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Our sun and the earth, and all the planets and moons and dwarf planets and asteroids and comets,
我们的太阳和地球以及所有行星、卫星、矮行星、小行星和彗星....
the Solar System, in short formed about 4.6 billion years ago from a nebulous cloud of swirling gas and dust
简称太阳系，形成于约46亿年前的一个模糊星云，
which coalesced thanks to the irresistibly attractive force of gravity.
它们由于不可抗拒的引力而结合在一起。
However, this nebula started off more or less as a big shapeless blob.
然而，这个星云开始时或多或少是一个大而不成形的团状物。
So how did our solar system end up with all the planets and their moons orbiting in a flat disk?
那么，我们的太阳系怎么演变成今天这样，所有的行星及其卫星都在一个盘子样扁平的轨道上的？
I mean, we've all seen the planetary model of the atom, which is definitely wrong when applied to atoms.
我的意思是，我们都见过原子的行星模型，当应用到原子上时，这个模型显然是错的。
But it also kind of suggests that planets might revolve around the sun every which way.
但这也表明行星也许曾以所有可能的方向围绕太阳旋转。
So is our solar system somehow special in its flatness? Or is the planetary model of the atom doubly wrong?
所以是不是因为我们太阳系的扁平形状在宇宙中比较特别？或者原子行星模型又错了？
Well, our solar system definitely isn't alone.
好吧，我们的太阳系绝不孤独。
Many exoplanets' star systems are flat, a lot of galaxies are flat, black hole accretion disks are flat, Saturn's rings are flat, etc.
很多系外行星的恒星系统是平的，许多星系是平的，黑洞吸积盘是平的，土星环是平的，等等。
So why, when there's all of 3D space to fill, does the universe have this preference for flatness?
所以，为什么物质可以向所有3D的方向延伸时，宇宙却偏好让物质在一个二维平面上旋转？
The answer has to do with two things: collisions and the fact that we live in three dimensions. Bear with me.
答案和两件事有关：碰撞以及我们生活在三维世界里的事实。请先容我讲完。
Anytime a bunch of objects held together by gravity are zooming and circling around, their individual paths are nearly impossible to predict.
任何时候，一堆物体被引力拉在一起的物质缩放和旋转时，它们各自的路径几乎是不可能预测的。
And yet, collected together, they have a single total amount that they spin about their center of mass.
但聚集在一起时，它们有一个单一的总的不变量，他们总是围绕着它们的质心旋转
It may be hard to figure out exactly what direction that rotation is in,
虽然很难搞清楚具体的转动方向，
but the mathematics implies there must be some plane in which the cloud, taken as a whole, spins.
但从数学上说，总有一些是平面的，其中星云，作为一个整体在旋转。
Now, in two dimensions, a cloud of particles rotating in a plane is flat by definition, it's in two dimensions.
现在，在二维空间中，一个在平面上旋转的粒子云是平的，根据上面的定义就很容易看出。
But in three dimensions, even though the rotation of the cloud is given by one plane, particles can whiz around far up and down from that plane.
但在三个维空间中，尽管星云的旋转是在一个平面上，但粒子可以在离那个平面很远的地方上下移动。
As the particles bump into each other, all the up and down motion tends to cancel out. It's energy lost in crashing and clumping.
当粒子相互碰撞时，所有向上和向下的运动都趋于抵消。它的能量在碰撞和结块过程中丢失。
Yet, the whole mass must continue spinning inexorably, because in our universe, the total amount of spinning in any isolated system always stays the same.
然而，整体的质量还是必须继续不停的旋转，因为在我们的宇宙中，旋转的总量对于一个孤立的系统始终保持不变。
So over time, through collisions and crashes, the cloud loses its loft and flattens
所以，随着时间的推移，通过碰撞和崩解，星云失去了它垂直于旋转平面上的粒子高度
into a spinning, roughly 2 dimensional disk shape, like a solar system or a spiral galaxy.
变成扁平状（大概是二维的圆盘）旋转，就像太阳系或螺旋星系。
However, in 4 spacial dimensions, the math works out such that there can be two separate and complementary planes of rotation,
但在4维空间中，数学推导发现可以有两个旋转分离和互补的平面，
which is both really really hard for our 3D-thinking brains to picture
我们三维的大脑真的很难想象这样的画面
and also means there's no up and down direction in which particles lose energy by collisions.
也意味着没有上和下的区分，在这些方向上粒子不会因为碰撞而相互抵消动量。
So a cloud of particles can continue being just that... a cloud.
所以一团粒子可以继续做一朵云。
And thus, only in three dimensions can a nebula or infant galaxy start out not flat and end up flat which is definitely a good thing,
因此，只有在三维空间星云和幼年星系在开始时并不平坦，并最终却走向扁平化，这绝对是一件好事，
because we need all that matter to clump together in order for stars and planets to form,
因为我们需要所有的物质聚集在一起使恒星和行星得以形成
and for us, even those of us who think atoms look like this, to exist.
使人类，甚至那些认为原子长这样的人，得以存在。
