来自《耶鲁大学开放课程：基础物理 第14集 第14集》

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  There's nothing privileged.
  没有什么普遍的规律
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  It's the one with the minus signs in it
  只有这个带负号的组合
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  that has the privileged role
  在点乘的计算中
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  of playing the role of the dot product.
  才会被优先考虑
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  So, space-time is not Euclidean.
  时空并不是欧氏空间
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  Euclidean space is the space in which we live,
  我们生活的空间是欧式空间
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  and distance squared is the sum of the squares of all the coordinates.
  距离的平方等于所有坐标的平方之和
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  So, it's a pseudo-Euclidean space
  所以在这个伪欧氏空间中
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  in which to find the invariant length,
  要找到不变的长度
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  you've got to square some components
  我们必须在某些分量的平方里
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  and subtract from them the square of some other components.
  减去一些其它分量的平方
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  Okay. So, finally,
  好 最后