来自《耶鲁大学开放课程：基础物理 第14集 第14集》

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  One can ask that question.
  人们可以问这样的问题
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  So, when we look at that,
  当我们考虑这个问题时
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  we'll find out that that's not the case;
  我们会发现情况并不是这样
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  t^2+x^2 is not invariant.
  t^2+x^2 并非是不变量
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  But even before you do that,
  但在我们考虑这个问题之前
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  you should shudder at the prospect of writing something like this. Right?
  还是应当三思而后行
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  What's wrong with this?
  它错在哪
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  Good. Everyone's on top of the units here.
  大家都注意到了单位
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  So, you cannot add t^2 and x^2.
  因此我们不能简单把 t^2 和 x^2 相加
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  There is no way this can be anything.
  这个式子不代表任何东西
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  So, we know how we have to fix that.
  我们知道该如何去修正它
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  We've got to have either both coordinates in space-time
  我们有了时空坐标