It has got two mirrors

它有两个镜面
and a light pulse just goes up and down between the mirrors

在这两镜面间有一道光脉冲上下窜动
and every time it completes a round trip,

每当它完成一个来回
it sets off some detector and it goes "click".

传感器会被触发并"滴答"响一声
Now, this is very important to mention that

这里提一点很重要的一点
this is my x coordinate and that's the y coordinate

这是我的 x 轴这是 y 轴
and I did not show you here there are logical arguments and why

我这里就不做逻辑推导说明原因了
"if you and I are moving along x,

"假设你我沿 x 轴移动
our y coordinates, if they agree in the beginning,

那我们的 y 坐标如果开始时相同
continue to agree."

那就一直都会相同"
That's because if you say something is two meters tall

这是因为假设有两个 2 米高的物件
and they pass each other,

如果它们相遇