if so, wouldn't no one notice because everything would scale up at the same relative size and

如果如此的話，大家就都不會注意到了，因為萬物都成比例地變大，然後

it would seem like nothing had changed?

就好像都沒有變大一樣？

Well, the truth is, the universe isn't expanding. Space is. Of course, everything else in the

事實上，宇宙並沒有在擴張，但是太空有。當然，在宇宙裡的萬物

universe is "in" space, and ignoring the influence of any other forces, when the space between

都在太空「裡面」，而且不受到其他力的影響，當物質之間的太空

things expands, those things will appear to be carried farther apart. But there are other

擴張了，那些物質就會看起來像是被帶著分離得更遠了。但是在宇宙中

forces in the universe, and their strengths aren't directly affected when space expands.

有其他的力，而且當太空擴張時，它們的強度並不會直接地被影響的

For example, gravity and electromagnetism are able to hold atoms and solar systems together

舉例來說，重力和電磁力可以把原子和太陽系拉再一起

because these forces are too strong for the slow expansion of space to overcome.

因為它們的重力超過了太空緩慢的擴張

Think of it like this - if you and your cat are standing on a piece of ground that starts

想像一下像這樣，如果你和你的貓站在同一片地上，並開始

to stretch horizontally between you, the ground will pull you and your cat away from each

在你們之間水平地擴張開來，那塊地就會把你和你的貓拉離彼此

other. But if the leash is strong enough - in particular, if it's stronger than the friction

但是如果鍊子夠強壯，尤其如果鍊子的強度比你的腳和

force between your feet and the ground - it'll hold you and your cat together as space expands

地板之間的磨擦力，這力量就會把你和你的貓拉在一起，就算太空同時在

beneath you.

你腳下擴張

In the real universe, it's the strength of the electromagnetic force that determines

在真實的宇宙之中，電磁力的強度會決定

the effective sizes of particles, atoms, and molecules so that even if the space they're

粒子、原子和分子的實際尺寸，所以就算太空正在

in expands, the molecules don't. Electromagnetism is basically the leash that holds atoms and

擴張中，分子也不會隨之擴張。電磁基本上就是那個鍊子，拉住原子和

molecules together - and gravity is the leash that holds solar systems and galaxies together.

分子在一起，而且重力就是將太陽系和銀河系拉在一起的鍊子

Only over super super long distances is gravity's attraction weak enough and the cumulative

只有當距離超級超級遠，重力的引力夠弱的時候，累積已久的

expansion of all that space big enough to cause galaxies and galactic clusters to actually

太空擴張程度大到有辦法形成星系和星系團，以至於實際上

grow more distant from one another over time - we know this because the light from distant

隨著時間在彼此間拉遠的距離，我們會知道的原因是因為從遙遠

galaxies gets red-shifted as it's stretched by the expansion of the space it's traveling

星系的光線會紅移，就是因為光線在太空中傳播的時候被太空的擴張而拉長了

through. Gravity certainly still pulls distant objects towards each other, but space expands

重力當然還是會將遙遠的物體拉近，但是太空擴張得

faster. Basically, the cat is off the leash.

更快。基本上，那隻貓的鍊子已經掉了