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It's kind of weird thinking about the universe having a shape. It's the universe, right?
宇宙也有形状？天方夜谭吧，毕竟，我们讨论的对象是宇宙，不是吗？
It's universe shaped. But it does have one. We just aren't sure what that shape is.
是宇宙哎，怎么会有形状呢？但宇宙确实有形状，我们只是还不确定宇宙的形状是怎样的。
And some cosmologists have proposed that of all things, it might be shaped kind of like a donut.
一些宇宙学家认为，宇宙的形状可能像个甜甜圈。
When we're talking about the shape of the universe, we're really talking about its topology.
我们谈论宇宙形状的时候，并不是真地在谈论宇宙的拓扑学。
The topology of an object has nothing to do with how many sides it has, or where parts of it are relative to one another.
某个物体的拓扑结构跟它有多少边没有关联，跟它与另一个物体之间的关系也没关联。
It only depends on the number of holes: a sphere has none, a donut has one, and so on.
只取决于孔洞的数量，比如球体没有孔洞，甜甜圈有一个洞，诸如此类。
A sphere and a donut are both round, but they have different topologies
球体和甜甜圈都是圆的，但他们的拓扑结构不同，
because there's no way to take a sphere and stretch or smash it to make it look like a donut without, shall we say, tearing it a new one.
因为无论怎样拉伸碰撞，球体也是无法变成甜甜圈的，除非撕破重组。
On the other hand, a lot of objects we think of as clearly being different shapes have exactly the same topology.
另一方面，有很多我们认为形状迥然不同的物体却有着一模一样的拓扑结构。
A coffee mug has one hole in the handle, so topologically speaking, there's no difference between a mug and a donut.
咖啡杯的手柄上有一个洞，所以从拓扑结构的角度讲，咖啡杯和甜甜圈是没有差别的。
There are a lot of proposed topologies that the universe could possibly have, from plain shapes with no holes to some that are much more complex.
宇宙可能有很多种拓扑结构，可以是无洞的简单结构，也可以是更为复杂的结构。
But one of the most common ideas is what's called a 3-torus.
但最常见的一种想法是宇宙是圆环面的拓扑结构。
It's impossible for our puny human minds to visualize what that looks like from the outside, because you'd need a fourth spatial dimension.
人类的大脑是无法想象出那种结构的样子的，因为需要四维能力。
A line is one dimensional. A flat plane: two. And with the third dimension you get depth.
一条线可以视为一维，平面就是二维，三维就很有深度了。
Usually, when we talk about the fourth dimension, we mean time. But in this case, it's a fourth spatial dimension, the next level up from 3D.
通常情况下，我们所说的四维是指时间，而我们现在所说的宇宙拓扑结构就是四维的，在三维的基础上再加一维。
And as beings who've lived our whole lives in 3D space, we can't really imagine what 4D would look like.
而一直生活在三维空间中的人类是无法想象四维的样子的。
That doesn't mean you can't model a 3-torus, though! You just have to step everything down a dimension.
但这并不意味着我们无法对圆环面建模，所以我们得把所有东西都降一个维度。
You can take our universe, with its 3 spatial dimensions, and model it as a two-dimensional flat plane,
我们可以将宇宙视为三维的结构，给宇宙建模成二维的平面，
think of it like a piece of paper, except pretend it stretches to infinity along both axes. If you take that piece of paper and wrap it into a cylinder,
比如把它想象成一张纸，只不过，纸是无法无线延伸的，如果将这张纸卷成圆柱形的话，
it stays infinite in one direction but becomes finite in the other, because eventually you loop around the cylinder.
那么它的一个方向是无限延伸的，另一个方向就是有限的了，因为有限的平面是一个回环。
If you then wrap the ends of the cylinder together, making it connect along both axes, you have yourself a 3-dimensional version of a 3-torus.
如果将圆柱形的首尾相接，沿着两轴连接，就可以实现三维的圆环面了。
There you go. Donut universe. Topology is different from geometry.
以甜甜圈为模型的宇宙就做成了，拓扑结构跟几何结构不是一码事。
It doesn't care if or how space is curved, or about the distance or angles between different objects.
拓扑结构与宇宙是否弯曲以及弯曲方式无关，与不同物体间的距离和角度也无关。
Geometry does; it knows the difference between our mug and donut, and we're actually pretty sure we know what the geometry of the universe is.
但几何结构却与这几方面有关，在几何学里，咖啡杯的洞和甜甜圈之间是有区别的，而对于宇宙的几何结构，我们是非常有把握的。
Measurements have shown it's almost definitely flat, as opposed to positively or negatively curved.
测量数据显示，宇宙几乎就是个平面，而不是弯曲的。
Which, again, gets a little brain-break-y, because flat is not normally an adjective we use to describe things in 3D.
而这再一次冲击了我们的世界观，因为平面这个词，我们一般会用来描述三维的事物。
But cosmologists talk about 3D space as being flat or curved
但在宇宙学家的字典里，平面或者曲面都可以用来形容三维空间，
because it's the best analogy we have for what we're trying to describe, which basically boils down to this:
因为这是描述宇宙最好的类比方式了，可以归结如下：
If you shine two laser beams in parallel, do they stay parallel? Or do they eventually cross or diverge?
如果我们打出2道平行的激光束，那么它们会保持平行吗？它们最后会相交吗？
When we say the universe is flat, we mean that the beams stay parallel, space isn't warped in a way that would curve them toward or away from each other.
我们说宇宙是平面的时候，就意味着激光束可以保持平行，就算空间再弯曲，也不会让它们彼此相交。
The ‘donut' universe is actually a topology with a flat geometry, which our simplified 3D model can't, well, model.
对于甜甜圈模型的宇宙来说，它的拓扑结构就是平面的几何结构，而我们简易的三维模型是不能无法很好的体现这一点。
But the idea is that instead of a simple plane that stretches to infinity in 3 dimensions, it connects at multiple points. In other words, it's finite.
但其理念是：它并不是一种可以在三个维度无限延伸的简单平面，它可以在不同的点进行连接，换言之，它是有限的。
But it mimics an infinite universe because light wraps all the way around the shape, so you get the effect of standing in one of those infinity light boxes.
但它模拟的是无限的宇宙，因为光会遍布其中，这样就能产生这样一种效果，即处于一种无限的灯箱中。
The 2D version of this is a classic video game where, if you move to one side of the screen, you pop up on the opposite side with the same speed and trajectory.
它的二维版本就像一种经典的游戏：移到屏幕的一端，就会以同样的速度和路径出现在另一端。
In 3D space, if the universe were incredibly small, and you looked directly up, you'd see the bottom of your own feet.
在三维空间中，如果宇宙非常小的话，抬头一看，就能看到脚底。
Or if you looked over your shoulder, you'd see yourself repeated in an infinitely long line.
或者你看过自己的肩膀，就能看见自己在无限长的一条线上不断重复出现。
But our universe isn't that small. So how could we possibly tell if we're living in a 3-torus?
不过，宇宙一点都不小，那么，我们怎样判断自己是否生活在圆环面中呢？
Basically, astronomers hunt for recurring patterns in astronomical images, the same cosmological structures showing up in multiple parts of the sky.
天文学家会在天文图像中寻找循环出现的模式，相同的天文结构出现在天空的不同地方。
That would mean light is wrapping around the universe, and we could run models on those patterns to determine its topology.
也就是说，光会出现在宇宙中的各个地方，而我们可以在这些模式中使用一些模型来确定拓扑结构。
Or at least, we could if the true size of universe is smaller than the distance it takes the light of everything to reach us.
或者这种情况下也可以：宇宙真正的大小要比物体发出的光线抵达地球短。
Because remember, light takes time to get to us. There could be things out there so far away that even though the light coming from them
因为毕竟光需要时间来传播，可能有些物体离我们非常遥远，以至于即便从他们产生之处
has been traveling almost since the beginning of time, it still hasn't reached us yet.
光线就开始传播了，但直到现在，我们也还没有收到它们发出的光线。
If there is stuff beyond where the earliest light we can see originated, the light wouldn't have the opportunity to wrap around and create any repeat images.
如果宇宙中的物体超过了最初抵达地球的光线所来自的物体，那么这些光线是不可能环绕宇宙中并产生图像的。
The universe could be a donut, and we'd never know. That said, results as recent as 2015, courtesy of the European Space Agency's now-defunct Planck satellite,
或许宇宙真的是甜甜圈的形状，但我们无从得知，虽然如此，最近的一次研究是在2015年，当时欧洲太空总署现已停止运行的普朗克卫星
have failed to find any evidence of a 3-torus topology. Or any topology.
也没能找到圆环面拓扑结构的证据，就连其他拓扑结构的证据也没找到。
So as far as we can tell, the universe is not a donut. But what if it were?
目前为止，我们可以得出的结论是：宇宙不是甜甜圈的拓扑结构，但如果宇宙是甜甜圈拓扑结构呢？
Well, in day-to-day life - even in certain fields of astronomy, it wouldn't matter very much.
在我们的日常生活甚至是在天文学的某些领域里，这个问题并不重要。
Just like there's a lot of physics we can still do with old-school Newtonian equations,
这就好像，我们可以用很多牛顿学说的方程式来解决很多物理问题，
changing how we think about the universe's topology wouldn't require rewriting the rules of stellar life cycles or planetary formation.
改变我们对于宇宙拓扑结构的看法并不需要改变恒星生命周期或者行星组成的规则。
But there is one bit of cosmology a donut universe definitely threatens: inflation.
但如果宇宙果真是甜甜圈的拓扑结构，那么确实会威胁到宇宙学的一个方面：膨胀。
Cosmologists think the universe suddenly expanded in size when it was super duper young.
宇宙学家认为，宇宙初期发生了突然膨胀的现象。
When we look at the light left over from the Big Bang, what's known as the cosmic microwave background, or CMB, we can see evidence of that inflation.
但我们观察大爆炸时期留下来的光的时候，也就是宇宙微波背景辐射，我们可以发现膨胀过的证据。
But there's nothing in the theory that would cause inflation to produce a universe big enough to house all the galaxies,
但这个理论中并未陈述有任何东西可以引发膨胀，继而产生一个巨大无比的宇宙，大到可以囊括所有的星系，
yet small enough to see itself repeated within the observable horizon.
同时又小到在可观察的空间里不断自我重复。
So knowing the universe was a donut would force astronomers to find a new explanation for why the CMB looks the way it does.
所以天文学家如果能确定宇宙是甜甜圈的拓扑结构，就能迫使他们寻找宇宙微波背景辐射的成因。
But one of the cooler consequences of a donut universe? Somewhere out there, some of the oodles of galaxies would actually be the Milky Way itself.
还会不会有更酷的发现呢？一些星系可能就是银河系本身。
But we wouldn't be able to tell just by looking at them, because they'd appear billions of years younger.
但光凭观测，我们是无法得出这一点的，因为这些星系可能比实际看起来要小很多。
So you could actually wave to yourself, but you'd be very dead before you received the message.
所以我们有可能是在跟自己打招呼，而且在离开这个世界之前也不会收到回应。
But hey, it's the thought that counts. Thanks for watching this episode of SciShow Space.
不过，重要的是想法，感谢收看本期的《太空科学秀》。
If you want to celebrate your love of the universe and drink coffee out of a topological donut every morning, we've got your covered!
如果你想表达自己对宇宙的热爱，也想每天早上一杯咖啡配甜甜圈的话，就来看看吧！
Head over the DFTBA.com/SciShow to get your own topological donut!
登录DFTBA.com/SciShow来获取属于你的那份甜甜圈！
It's a coffee mug. We hope you like it. And thanks for supporting SciShow Space!
是个咖啡杯哦，我相信大家一定会喜欢的，感谢支持《太空科学秀》！
