最近一个周期中，M82的重力增强了M81周围密度波的脉动，导致M81的旋臂更浓密。

Last go-round, M82's gravity likely raised density waves rippling around M81, resulting in the richness of M81's spiral arms.

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锂是元素周期表第三“轻”的元素，这使得锂具备更高的能量密度。

Lithium is the third-lightest element on the periodic table, which allows for far greater energy density.

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在这个周期的末尾就要进入人们的视线的时候，你已经准备好回到那个你曾经呆过的更高的密度。

As the end time of this cycle comes into sight, you are preparing to move back to the higher dimensions from whence you came.

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So, instead of having the periodic variation of water, or the periodic variation of air density here we're talking about an electric field.

不像水或者空气密度的,周期变化，我们这里,讨论的是电场。

麻省理工公开课 - 化学原理课程节选

So, for example, we have this average level, and then it can go high where we have the peak, or it can go very low. We can also discuss sound waves, so again it's just the periodic variation of some property -- in this case we're talking about density, so we have high density areas and low density areas.

例如这是平均位置，在峰的位置水位高，在谷水位低，我们也可以,讨论声波它也是某种,量的周期变化，在这里我们,讨论的是密度,我们有高密度区和低密度区。

麻省理工公开课 - 化学原理课程节选