well, sorta, but not really. Technically, the law of gravity is an equation.
嗯，差不多吧，但不完全如此。从学术的角度来说，重力是一条公式。
It is: F=GxM1xM2/R^2, where G is the universal gravitational constant,
F等于G乘M1乘M2除以R的平方，
M1 and M2 are the masses of the two objects, and R is the distance between them squared.
而G是重力常数，M1和M2是两个物体的质量，R是它俩之间的距离，被平方了。
That was easy, right? Probably not. What does this actually mean?
看起来很容易，对吗？恐怕不是。这个公式到底是什么意思呢？
Well it means that - well, everything is attracted to everything else.
它的意思是：任何物体彼此间都会互相吸引。
What I mean by that is if you have two objects, any two objects, they are attracted to each other. OK.
我是说，如果你有两个物体，随便两个东西，它们都会互相吸引，就这样。
Let's try and wrap our minds around this. What happens when you drop a rock off a cliff?
我们来想想看。你在悬崖边扔下一块石头，会出现什么情况？
It falls to the earth. Right? Well, yes, but something else happens.
石头会掉在地上，对吗？没错，但还有其他事情发生。
You see, the law of gravity says that both objects, the rock and the earth, are attracted to each other.
很明显，重力定律说两个物体，也就是石头和地球，会互相吸引。
This means that the rock falls towards the earth, and the earth falls towards the rock.
意思是石头会落向地球，而地球也会“落”向石头。
Wait a second - you mean to tell me that if - the earth falls up to meet the rock?
等一等，你意思是，地球会迎着石头向上“落”吗？
Yes, that's exactly what I'm saying.
是的，我就是这个意思。
And the reason you don't see the earth fall up to meet the rock
你看不到地球迎着石头往上“落”的原因，
is because the objects move towards each other proportionate to their respective masses.
是因为两个物体会按它们质量的比例向对方移动。
The earth is much much much more massive than the rock, so it moves a very very small distance,
地球的质量比石头大得多，所以它只移动了一个非常小的距离，
and the rock is much less massive, so it moves farther with respect to the earth.
而石头的质量小的多，因此它相对于地球而言移动得远一点。
Maybe a better way to understand gravity is to take two teenagers in spacesuits,
可能更容易明白重力的方法，是叫两个年轻人，穿上宇航服，
and place them far out in space - away from all the planets and the stars.
把他们放在外层空间--远离所有行星和恒星。
It turns out they will be attracted to each other.
我们会发现他俩会互相吸引。
I'm not talking about that kind of attraction.
我说的不是这种（感情上的）吸引力。

See, they have mass, and since they have mass, they will move towards each other. They are attracted to each other.
我是说，他们有质量，而正因为如此，他们会移向对方。他们会被对方吸引。
Maybe one more thing might help. Have you ever played with two magnets?
或许另一个例子更容易理解。你试过同时玩两块磁铁吗？
You know, the magnets with the north and the south poles?
就是有南极和北极的磁铁。
When you take the magnets and put them closer to each other, they move together. They are attracted to each other.
当你让两块磁铁彼此靠近，它们会对接起来，会互相吸引。
And the closer they are, the stronger the attraction.
它们的距离越近，吸引力越大。
Think of the mass of the object like the strength of a magnet
试着把物体的质量想象为磁铁的强度，
and the distance between the objects like the distance between the two magnets.
而物体间的距离就好比两块磁铁之间的距离。
Now understand, I'm not saying that gravity and magnetism are the same, they just behave in a similar way.
不要理解错了，我不是说重力和磁力是一样的东西，只不过它们有类似的行为。
Let's think of one other thing - astronauts.
让我们想想另一样东西--航天员。
You know, astronauts, they weigh less on the moon than on the earth. Why is that?
航天员在月球上的体重比地球上的轻。为什么呢？
Well you see, the moon is less massive than the earth.
我们知道，月球的质量比地球的少得多。
Therefore it has a smaller gravitational pull on the astronaut.
因此它对航天员的引力就比较小。
It's like the moon is a weaker magnet. They aren't as attracted to each other.
就好像月球是一块较弱的磁铁。它们之间的吸引力没那么大。
Distance also plays a role. Think back to playing with a magnet.
距离也很重要。回想一下玩磁铁的时候。
The pull of the magnets towards each other are stronger when they are closer together. The same is true of gravity.
磁铁间的吸引力在它们彼此接近的时候比较大。同样的情况也适用于重力。
For example, the sun is the most massive object near the earth.
例如，太阳是地球附近质量最大的物体。
It dictates most of the gravitational forces in our solar system. It is very very massive.
它操控了我们太阳系内的大部分重力。它的质量非常庞大。
But it is relatively far away, so even though the sun is a much stronger magnet, so to speak, it is a long ways away.
但由于它相对地球较远，所以即便说太阳是一块强得多的磁铁，但它离我们太远了。
Therefore the attraction isn't as strong. So let's look back at that law of gravity.
因此它的吸引力并不那么强。再复习一下重力定律。
The equation: F=GxM1xM2/R^2. You see the force of gravity is equal to a number.
公式是：F等于G乘M1乘M2，除以R的平方。可以看到重力对应一个数值。
That's that universal gravitational constant G times the mass of object one, times the mass of object two.
那就是重力常数G，乘第一个物体的质量，再乘第二个物体的质量。
Think of M1 being the mass of the sun and M2 being the mass of the earth.
假设M1是太阳的质量，而M2是地球的质量。
And then we divide by the distance between them squared.
然后我们除以它们之间距离的平方。
This determines the force of attraction between the sun and earth.
这样就能计算出太阳和地球之间的重力。
You could just as easily plug in your mass and the earth's mass and the distance between you and the center of the earth,
你也可以代入你自己的质量和地球的质量，以及你和地心之间的距离，
and find out how much you are attracted to the earth, and the earth attracted to you.
从而得出你受到来自地球的引力，和地球被你吸引的力。
So, what's gravity? Everything is attracted to everything else. Everything.
那么重力究竟是什么？任何东西之间都会被互相吸引。任何东西。
Oh, one last thing, just to make you wonder. What causes gravity?
噢，还有一件事，你可能会很好奇。重力是怎么形成的？
Why are two objects with mass attracted to each other?
为什么两个有质量的物体会彼此吸引？
Well, the answer is - We don't know.
答案是--我们不知道。
The cause of gravity remains a mystery to scientists.
重力的成因对科学家来说仍是个谜。
We don't really know conclusively what causes gravity.
关于重力的成因，我们始终没有得出一个确凿的结论。
It is one of the great mysteries of science.
这是科学界最大的未解之谜之一。